ISO/IEC 10646:2003: Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor
If
mis less than zero, return the String concatenation of the String
"-"and
ToString(-
m).
If
mis
+∞, return the String
"Infinity".
Otherwise, let
n,
k, and
sbe integers such that
k≥ 1, 10
k-1≤
s< 10
k, the Number value for
s× 10
n-
kis
m, and
kis as small as possible. Note that
kis the number of digits in the decimal representation of
s, that
sis not divisible by 10, and that the least significant digit of
sis not necessarily uniquely determined by these criteria.
If
k≤
n≤ 21, return the String consisting of the code units of the
kdigits of the decimal representation of
s(in order, with no leading zeroes), followed by
n-
koccurrences of the code unit 0x0030 (DIGIT ZERO).
If 0 <
n≤ 21, return the String consisting of the code units of the most significant
ndigits of the decimal representation of
s, followed by the code unit 0x002E (FULL STOP), followed by the code units of the remaining
k-
ndigits of the decimal representation of
s.
If -6 <
n≤ 0, return the String consisting of the code unit 0x0030 (DIGIT ZERO), followed by the code unit 0x002E (FULL STOP), followed by -
noccurrences of the code unit 0x0030 (DIGIT ZERO), followed by the code units of the
kdigits of the decimal representation of
s.
Otherwise, if
k= 1, return the String consisting of the code unit of the single digit of
s, followed by code unit 0x0065 (LATIN SMALL LETTER E), followed by the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether
n-1 is positive or negative, followed by the code units of the decimal representation of the integer
abs(
n-1) (with no leading zeroes).
Return the String consisting of the code units of the most significant digit of the decimal representation of
s, followed by code unit 0x002E (FULL STOP), followed by the code units of the remaining
k-1 digits of the decimal representation of
s, followed by code unit 0x0065 (LATIN SMALL LETTER E), followed by code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether
n-1 is positive or negative, followed by the code units of the decimal representation of the integer
abs(
n-1) (with no leading zeroes).
否则，让 n ， k 和 s 是整数，使得 k ≥1,10 var> k -1 ≤ s ＆lt; 10 k ， s的数值×10 n - k 是 m ， k 尽可能小。 如果 s 有多种可能，请选择 s 的值 x 10 n - k 的值最接近 m 。 如果有 s 的两个这样的值，请选择一个是均匀的。 请注意， k 是 s 的十进制表示中的位数，并且 s 不能被10整除。
注意 3
ECMAScript的实现者可能会发现David M. Gay为浮点数的二进制到十进制转换而编写的论文和代码很有用:
If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return true; otherwise, return false.
比较 x ＆lt; y ，其中 x 和 y 是值，生成 true ， false 或 undefined （表示至少有一个操作数是 NaN ）。除了 x 和 y ，算法将一个名为 LeftFirst 的布尔标志作为参数。该标志用于控制在 x 和 y 上执行具有潜在可见副作用的操作的顺序。这是必要的，因为ECMAScript指定从左到右的表达式的执行。 LeftFirst 的默认值为 true ，表示 x 参数对应于 y 参数的相应表达式。如果 LeftFirst 是 false ，则反之亦然，必须在 y 之前执行操作，然后再在 x 。这样的比较如下进行:
If py is a prefix of px, return false. (A String value p is a prefix of String value q if q can be the result of concatenating p and some other String r. Note that any String is a prefix of itself, because r may be the empty String.)
If px is a prefix of py, return true.
Let k be the smallest nonnegative integer such that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.)
Let m be the integer that is the code unit value at index k within px.
Let n be the integer that is the code unit value at index k within py.
If m < n, return true. Otherwise, return false.
Else,
Let nx be ? ToNumber(px). Because px and py are primitive values evaluation order is not important.
If nx and ny are the same Number value, return false.
If nx is +0 and ny is -0, return false.
If nx is -0 and ny is +0, return false.
If nx is +∞, return false.
If ny is +∞, return true.
If ny is -∞, return false.
If nx is -∞, return true.
If the mathematical value of nx is less than the mathematical value of ny —note that these mathematical values are both finite and not both zero—return true. Otherwise, return false.
抽象操作DefinePropertyOrThrow用于调用对象的[[DefineOwnProperty]]内部方法，方法是在不能执行请求的属性更新时引发 TypeError 异常。 使用参数 O ， P 和 desc 调用操作，其中 O 是对象， P 是属性键， desc 是属性描述符(Property Descriptor)。 此抽象操作执行以下步骤:
抽象操作Invoke用于调用 ECMAScript语言值(ECMAScript language value)。 使用参数 V ， P 和可选的 argumentsList 调用该操作，其中 V 用作查找属性和提供调用的this值， P 是属性键， argumentsList 是传递的参数值的列表 的方法。 如果 argumentsList 不存在，则新的空List用作其值。 此抽象操作执行以下步骤:
If desc.[[Enumerable]] is true, append key to names.
Order the elements of names so they are in the same relative order as would be produced by the Iterator that would be returned if the EnumerateObjectProperties internal method was invoked with O.
Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
在envRec中为N创建可变绑定，并记录它是未初始化的。 如果D为true，则记录新创建的绑定可能会被随后的删除绑定调用删除(Create a mutable binding in envRec for N and record that it is uninitialized. If D is true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.)
具体的环境记录Environment Record方法CreateImmutableBinding用于声明性环境记录创建一个 未初始化的名称 N 的新的不可变绑定。 环境记录中必须不存在N。 如果布尔参数 S 具有值 true ，则新绑定被标记为严格绑定.
Let envRec be the declarative Environment Record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized. If S is true, record that the newly created binding is a strict binding.
具体的环境记录Environment RecordSetMutableBinding方法用于声明性环境记录的尝试将名称为参数 N 的值的标识符的当前绑定的绑定值更改为参数 V 的值。 通常已经存在 N 的绑定，但在极少数情况下可能不存在。 如果绑定是不可变绑定，则如果 S 为 true ，则抛出 TypeError .
Let envRec be the declarative Environment Record for which the method was invoked.
For each property of the Global Object specified in clause 18, do
Let name be the String value of the property name.
Let desc be the fully populated data property descriptor for the property containing the specified attributes for the property. For properties listed in 18.2, 18.3, or 18.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
Let callerScriptOrModule be callerContext's ScriptOrModule.
Let pending be PendingJob{ [[Job]]: job, [[Arguments]]: arguments, [[Realm]]: callerRealm, [[ScriptOrModule]]: callerScriptOrModule, [[HostDefined]]: undefined }.
Perform any implementation or host environment defined processing of pending. This may include modifying the [[HostDefined]] field or any other field of pending.
Add pending at the back of the Job Queue named by queueName.
If the host requires use of an exotic object to serve as realm's global object, let global be such an object created in an implementation defined manner. Otherwise, let global be undefined, indicating that an ordinary object should be created as the global object.
If the host requires that the this binding in realm's global scope return an object other than the global object, let thisValue be such an object created in an implementation defined manner. Otherwise, let thisValue be undefined, indicating that realm's global this binding should be the global object.
Create any implementation defined global object properties on globalObj.
In an implementation dependent manner, obtain the ECMAScript source texts (see clause 10) and any associated host-defined values for zero or more ECMAScript scripts and/or ECMAScript modules. For each such sourceText and hostDefined,
If sourceText is the source code of a script, then
If O is not undefined, create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
If O is not undefined, create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
Return true.
Return true, if every field in Desc is absent.
Return true, if every field in Desc also occurs in current and the value of every field in Desc is the same value as the corresponding field in current when compared using the SameValue algorithm.
If the [[Configurable]] field of current is false, then
Return false, if the [[Configurable]] field of Desc is true.
Return false, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other.
If O is not undefined, convert the property named P of object O from a data property to an accessor property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
Else,
If O is not undefined, convert the property named P of object O from an accessor property to a data property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to their default values.
Assert: functionKind is either "normal", "non-constructor" or "generator".
If functionKind is "normal", let needsConstruct be true.
Else, let needsConstruct be false.
If functionKind is "non-constructor", let functionKind be "normal".
Let F be a newly created ECMAScript function object with the internal slots listed in Table 27. All of those internal slots are initialized to undefined.
Set F's essential internal methods to the default ordinary object definitions specified in 9.1.
Set F's [[Call]] internal method to the definition specified in 9.2.1.
If needsConstruct is true, then
Set F's [[Construct]] internal method to the definition specified in 9.2.2.
Set the [[ConstructorKind]] internal slot of F to "base".
Set the [[Strict]] internal slot of F to strict.
Set the [[FunctionKind]] internal slot of F to functionKind.
Set the [[Prototype]] internal slot of F to functionPrototype.
Set the [[Extensible]] internal slot of F to true.
Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded be true.
If the value of the [[ThisMode]] internal slot of func is lexical, then
NOTE Arrow functions never have an arguments objects.
Let argumentsObjectNeeded be false.
Else if "arguments" is an element of parameterNames, then
Let argumentsObjectNeeded be false.
Else if hasParameterExpressions is false, then
If "arguments" is an element of functionNames or if "arguments" is an element of lexicalNames, then
Let argumentsObjectNeeded be false.
For each String paramName in parameterNames, do
Let alreadyDeclared be envRec.HasBinding(paramName).
NOTE Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
NOTE mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
Let iteratorRecord be Record {[[Iterator]]: CreateListIterator(argumentsList), [[Done]]: false}.
If hasDuplicates is true, then
Perform ? IteratorBindingInitialization for formals with iteratorRecord and undefined as arguments.
Else,
Perform ? IteratorBindingInitialization for formals with iteratorRecord and env as arguments.
If hasParameterExpressions is false, then
NOTE Only a single lexical environment is needed for the parameters and top-level vars.
Let instantiatedVarNames be a copy of the ListparameterNames.
For each n in varNames, do
If n is not an element of instantiatedVarNames, then
Append n to instantiatedVarNames.
Perform ! envRec.CreateMutableBinding(n, false).
Call envRec.InitializeBinding(n, undefined).
Let varEnv be env.
Let varEnvRec be envRec.
Else,
NOTE A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
NOTE: Non-strict functions use a separate lexical Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
Set the LexicalEnvironment of calleeContext to lexEnv.
Let lexDeclarations be the LexicallyScopedDeclarations of code.
For each element d in lexDeclarations do
NOTE A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
Let result be the Completion Record that is the result of evaluating F in an implementation defined manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
Let result be the Completion Record that is the result of evaluating F in an implementation defined manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.
Assert: steps is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.
Let func be a new built-in function object that when called performs the action described by steps. The new function object has internal slots whose names are the elements of internalSlotsList. The initial value of each of those internal slots is undefined.
Let target be the value of F's [[BoundTargetFunction]] internal slot.
Let boundThis be the value of F's [[BoundThis]] internal slot.
Let boundArgs be the value of F's [[BoundArguments]] internal slot.
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
Let target be the value of F's [[BoundTargetFunction]] internal slot.
Assert: target has a [[Construct]] internal method.
Let boundArgs be the value of F's [[BoundArguments]] internal slot.
Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
If SameValue(F, newTarget) is true, let newTarget be target.
Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
The abstract operation CreateMappedArgumentsObject is called with object func, parsed grammar phrase formals, ListargumentsList, and Environment Recordenv. The following steps are performed:
Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
Let len be the number of elements in argumentsList.
Let obj be a newly created arguments exotic object with a [[ParameterMap]] internal slot.
Set the [[GetOwnProperty]] internal method of obj as specified in 9.4.4.1.
Set the [[DefineOwnProperty]] internal method of obj as specified in 9.4.4.2.
Set the [[Get]] internal method of obj as specified in 9.4.4.3.
Set the [[Set]] internal method of obj as specified in 9.4.4.4.
Set the [[HasProperty]] internal method of obj as specified in 9.4.4.5.
Set the [[Delete]] internal method of obj as specified in 9.4.4.6.
Set the remainder of obj's essential internal methods to the default ordinary object definitions specified in 9.1.
The abstract operation MakeArgGetter called with String name and Environment Recordenv creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps:
An ArgGetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgGetter function f that expects no arguments is called it performs the following steps:
Let name be the value of f's [[Name]] internal slot.
Let env be the value of f's [[Env]] internal slot.
Return env.GetBindingValue(name, false).
Note
ArgGetter functions are never directly accessible to ECMAScript code.
The abstract operation MakeArgSetter called with String name and Environment Recordenv creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps:
An ArgSetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgSetter function f is called with argument value it performs the following steps:
Let name be the value of f's [[Name]] internal slot.
Let env be the value of f's [[Env]] internal slot.
Return env.SetMutableBinding(name, value, false).
Note
ArgSetter functions are never directly accessible to ECMAScript code.
When the [[DefineOwnProperty]] internal method of an Integer Indexed exotic object O is called with property key P, and Property DescriptorDesc, the following steps are taken:
When the [[Get]] internal method of an Integer Indexed exotic object O is called with property key P and ECMAScript language valueReceiver, the following steps are taken:
When the [[Set]] internal method of an Integer Indexed exotic object O is called with property key P, value V, and ECMAScript language valueReceiver, the following steps are taken:
The abstract operation IntegerIndexedObjectCreate with arguments prototype and internalSlotsList is used to specify the creation of new Integer Indexed exotic objects. The argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. IntegerIndexedObjectCreate performs the following steps:
Assert: internalSlotsList contains the names [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]].
Let A be a newly created object with an internal slot for each name in internalSlotsList.
Set A's essential internal methods to the default ordinary object definitions specified in 9.1.
Set the [[GetOwnProperty]] internal method of A as specified in 9.4.5.1.
Set the [[HasProperty]] internal method of A as specified in 9.4.5.2.
Set the [[DefineOwnProperty]] internal method of A as specified in 9.4.5.3.
Set the [[Get]] internal method of A as specified in 9.4.5.4.
Set the [[Set]] internal method of A as specified in 9.4.5.5.
Set the [[OwnPropertyKeys]] internal method of A as specified in 9.4.5.6.
Set the [[Prototype]] internal slot of A to prototype.
Set the [[Extensible]] internal slot of A to true.
When the [[DefineOwnProperty]] internal method of a module namespace exotic object O is called with property key P and Property DescriptorDesc, the following steps are taken:
When the [[Get]] internal method of a module namespace exotic object O is called with property key P and ECMAScript language valueReceiver, the following steps are taken:
When the [[Set]] internal method of a module namespace exotic object O is called with property key P, value V, and ECMAScript language valueReceiver, the following steps are taken:
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target »)).
Let targetResult be ? target.[[IsExtensible]]().
If SameValue(booleanTrapResult, targetResult) is false, throw a TypeError exception.
Return booleanTrapResult.
Note
[[IsExtensible]] for proxy objects enforces the following invariant:
The result of [[IsExtensible]] is a Boolean value.
[[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's target object with the same argument.
If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
Throw a TypeError exception.
Return resultDesc.
Note
[[GetOwnProperty]] for proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.
A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.
When the [[DefineOwnProperty]] internal method of a Proxy exotic object O is called with property key P and Property DescriptorDesc, the following steps are taken:
If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
Return true.
Note
[[DefineOwnProperty]] for proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]] is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
When the [[Get]] internal method of a Proxy exotic object O is called with property key P and ECMAScript language valueReceiver, the following steps are taken:
Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined, then
If IsDataDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false and targetDesc.[[Get]] is undefined, then
If trapResult is not undefined, throw a TypeError exception.
Return trapResult.
Note
[[Get]] for proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.
When the [[Set]] internal method of a Proxy exotic object O is called with property key P, value V, and ECMAScript language valueReceiver, the following steps are taken:
Let booleanTrapResult be ToBoolean(? Call(trap, handler, « target, P, V, Receiver »)).
If booleanTrapResult is false, return false.
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined, then
If IsDataDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
If SameValue(V, targetDesc.[[Value]]) is false, throw a TypeError exception.
If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Configurable]] is false, then
If targetDesc.[[Set]] is undefined, throw a TypeError exception.
Return true.
Note
[[Set]] for proxy objects enforces the following invariants:
The result of [[Set]] is a Boolean value.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.
The [[Call]] internal method of a Proxy exotic object O is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:
Let handler be the value of the [[ProxyHandler]] internal slot of O.
A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
The [[Construct]] internal method of a Proxy exotic object O is called with parameters argumentsList which is a possibly empty List of ECMAScript language values and newTarget. The following steps are taken:
Let handler be the value of the [[ProxyHandler]] internal slot of O.
Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
If Type(newObj) is not Object, throw a TypeError exception.
Return newObj.
Note 1
A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for proxy objects enforces the following invariants:
The TRV of UnicodeEscapeSequence::u{HexDigits} is the sequence consisting of code unit value 0x0075 followed by code unit value 0x007B followed by TRV of HexDigits followed by code unit value 0x007D.
CreateDataProperty is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].
Return a List whose first element is siteObj, whose second elements is firstSub, and whose subsequent elements are the elements of restSub, in order. restSub may contain no elements.
Append the Record{[[Strings]]: rawStrings, [[Array]]: template} to templateRegistry.
Return template.
Note 1
The creation of a template object cannot result in an abrupt completion.
Note 2
Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (12.2.9.5). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template objects.
The abstract operation EvaluateCall takes as arguments a value ref, a syntactic grammar production arguments, and a Boolean argument tailPosition. It performs the following steps:
The abstract operation EvaluateDirectCall takes as arguments a value func, a value thisValue, a syntactic grammar production arguments, and a Boolean argument tailPosition. It performs the following steps:
Let argList be ? ArgumentListEvaluation(arguments).
If Type(func) is not Object, throw a TypeError exception.
If IsCallable(func) is false, throw a TypeError exception.
Assert: If tailPosition is true, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
Return a value of type Reference that is a Super Reference whose base value is bv, whose referenced name is propertyKey, whose thisValue is actualThis, and whose strict reference flag is strict.
The sign of the result is positive if both operands have the same sign, negative if the operands have different signs.
Division of an infinity by an infinity results in NaN.
Division of an infinity by a zero results in an infinity. The sign is determined by the rule already stated above.
Division of an infinity by a nonzero finite value results in a signed infinity. The sign is determined by the rule already stated above.
Division of a finite value by an infinity results in zero. The sign is determined by the rule already stated above.
Division of a zero by a zero results in NaN; division of zero by any other finite value results in zero, with the sign determined by the rule already stated above.
Division of a nonzero finite value by a zero results in a signed infinity. The sign is determined by the rule already stated above.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the quotient is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows; the result is then an infinity of appropriate sign. If the magnitude is too small to represent, the operation underflows and the result is a zero of the appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.
If the dividend is finite and the divisor is an infinity, the result equals the dividend.
If the dividend is a zero and the divisor is nonzero and finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, the floating-point remainder r from a dividend n and a divisor d is defined by the mathematical relation r = n - (d × q) where q is an integer that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d. r is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode.
Returns an implementation-dependent approximation of the result of raising base to the power exponent.
If exponent is NaN, the result is NaN.
If exponent is +0, the result is 1, even if base is NaN.
If exponent is -0, the result is 1, even if base is NaN.
If base is NaN and exponent is nonzero, the result is NaN.
If abs(base) > 1 and exponent is +∞, the result is +∞.
If abs(base) > 1 and exponent is -∞, the result is +0.
If abs(base) is 1 and exponent is +∞, the result is NaN.
If abs(base) is 1 and exponent is -∞, the result is NaN.
If abs(base) < 1 and exponent is +∞, the result is +0.
If abs(base) < 1 and exponent is -∞, the result is +∞.
If base is +∞ and exponent > 0, the result is +∞.
If base is +∞ and exponent < 0, the result is +0.
If base is -∞ and exponent > 0 and exponent is an odd integer, the result is -∞.
If base is -∞ and exponent > 0 and exponent is not an odd integer, the result is +∞.
If base is -∞ and exponent < 0 and exponent is an odd integer, the result is -0.
If base is -∞ and exponent < 0 and exponent is not an odd integer, the result is +0.
If base is +0 and exponent > 0, the result is +0.
If base is +0 and exponent < 0, the result is +∞.
If base is -0 and exponent > 0 and exponent is an odd integer, the result is -0.
If base is -0 and exponent > 0 and exponent is not an odd integer, the result is +0.
If base is -0 and exponent < 0 and exponent is an odd integer, the result is -∞.
If base is -0 and exponent < 0 and exponent is not an odd integer, the result is +∞.
If base < 0 and base is finite and exponent is finite and exponent is not an integer, the result is NaN.
Note
The result of base**exponent when base is 1 or -1 and exponent is +Infinity or -Infinity differs from IEEE 754-2008. The first edition of ECMAScript specified a result of NaN for this operation, whereas later versions of IEEE 754-2008 specified 1. The historical ECMAScript behaviour is preserved for compatibility reasons.
Return the result of applying the addition operation to lnum and rnum. See the Note below 12.8.5.
Note 1
No hint is provided in the calls to ToPrimitive in steps 5 and 6. All standard objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Exotic objects may handle the absence of a hint in some other manner.
Note 2
Step 7 differs from step 5 of the Abstract Relational Comparison algorithm, by using the logical-or operation instead of the logical-and operation.
The + operator performs addition when applied to two operands of numeric type, producing the sum of the operands. The - operator performs subtraction, producing the difference of two numeric operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE 754-2008 binary double-precision arithmetic:
If either operand is NaN, the result is NaN.
The sum of two infinities of opposite sign is NaN.
The sum of two infinities of the same sign is the infinity of that sign.
The sum of an infinity and a finite value is equal to the infinite operand.
The sum of two negative zeroes is -0. The sum of two positive zeroes, or of two zeroes of opposite sign, is +0.
The sum of a zero and a nonzero finite value is equal to the nonzero operand.
The sum of two nonzero finite values of the same magnitude and opposite sign is +0.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and the operands have the same sign or have different magnitudes, the sum is computed and rounded to the nearest representable value using IEEE 754-2008 round to nearest, ties to even mode. If the magnitude is too large to represent, the operation overflows and the result is then an infinity of appropriate sign. The ECMAScript language requires support of gradual underflow as defined by IEEE 754-2008.
Note
The - operator performs subtraction when applied to two operands of numeric type, producing the difference of its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric operands a and b, it is always the case that a-b produces the same result as a+(-b).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The result is a signed 32-bit integer.
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum & 0x1F.
Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The result is an unsigned 32-bit integer.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
The abstract operation InstanceofOperator(O, C) implements the generic algorithm for determining if an object O inherits from the inheritance path defined by constructor C. This abstract operation performs the following steps:
If Type(C) is not Object, throw a TypeError exception.
Let instOfHandler be ? GetMethod(C, @@hasInstance).
Steps 5 and 6 provide compatibility with previous editions of ECMAScript that did not use a @@hasInstance method to define the instanceof operator semantics. If a function object does not define or inherit @@hasInstance it uses the default instanceof semantics.
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
If r is true, return false. Otherwise, return true.
Note 1
Given the above definition of equality:
String comparison can be forced by: "" + a == "" + b.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:
new String("a") == "a" and "a" == new String("a") are both true.
new String("a") == new String("a") is false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.
The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.
The grammar for a ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.
When an assignment occurs within strict mode code, it is an runtime error if lref in step 1.f of the first algorithm or step 7 of the second algorithm it is an unresolvable reference. If it is, a ReferenceError exception is thrown. The LeftHandSideExpression also may not be a reference to a data property with the attribute value {[[Writable]]: false}, to an accessor property with the attribute value {[[Set]]: undefined}, nor to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.
Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.
The value of a StatementList is the value of the last value producing item in the StatementList. For example, the following calls to the eval function all return the value 1:
When a Block or CaseBlock production is evaluated a new declarative Environment Record is created and bindings for each block scoped variable, constant, function, generator function, or class declared in the block are instantiated in the Environment Record.
BlockDeclarationInstantiation is performed as follows using arguments code and env. code is the grammar production corresponding to the body of the block. env is the Lexical Environment in which bindings are to be created.
If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's object Environment Record, then step 6 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
With parameters value, environment, and propertyName.
Note
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.
undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see 9.2.12). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
The abstract operation ForIn/OfHeadEvaluation is called with arguments TDZnames, expr, and iterationKind. The value of iterationKind is either enumerate or iterate.
The abstract operation ForIn/OfBodyEvaluation is called with arguments lhs, stmt, iterator, lhsKind, and labelSet. The value of lhsKind is either assignment, varBinding or lexicalBinding.
Return an Iterator object (25.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.
The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.
Note
The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function* EnumerateObjectProperties(obj) {
let visited = newSet;
for (let key ofReflect.ownKeys(obj)) {
if (typeof key === "string") {
let desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc && !visited.has(key)) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
}
let proto = Reflect.getPrototypeOf(obj)
if (proto === null) return;
for (let protoName of EnumerateObjectProperties(proto)) {
if (!visited.has(protoName)) yield protoName;
}
}
It is a Syntax Error if this production is not nested, directly or indirectly (but not crossing function boundaries), within an IterationStatement or a SwitchStatement.
A return statement causes a function to cease execution and return a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression.
The with statement adds an object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.
No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.
CaseSelectorEvaluation does not execute the associated StatementList. It simply evaluates the Expression and returns the value, which the CaseBlock algorithm uses to determine which StatementList to start executing.
A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.
The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.
14.1.1Directive Prologues and the Use Strict Directive(指令序言和使用严格指令)#
A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the initial StatementListItem or ModuleItem productions of a FunctionBody, a ScriptBody, or a ModuleBody and where each ExpressionStatement in the sequence consists entirely of a StringLiteral token followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.(指令序言是作为FunctionBody，ScriptBody或ModuleBody的初始声明列表项或模块项产生而出现的最长的表达式语句产生序列，并且序列中的每个表达式语句完全由字符串字面值标记后跟分号组成。 分号可能会显式出现，或者可能通过自动分号插入。 指令序言可能是一个空的序列。)
Multiple occurrences of the same BindingIdentifier in a FormalParameterList is only allowed for functions and generator functions which have simple parameter lists and which are not defined in strict mode code.
The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional with undefined as their default value.
14.1.10Static Semantics: IsAnonymousFunctionDefinition ( production )(匿名函数定义)#
The abstract operation IsAnonymousFunctionDefinition determines if its argument is a function definition that does not bind a name. The argument production is the result of parsing an AssignmentExpression or Initializer. The following steps are taken:
If IsFunctionDefinition of production is false, return false.
Let hasName be the result of HasName of production.
If ContainsExpression of BindingElement is false, return the result of performing IteratorBindingInitialization for BindingElement using iteratorRecord and environment as the arguments.
If ContainsExpression of BindingRestElement is false, return the result of performing IteratorBindingInitialization for BindingRestElement using iteratorRecord and environment as the arguments.
A prototype property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.
Normally, Contains does not look inside most function forms. However, Contains is used to detect new.target, this, and super usage within an ArrowFunction.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 4 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the scope that is captured by the function object of the ArrowFunction.
注解：
function a(...list) {
'use strict'
console.log(list);
}
在chrome中会报语法错误:Uncaught SyntaxError: Illegal 'use strict' directive in function with non-simple parameter list
Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
If Type(innerResult) is not Object, throw a TypeError exception.
NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
如果ClassElementList的PrototypePropertyNameList包含多个“构造函数”，则是一个语法错误。
class test {
constructor () {}
constructor (arg1) {}
}
Uncaught SyntaxError: A class may only have one constructor
如果MethodDefinition的PropName不是“构造函数”，并且MethodDefinition的HasDirectSuper为true，那么这是一个语法错误。
意思是非构造函数不能有super语句
class ClassA {
}
class ClassB extends ClassA {
say () {
super();
}
}
Uncaught SyntaxError: 'super' keyword unexpected here
如果MethodDefinition的PropName是“构造函数”，并且MethodDefinition的SpecialMethod是true，那么它是一个语法错误。
意思是构造函数不能是generator函数，get函数，set函数，否则报错
class ClassC {
* constructor() {}
}
Uncaught SyntaxError: Class constructor may not be a generator
如果MethodDefinition的HasDirectSuper为true，那么这是一个语法错误。
静态方法不能有super语句
class ClassSt extends ClassA {
constructor() {}
static say () {
super();
}
}
Uncaught SyntaxError: 'super' keyword unexpected here
如果MethodDefinition的PropName是“prototype”，那么它是一个语法错误。
class ClassP {
constructor() {}
static prototype () {
}
}
Classes may not have static property named prototype
Let constructorInfo be the result of performing DefineMethod for constructor with arguments proto and constructorParent as the optional functionPrototype argument.
Return the result of HasProductionInTailPosition of body with argument nonterminal.
Note
Tail Position calls are only defined in strict mode code because of a common non-standard language extension (see 9.2.7) that enables observation of the chain of caller contexts.
nonterminal is a parsed grammar production that represents a specific range of source text. When the following algorithms compare nonterminal to other grammar symbols they are testing whether the same source text was matched by both symbols.
A potential tail position call that is immediately followed by return GetValue of the call result is also a possible tail position call. Function calls cannot return reference values, so such a GetValue operation will always returns the same value as the actual function call result.
Assert: leafContext has no further use. It will never be activated as the running execution context.
A tail position call must either release any transient internal resources associated with the currently executing function execution context before invoking the target function or reuse those resources in support of the target function.
For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.
It is a Syntax Error if StatementList Contains super unless the source code containing super is eval code that is being processed by a direct eval that is contained in function code that is not the function code of an ArrowFunction.
It is a Syntax Error if StatementList Contains NewTarget unless the source code containing NewTarget is eval code that is being processed by a direct eval that is contained in function code that is not the function code of an ArrowFunction.
The abstract operation ParseScript with arguments sourceText, realm, and hostDefined creates a Script Record based upon the result of parsing sourceText as a Script. ParseScript performs the following steps:
Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Script as the goal symbol and analyze the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError or ReferenceError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation dependent, but at least one must be present.
An implementation may parse script source text and analyze it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.
When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.
GlobalDeclarationInstantiation is performed as follows using arguments script and env. script is the ScriptBody for which the execution context is being established. env is the global lexical environment in which bindings are to be created.
If vn is not an element of declaredFunctionNames, then
Let vnDefinable be ? envRec.CanDeclareGlobalVar(vn).
If vnDefinable is false, throw a TypeError exception.
If vn is not an element of declaredVarNames, then
Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object. However, if the global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex B.3.3.2 adds additional steps at this point.
Let lexDeclarations be the LexicallyScopedDeclarations of script.
For each element d in lexDeclarations do
NOTE Lexically declared names are only instantiated here but not initialized.
Early errors specified in 15.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more than one Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if the global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an abrupt completion and some global declarations not being instantiated. If this occurs, the code for the Script is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the global object result in global bindings that may be shadowed by let/const/class declarations.
It is a Syntax Error if the LexicallyDeclaredNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the LexicallyDeclaredNames of ModuleItemList also occurs in the VarDeclaredNames of ModuleItemList.
It is a Syntax Error if the ExportedNames of ModuleItemList contains any duplicate entries.
It is a Syntax Error if any element of the ExportedBindings of ModuleItemList does not also occur in either the VarDeclaredNames of ModuleItemList, or the LexicallyDeclaredNames of ModuleItemList.
The duplicate ExportedNames rule implies that multiple export defaultExportDeclaration items within a ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of a Module. If any such errors are detected the Module is not evaluated.
The abstract operation ImportedLocalNames with argument importEntries creates a List of all of the local name bindings defined by a List of ImportEntry Records (see Table 40). ImportedLocalNames performs the following steps:
A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.
For specification purposes Module Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with concrete subclasses. This specification only defines a single Module Record concrete subclass named Source Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.
Module Record defines the fields listed in Table 37. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list in Table 38. All Module definition subclasses must provide concrete implementations of these abstract methods.
模块记录封装有关单个模块的导入和导出的结构信息。此信息用于链接已连接模块集的导入和导出。模块记录包括仅在执行模块时使用的四个字段。
出于规范目的，模块记录值是记录规范类型的值，可以认为是存在于简单的面向对象的层次结构中，其中模块记录是具有具体子类的抽象类。此规范仅定义了名为Source Text Module Record的单个Module Record具体子类。其他规范和实现可以定义与它们定义的替代模块定义工具相对应的附加模块记录子类。
模块记录定义表37中列出的字段。所有模块定义子类至少包括那些字段。模块记录还定义了表38中的抽象方法列表。所有模块定义子类必须提供这些抽象方法的具体实现。
The Lexical Environment containing the top level bindings for this module. This field is set when the module is instantiated.
Lexical Environment包含此模块的顶级绑定。 在实例化模块时设置此字段。
[[Namespace]]
Object | undefined
The Module Namespace Object (26.3) if one has been created for this module. Otherwise undefined.
模块命名空间对象（26.3）（如果已为此模块创建了一个）。 否则为undefined。
[[Evaluated]]
Boolean
Initially false, true if evaluation of this module has started. Remains true when evaluation completes, even if it is an abrupt completion.
最初为false，如果已开始执行模块，则为true。 执行完成时仍然为true，即使它是突然完成。
[[HostDefined]]
Any, default value is undefined.
Field reserved for use by host environments that need to associate additional information with a module.
用于需要将附加信息与模块相关联的host环境使用的保留字段。
Table 38: Abstract Methods of Module Records（模块记录的抽象方法）
Method
Purpose
GetExportedNames(exportStarSet)
Return a list of all names that are either directly or indirectly exported from this module.
返回直接或间接从此模块导出的所有名称的列表。
Return the binding of a name exported by this module. Bindings are represented by a Record of the form {[[Module]]: Module Record, [[BindingName]]: String}
返回此模块导出的名称的绑定。 绑定是一条记录形如：{[[Module]]: Module Record, [[BindingName]]: String}
ModuleDeclarationInstantiation()
Transitively resolve all module dependencies and create a module Environment Record for the module.
传递性地解析所有模块依赖关系并为模块创建模块环境记录。
ModuleEvaluation()
Do nothing if this module has already been evaluated. Otherwise, transitively evaluate all module dependences of this module and then evaluate this module.
ModuleDeclarationInstantiation must be completed prior to invoking this method.
A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (10) that was parsed using the goal symbol Module. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, instantiate, and evaluate the module.
In addition to the fields, defined in Table 37, Source Text Module Records have the additional fields listed in Table 39. Each of these fields initially has the value undefined.
A List of all the ModuleSpecifier strings used by the module represented by this record to request the importation of a module. The List is source code occurrence ordered.
此记录表示模块使用的所有ModuleSpecifier字符串的列表，用于请求导入模块。 List是已排序的源代码。
A List of ExportEntry records derived from the code of this module that correspond to declarations that occur within the module.
从此模块的代码派生的ExportEntry记录列表，对应于模块中发生的声明。
A List of ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module.
从此模块的代码派生的ExportEntry记录列表，对应于模块中出现的重新导出的导入。
A List of ExportEntry records derived from the code of this module that correspond to export * declarations that occur within the module.
从此模块的代码派生的ExportEntry记录列表，对应于模块中出现的export *声明。
An ImportEntry Record is a Record that digests information about a single declarative import. Each ImportEntry Record has the fields defined in Table 40:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value "*" indicates that the import request is for the target module's namespace object.
由[[ModuleRequest]]标识的模块导出所需绑定的名称。 值“*”表示导入请求是针对目标模块的命名空间对象的。
[[LocalName]]
String
The name that is used to locally access the imported value from within the importing module.
本地访问导入模块导入值的名称。
Note 1
Table 41 gives examples of ImportEntry records fields used to represent the syntactic import forms:
表41给出了用于表示语法import格式的ImportEntry记录字段的示例：
Table 41 (Informative): Import Forms Mappings to ImportEntry Records
Import Statement Form
import语句格式
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
import v from "mod";
"mod"
"default"
"v"
import * as ns from "mod";
"mod"
"*"
"ns"
import {x} from "mod";
"mod"
"x"
"x"
import {x as v} from "mod";
"mod"
"x"
"v"
import "mod";
An ImportEntry Record is not created.
未创建ImportEntry记录。
An ExportEntry Record is a Record that digests information about a single declarative export. Each ExportEntry Record has the fields defined in Table 42:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration does not have a ModuleSpecifier. "*" indicates that the export request is for all exported bindings.
由[[ModuleRequest]]标识的模块导出所需绑定的名称。 如果ExportDeclaration没有ModuleSpecifier，则返回null。 “*”表示导出请求适用于所有导出的绑定。
[[LocalName]]
String | null
The name that is used to locally access the exported value from within the importing module. null if the exported value is not locally accessible from within the module.
用于从导入模块中本地访问导出值的名称。 如果无法从模块中本地访问导出的值，则返回null。
Note 2
Table 43 gives examples of the ExportEntry record fields used to represent the syntactic export forms:
表43给出了用于表示export语法格式的ExportEntry记录字段的示例：
Table 43 (Informative): Export Forms Mappings to ExportEntry Records（export格式映射到ExportEntry记录）
Export Statement Form
export语句格式
[[ExportName]]
[[ModuleRequest]]
[[ImportName]]
[[LocalName]]
export var v;
"v"
null
null
"v"
export default function f(){}
"default"
null
null
"f"
export default function(){}
"default"
null
null
"*default*"
export default 42;
"default"
null
null
"*default*"
export {x};
"x"
null
null
"x"
export {v as x};
"x"
null
null
"v"
export {x} from "mod";
"x"
"mod"
"x"
null
export {v as x} from "mod";
"x"
"mod"
"v"
null
export * from "mod";
null
"mod"
"*"
null
The following definitions specify the required concrete methods and other abstract operations for Source Text Module Records
The abstract operation ParseModule with arguments sourceText, realm, and hostDefined creates a Source Text Module Record based upon the result of parsing sourceText as a Module. ParseModule performs the following steps:
Assert: sourceText is an ECMAScript source text (see clause 10).
Parse sourceText using Module as the goal symbol and analyze the parse result for any Early Error conditions. If the parse was successful and no early errors were found, let body be the resulting parse tree. Otherwise, let body be a List of one or more SyntaxError or ReferenceError objects representing the parsing errors and/or early errors. Parsing and early error detection may be interweaved in an implementation dependent manner. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation dependent, but at least one must be present.
If ee.[[LocalName]] is not an element of importedBoundNames, then
Append ee to localExportEntries.
Else,
Let ie be the element of importEntries whose [[LocalName]] is the same as ee.[[LocalName]].
If ie.[[ImportName]] is "*", then
Assert: this is a re-export of an imported module namespace object.
Append ee to localExportEntries.
Else, this is a re-export of a single name
Append to indirectExportEntries the Record {[[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]: null, [[ExportName]]: ee.[[ExportName]] }.
An implementation may parse module source text and analyze it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.
The ResolveExport concrete method of a Source Text Module Record with arguments exportName, resolveSet, and exportStarSet performs the following steps:
Let resolution be ? importedModule.ResolveExport(exportName, resolveSet, exportStarSet).
If resolution is "ambiguous", return "ambiguous".
If resolution is not null, then
If starResolution is null, let starResolution be resolution.
Else,
Assert: there is more than one * import that includes the requested name.
If resolution.[[Module]] and starResolution.[[Module]] are not the same Module Record or SameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) is false, return "ambiguous".
Return starResolution.
Note
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by the Module Record this method was invoked on or some other module that is imported by that module. The parameter resolveSet is use to detect unresolved circular import/export paths. If a pair consisting of specific Module Record and exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.
If a defining module is found a Record {[[Module]], [[BindingName]]} is returned. This record identifies the resolved binding of the originally requested export. If no definition was found or the request is found to be circular, null is returned. If the request is found to be ambiguous, the string "ambiguous" is returned.
For each String required that is an element of module.[[RequestedModules]] do,
NOTE: Before instantiating a module, all of the modules it requested must be available. An implementation may perform this test at any time prior to this point.
HostResolveImportedModule is an implementation defined abstract operation that provides the concrete Module Record subclass instance that corresponds to the ModuleSpecifier String, specifier, occurring within the context of the module represented by the Module RecordreferencingModule.
The implementation of HostResolveImportedModule must conform to the following requirements:
The normal return value must be an instance of a concrete subclass of Module Record.
If a Module Record corresponding to the pair referencingModule, specifier does not exist or cannot be created, an exception must be thrown.
This operation must be idempotent if it completes normally. Each time it is called with a specific referencingModule, specifier pair as arguments it must return the same Module Record instance.
Multiple different referencingModule, specifier pairs may map to the same Module Record instance. The actual mapping semantic is implementation defined but typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as alphabetic case folding and expansion of relative and abbreviated path specifiers.
An implementation may parse a sourceText as a Module, analyze it for Early Error conditions, and instantiate it prior to the execution of the TopLevelModuleEvaluationJob for that sourceText. An implementation may also resolve, pre-parse and pre-analyze, and pre-instantiate module dependencies of sourceText. However, the reporting of any errors detected by these actions must be deferred until the TopLevelModuleEvaluationJob is actually executed.
For each IdentifierNamen in ReferencedBindings of ExportClause: It is a Syntax Error if StringValue of n is a ReservedWord or if the StringValue of n is one of: "implements", "interface", "let", "package", "private", "protected", "public", or "static".
Return a new List containing the Record {[[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
Let sourceName be the StringValue of the first IdentifierName.
Let exportName be the StringValue of the second IdentifierName.
If module is null, then
Let localName be sourceName.
Let importName be null.
Else,
Let localName be null.
Let importName be sourceName.
Return a new List containing the Record {[[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.
It is not necessary to treat export defaultAssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an early error prevents the evaluation of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.
An implementation must report as an early error any occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in 16.2, an implementation may extend Script syntax, Module syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing SyntaxError when they encounter an implementation-defined extension to the script syntax or regular expression pattern or flag syntax.
Except as restricted in 16.2, an implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to have implementation-defined behaviour instead of throwing an error (such as ReferenceError).
An implementation may define behaviour other than throwing RangeError for toFixed, toExponential, and toPrecision when the fractionDigits or precision argument is outside the specified range.
HostReportErrors is an implementation-defined abstract operation that allows host environments to report parsing errors, early errors, and runtime errors.
An implementation of HostReportErrors must complete normally in all cases. The default implementation of HostReportErrors is to do nothing.
Note
errorList will be a List of ECMAScript language values. If the errors are parsing errors or early errors, these will always be SyntaxError or ReferenceError objects. Runtime errors, however, can be any ECMAScript value.
An implementation must not extend this specification in the following ways:
Other than as defined in this specification, ECMAScript Function objects defined using syntactic constructors in strict mode code must not be created with own properties named "caller" or "arguments" other than those that are created by applying the AddRestrictedFunctionProperties abstract operation to the function. Such own properties also must not be created for function objects defined using an ArrowFunction, MethodDefinition, GeneratorDeclaration, GeneratorExpression, ClassDeclaration, or ClassExpression regardless of whether the definition is contained in strict mode code. Built-in functions, strict mode functions created using the Function constructor, generator functions created using the Generator constructor, and functions created using the bind method also must not be created with such own properties.
If an implementation extends non-strict or built-in function objects with an own property named "caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a strict function object. If it is an accessor property, the function that is the value of the property's [[Get]] attribute must never return a strict function when called.
The behaviour of the following methods must not be extended except as specified in ECMA-402: Object.prototype.toLocaleString, Array.prototype.toLocaleString, Number.prototype.toLocaleString, Date.prototype.toLocaleDateString, Date.prototype.toLocaleString, Date.prototype.toLocaleTimeString, String.prototype.localeCompare, %TypedArray%.prototype.toLocaleString.
The RegExp pattern grammars in 21.2.1 and B.1.4 must not be extended to recognize any of the source characters A-Z or a-z as IdentityEscape[U] when the U grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediate follow source text that matches the BindingIdentifier nonterminal symbol.
There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object, is part of the lexical environment of the executing program. Others are accessible as initial properties of the global object or indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in Function object with the characteristics described in 9.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value true. Every built-in Function object has a [[Realm]] internal slot whose value is the Realm Record of the realm for which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of that function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “this value” and “NewTarget” have the meanings given in 9.3.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor described is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (19.2.3), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (19.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.
Unless otherwise specified, each built-in function defined in this specification is created as if by calling the CreateBuiltinFunction abstract operation (9.3.3).
Every built-in Function object, including constructors, has a length property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description. Optional parameters (which are indicated with brackets: []) or rest parameters (which are shown using the form «...name») are not included in the default argument count.
Note 2
For example, the function object that is the initial value of the map property of the Array prototype object is described under the subclause heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value of the length property of that Function object is 1.
Unless otherwise specified, the length property of a built-in Function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every built-in Function object, including constructors, that is not identified as an anonymous function has a name property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. For functions that are specified as properties of objects, the name value is the property name string used to access the function. Functions that are specified as get or set accessor functions of built-in properties have "get " or "set " prepended to the property name string. The value of the name property is explicitly specified for each built-in functions whose property key is a Symbol value.
Unless otherwise specified, the name property of a built-in Function object, if it exists, has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every other data property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.
Every accessor property described in clauses 18 through 26 and in Annex B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value, undefined. If only a set accessor is described the get accessor is the default value, undefined.
The global object does not have a [[Construct]] internal method; it is not possible to use the global object as a constructor with the new operator.
The global object does not have a [[Call]] internal method; it is not possible to invoke the global object as a function.
The value of the [[Prototype]] internal slot of the global object is implementation-dependent.
In addition to the properties defined in this specification the global object may have additional host defined properties. This may include a property whose value is the global object itself; for example, in the HTML document object model the window property of the global object is the global object itself.
The value of undefined is undefined (see 6.1.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Let script be the ECMAScript code that is the result of parsing x, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, for the goal symbol Script. If the parse fails, throw a SyntaxError exception. If any early errors are detected, throw a SyntaxError or a ReferenceError exception, depending on the type of the error (but see also clause 16). Parsing and early error detection may be interweaved in an implementation dependent manner.
If script Contains ScriptBody is false, return undefined.
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if the calling context is evaluating formal parameter initializers or if either the code of the calling context or the eval code is strict code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.
Let vnDefinable be ? varEnvRec.CanDeclareGlobalVar(vn).
If vnDefinable is false, throw a TypeError exception.
If vn is not an element of declaredVarNames, then
Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step unless varEnvRec is a global Environment Record and the global object is a Proxy exotic object.
Let lexDeclarations be the LexicallyScopedDeclarations of body.
For each element d in lexDeclarations do
NOTE Lexically declared names are only instantiated here but not initialized.
The isFinite function is the %isFinite% intrinsic object. When the isFinite function is called with one argument number, the following steps are taken:
A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X !== X. The result will be true if and only if X is a NaN.
The parseFloat function produces a Number value dictated by interpretation of the contents of the string argument as a decimal literal.
The parseFloat function is the %parseFloat% intrinsic object. When the parseFloat function is called with one argument string, the following steps are taken:
Let trimmedString be a substring of inputString consisting of the leftmost code unit that is not a StrWhiteSpaceChar and all code units to the right of that code unit. (In other words, remove leading white space.) If inputString does not contain any such code units, let trimmedString be the empty string.
If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a StrDecimalLiteral (see 7.1.3.1), return NaN.
Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of a StrDecimalLiteral.
Let mathFloat be MV of numberString.
If mathFloat=0, then
If the first code unit of trimmedString is "-", return -0.
Return +0.
Return the Number value for mathFloat.
Note
parseFloat may interpret only a leading portion of string as a Number value; it ignores any code units that cannot be interpreted as part of the notation of an decimal literal, and no indication is given that any such code units were ignored.
The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the code unit pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the code unit pairs 0x or 0X.
The parseInt function is the %parseInt% intrinsic object. When the parseInt function is called, the following steps are taken:
Let S be a newly created substring of inputString consisting of the first code unit that is not a StrWhiteSpaceChar and all code units following that code unit. (In other words, remove leading white space.) If inputString does not contain any such code unit, let S be the empty string.
Let sign be 1.
If S is not empty and the first code unit of S is 0x002D (HYPHEN-MINUS), let sign be -1.
If S is not empty and the first code unit of S is 0x002B (PLUS SIGN) or 0x002D (HYPHEN-MINUS), remove the first code unit from S.
If the length of S is at least 2 and the first two code units of S are either "0x" or "0X", remove the first two code units from S and let R be 16.
If S contains a code unit that is not a radix-R digit, let Z be the substring of S consisting of all code units before the first such code unit; otherwise, let Z be S.
If Z is empty, return NaN.
Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)
If mathInt = 0, then
If sign = -1, return -0.
Return +0.
Let number be the Number value for mathInt.
Return sign × number.
Note
parseInt may interpret only a leading portion of string as an integer value; it ignores any code units that cannot be interpreted as part of the notation of an integer, and no indication is given that any such code units were ignored.
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in 18.2.6.2, 18.2.6.3, 18.2.6.4 and 18.2.6.5
Note
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.
A URI is composed of a sequence of components separated by component separators. The general form is:
Scheme:First/Second;Third?Fourth
where the italicized names represent components and “:”, “/”, “;” and “?” are reserved for use as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code units when the component is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
Runtime Semantics
When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form "%xx".
The unescaping and decoding process is described by the abstract operation Decode taking two String arguments string and reservedSet.
Let strLen be the number of code units in string.
Let R be the empty String.
Let k be 0.
Repeat
If k equals strLen, return R.
Let C be the code unit at index k within string.
If C is not "%", then
Let S be the String containing only the code unit C.
Else C is "%",
Let start be k.
If k + 2 is greater than or equal to strLen, throw a URIError exception.
If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
Increment k by 2.
If the most significant bit in B is 0, then
Let C be the code unit with code unit value B.
If C is not in reservedSet, then
Let S be the String containing only the code unit C.
Else C is in reservedSet,
Let S be the substring of string from index start to index k inclusive.
Else the most significant bit in B is 1,
Let n be the smallest nonnegative integer such that (B << n) & 0x80 is equal to 0.
If n equals 1 or n is greater than 4, throw a URIError exception.
Let Octets be an array of 8-bit integers of size n.
Put B into Octets at index 0.
If k + (3 × (n - 1)) is greater than or equal to strLen, throw a URIError exception.
Let j be 1.
Repeat, while j < n
Increment k by 1.
If the code unit at index k within string is not "%", throw a URIError exception.
If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
If the two most significant bits in B are not 10, throw a URIError exception.
Increment k by 2.
Put B into Octets at index j.
Increment j by 1.
Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from an array of octets into a 21-bit value. If Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw a URIError exception.
If V < 0x10000, then
Let C be the code unit V.
If C is not in reservedSet, then
Let S be the String containing only the code unit C.
Else C is in reservedSet,
Let S be the substring of string from index start to index k inclusive.
Else V ≥ 0x10000,
Let L be (((V - 0x10000) & 0x3FF) + 0xDC00).
Let H be ((((V - 0x10000) >> 10) & 0x3FF) + 0xD800).
Let S be the String containing the two code units H and L.
Let R be a new String value computed by concatenating the previous value of R and S.
Increase k by 1.
Note
This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in Table 44.
Table 44 (Informative): UTF-8 Encodings
Code Unit Value
Representation
1st Octet
2nd Octet
3rd Octet
4th Octet
0x0000 - 0x007F
00000000 0zzzzzzz
0zzzzzzz
0x0080 - 0x07FF
00000yyy yyzzzzzz
110yyyyy
10zzzzzz
0x0800 - 0xD7FF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
0xD800 - 0xDBFF
followed by
0xDC00 - 0xDFFF
110110vv vvwwwwxx
followed by
110111yy yyzzzzzz
11110uuu
10uuwwww
10xxyyyy
10zzzzzz
0xD800 - 0xDBFF
not followed by
0xDC00 - 0xDFFF
causes URIError
0xDC00 - 0xDFFF
causes URIError
0xE000 - 0xFFFF
xxxxyyyy yyzzzzzz
1110xxxx
10yyyyyy
10zzzzzz
Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in Surrogates, section 3.8, of the Unicode Standard.
The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UTF-16 surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs the surrogate pair.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.
The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.
The decodeURI function is the %decodeURI% intrinsic object. When the decodeURI function is called with one argument encodedURI, the following steps are taken:
The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.
The decodeURIComponent function is the %decodeURIComponent% intrinsic object. When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:
Let componentString be ? ToString(encodedURIComponent).
The encodeURI function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code points.
The encodeURI function is the %encodeURI% intrinsic object. When the encodeURI function is called with one argument uri, the following steps are taken:
The encodeURIComponent function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.
The encodeURIComponent function is the %encodeURIComponent% intrinsic object. When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:
The Object constructor is the %Object% intrinsic object and the initial value of the Object property of the global object. When called as a constructor it creates a new ordinary object. When Object is called as a function rather than as a constructor, it performs a type conversion.
The Object constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition.
The assign function is used to copy the values of all of the enumerable own properties from one or more source objects to a target object. When the assign function is called, the following steps are taken:
19.1.2.3Object.defineProperties ( O, Properties )#
The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:
19.1.2.4Object.defineProperty ( O, P, Attributes )#
The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:
If Type(O) is not Object, throw a TypeError exception.
19.1.2.8.1Runtime Semantics: GetOwnPropertyKeys ( O, Type )#
The abstract operation GetOwnPropertyKeys is called with arguments O and Type where O is an Object and Type is one of the ECMAScript specification types String or Symbol. The following steps are taken:
If an implementation defines a specific order of enumeration for the for-in statement, the same order must be used for the elements of the array returned in step 3.
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.
19.1.3.4Object.prototype.propertyIsEnumerable ( V )#
When the propertyIsEnumerable method is called with argument V, the following steps are taken:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocalString functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.
Note 1
This function provides a generic toLocaleString implementation for objects that have no locale-specific toString behaviour. Array, Number, Date, and Typed Arrays provide their own locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
If Type(tag) is not String, let tag be builtinTag.
Return the String that is the result of concatenating "[object ", tag, and "]".
This function is the %ObjProto_toString% intrinsic object.
Note
Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use @@toStringTag in ways that will invalidate the reliability of such legacy type tests.
The Function constructor is the %Function% intrinsic object and the initial value of the Function property of the global object. When Function is called as a function rather than as a constructor, it creates and initializes a new Function object. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
The Function constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Function constructor to create and initialize a subclass instances with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in Generator Function subclass.
The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.
When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:
It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:
The abstract operation CreateDynamicFunction is called with arguments constructor, newTarget, kind, and args. constructor is the constructor function that is performing this action, newTarget is the constructor that new was initially applied to, kind is either "normal" or "generator", and args is a List containing the actual argument values that were passed to constructor. The following steps are taken:
If newTarget is undefined, let newTarget be constructor.
Let parameters be the result of parsing P, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, using parameterGoal as the goal symbol. Throw a SyntaxError exception if the parse fails.
Let body be the result of parsing bodyText, interpreted as UTF-16 encoded Unicode text as described in 6.1.4, using goal as the goal symbol. Throw a SyntaxError exception if the parse fails.
If bodyText is strict mode code, then let strict be true, else let strict be false.
If any static semantics errors are detected for parameters or body, throw a SyntaxError or a ReferenceError exception, depending on the type of the error. If strict is true, the Early Error rules for StrictFormalParameters:FormalParameters are applied. Parsing and early error detection may be interweaved in an implementation dependent manner.
If ContainsUseStrict of body is true and IsSimpleParameterList of parameters is false, throw a SyntaxError exception.
If any element of the BoundNames of parameters also occurs in the LexicallyDeclaredNames of body, throw a SyntaxError exception.
If body Contains SuperCall is true, throw a SyntaxError exception.
If parameters Contains SuperCall is true, throw a SyntaxError exception.
If body Contains SuperProperty is true, throw a SyntaxError exception.
If parameters Contains SuperProperty is true, throw a SyntaxError exception.
If kind is "generator", then
If parameters Contains YieldExpression is true, throw a SyntaxError exception.
If strict is true, then
If BoundNames of parameters contains any duplicate elements, throw a SyntaxError exception.
A prototype property is automatically created for every function created using CreateDynamicFunction, to provide for the possibility that the function will be used as a constructor.
The Function constructor is itself a built-in function object. The value of the [[Prototype]] internal slot of the Function constructor is the intrinsic object %FunctionPrototype%.
The value of the [[Extensible]] internal slot of the Function constructor is true.
The Function constructor has the following properties:
The value of Function.prototype is %FunctionPrototype%, the intrinsic Function prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.2.3Properties of the Function Prototype Object#
The Function prototype object is the intrinsic object %FunctionPrototype%. The Function prototype object is itself a built-in function object. When invoked, it accepts any arguments and returns undefined. It does not have a [[Construct]] internal method so it is not a constructor.
Note
The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
The value of the [[Prototype]] internal slot of the Function prototype object is the intrinsic object %ObjectPrototype%. The initial value of the [[Extensible]] internal slot of the Function prototype object is true.
The Function prototype object does not have a prototype property.
The value of the length property of the Function prototype object is 0.
The value of the name property of the Function prototype object is the empty String.
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is an arrow function or a bound function then the thisArg will be ignored by the function [[Call]] in step 5.
If this method was called with more than one argument, then in left to right order, starting with the second argument, append each argument as the last element of argList.
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is an arrow function or a bound function then the thisArg will be ignored by the function [[Call]] in step 5.
When the toString method is called on an object func, the following steps are taken:
If func is a Bound Function exotic object, then
Return an implementation-dependent String source code representation of func. The representation must conform to the rules below. It is implementation dependent whether the representation includes bound function information or information about the target function.
If Type(func) is Object and is either a built-in function object or has an [[ECMAScriptCode]] internal slot, then
Return an implementation-dependent String source code representation of func. The representation must conform to the rules below.
The use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent.
If the object was defined using ECMAScript code and the returned string representation is not in the form of a MethodDefinition or GeneratorMethod then the representation must be such that if the string is evaluated, using eval in a lexical context that is equivalent to the lexical context used to create the original object, it will result in a new functionally equivalent object. In that case the returned source code must not mention freely any variables that were not mentioned freely by the original function's source code, even if these “extra” names were originally in scope.
If the implementation cannot produce a source code string that meets these criteria then it must return a string for which eval will throw a SyntaxError exception.
19.2.3.6Function.prototype [ @@hasInstance ] ( V )#
When the @@hasInstance method of an object F is called with value V, the following steps are taken:
The value of the name property of this function is "[Symbol.hasInstance]".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Note
This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specific constructor. An expression such as
v instanceof F
evaluates as
F[@@hasInstance](v)
A constructor function can control which objects are recognized as its instances by instanceof by exposing a different @@hasInstance method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.
Every function instance is an ECMAScript function object and has the internal slots listed in Table 27. Function instances created using the Function.prototype.bind method (19.2.3.2) have the internal slots listed in Table 28.
The Function instances have the following properties:
The value of the length property is an integer that indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its length property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
The value of the name property is an String that is descriptive of the function. The name has no semantic significance but is typically a variable or property name that is used to refer to the function at its point of definition in ECMAScript code. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Anonymous functions objects that do not have a contextual name associated with them by this specification do not have a name own property but inherit the name property of %FunctionPrototype%.
Function instances that can be used as a constructor have a prototype property. Whenever such a function instance is created another ordinary object is also created and is the initial value of the function's prototype property. Unless otherwise specified, the value of the prototype property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as a constructor.
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Function objects created using Function.prototype.bind, or by evaluating a MethodDefinition (that are not a GeneratorMethod) or an ArrowFunction grammar production do not have a prototype property.
The Boolean constructor is the %Boolean% intrinsic object and the initial value of the Boolean property of the global object. When called as a constructor it creates and initializes a new Boolean object. When Boolean is called as a function rather than as a constructor, it performs a type conversion.
The Boolean constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Boolean constructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.
The Boolean prototype object is the intrinsic object %BooleanPrototype%. The Boolean prototype object is an ordinary object. The Boolean prototype is itself a Boolean object; it has a [[BooleanData]] internal slot with the value false.
The value of the [[Prototype]] internal slot of the Boolean prototype object is the intrinsic object %ObjectPrototype%.
Boolean instances are ordinary objects that inherit properties from the Boolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.
The Symbol constructor is the %Symbol% intrinsic object and the initial value of the Symbol property of the global object. When Symbol is called as a function, it returns a new Symbol value.
The Symbol constructor is not intended to be used with the new operator or to be subclassed. It may be used as the value of an extends clause of a class definition but a super call to the Symbol constructor will cause an exception.
For each element e of the GlobalSymbolRegistry List,
If SameValue(e.[[Key]], stringKey) is true, return e.[[Symbol]].
Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
Let newSymbol be a new unique Symbol value whose [[Description]] value is stringKey.
Append the Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistry List.
Return newSymbol.
The GlobalSymbolRegistry is a List that is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code it is initialized as a new empty List. Elements of the GlobalSymbolRegistry are Records with the structure defined in Table 45.
The Symbol prototype object is the intrinsic object %SymbolPrototype%. The Symbol prototype object is an ordinary object. It is not a Symbol instance and does not have a [[SymbolData]] internal slot.
The value of the [[Prototype]] internal slot of the Symbol prototype object is the intrinsic object %ObjectPrototype%.
If Type(s) is not Object, throw a TypeError exception.
If s does not have a [[SymbolData]] internal slot, throw a TypeError exception.
Return the value of s's [[SymbolData]] internal slot.
19.4.3.4Symbol.prototype [ @@toPrimitive ] ( hint )#
This function is called by ECMAScript language operators to convert a Symbol object to a primitive value. The allowed values for hint are "default", "number", and "string".
When the @@toPrimitive method is called with argument hint, the following steps are taken:
Symbol instances are ordinary objects that inherit properties from the Symbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.
The Error constructor is the %Error% intrinsic object and the initial value of the Error property of the global object. When Error is called as a function rather than as a constructor, it creates and initializes a new Error object. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
The Error constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Error constructor to create and initialize subclass instances with a [[ErrorData]] internal slot.
The Error prototype object is the intrinsic object %ErrorPrototype%. The Error prototype object is an ordinary object. It is not an Error instance and does not have an [[ErrorData]] internal slot.
The value of the [[Prototype]] internal slot of the Error prototype object is the intrinsic object %ObjectPrototype%.
Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified uses of [[ErrorData]] is to identify Error and NativeError instances as Error objects within Object.prototype.toString.
A new instance of one of the NativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in 19.5.6.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in 19.5.5. Each of these objects has the structure described below, differing only in the name used as the constructor name instead of NativeError, in the name property of the prototype object, and in the implementation-defined message property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 19.5.5.
When a NativeError constructor is called as a function rather than as a constructor, it creates and initializes a new NativeError object. A call of the object as a function is equivalent to calling it as a constructor with the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
Each NativeError constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeError constructor to create and initialize subclass instances with a [[ErrorData]] internal slot.
The actual value of the string passed in step 2 is either "%EvalErrorPrototype%", "%RangeErrorPrototype%", "%ReferenceErrorPrototype%", "%SyntaxErrorPrototype%", "%TypeErrorPrototype%", or "%URIErrorPrototype%" corresponding to which NativeError constructor is being defined.
19.5.6.2Properties of the NativeError Constructors#
The value of the [[Prototype]] internal slot of a NativeError constructor is the intrinsic object %Error%.
Each NativeError constructor has a name property whose value is the String value `"NativeError"`.
Each NativeError constructor has the following properties:
The initial value of the constructor property of the prototype for a given NativeError constructor is the corresponding intrinsic object %NativeError% (19.5.6.1).
The initial value of the name property of the prototype for a given NativeError constructor is a string consisting of the name of the constructor (the name used instead of NativeError).
NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (19.1.3.6) to identify Error or NativeError instances.
The Number constructor is the %Number% intrinsic object and the initial value of the Number property of the global object. When called as a constructor, it creates and initializes a new Number object. When Number is called as a function rather than as a constructor, it performs a type conversion.
The Number constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Number constructor to create and initialize the subclass instance with a [[NumberData]] internal slot.
The value of Number.EPSILON is the difference between 1 and the smallest value greater than 1 that is representable as a Number value, which is approximately 2.2204460492503130808472633361816 x 10-16.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
This function differs from the global isNaN function (18.2.3) in that it does not convert its argument to a Number before determining whether it is NaN.
The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately 5 × 10-324.
In the IEEE 754-2008 double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of the Number.parseFloat data property is the same built-in function object that is the value of the parseFloat property of the global object defined in 18.2.4.
The value of the Number.parseInt data property is the same built-in function object that is the value of the parseInt property of the global object defined in 18.2.5.
The Number prototype object is the intrinsic object %NumberPrototype%. The Number prototype object is an ordinary object. The Number prototype is itself a Number object; it has a [[NumberData]] internal slot with the value +0.
The value of the [[Prototype]] internal slot of the Number prototype object is the intrinsic object %ObjectPrototype%.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an object that has a [[NumberData]] internal slot that has been initialized to a Number value.
The abstract operation thisNumberValue(value) performs the following steps:
If Type(value) is Object and value has a [[NumberData]] internal slot, then
Assert: value's [[NumberData]] internal slot is a Number value.
Return the value of value's [[NumberData]] internal slot.
Throw a TypeError exception.
The phrase “this Number value” within the specification of a method refers to the result returned by calling the abstract operation thisNumberValue with the this value of the method invocation passed as the argument.
Return a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:
Return the concatenation of the Strings s and "Infinity".
If f < 0 or f > 20, throw a RangeError exception. However, an implementation is permitted to extend the behaviour of toExponential for values of f less than 0 or greater than 20. In this case toExponential would not necessarily throw RangeError for such values.
If x = 0, then
Let m be the String consisting of f+1 occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else x ≠ 0,
If fractionDigits is not undefined, then
Let e and n be integers such that 10f ≤ n < 10f+1 and for which the exact mathematical value of n × 10e-f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e-f is larger.
Else fractionDigits is undefined,
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the Number value for n × 10e-f is x, and f is as small as possible. Note that the decimal representation of n has f+1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let a be the first element of m, and let b be the remaining f elements of m.
Let m be the concatenation of the three Strings a, ".", and b.
If e = 0, then
Let c be "+".
Let d be "0".
Else,
If e > 0, let c be "+".
Else e ≤ 0,
Let c be "-".
Let e be -e.
Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Let m be the concatenation of the four Strings m, "e", c, and d.
Return the concatenation of the Strings s and m.
If the toExponential method is called with more than one argument, then the behaviour is undefined (see clause 17).
Note
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 10.b.i be used as a guideline:
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the Number value for n × 10e-f is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which n × 10e-f is closest in value to x. If there are two such possible values of n, choose the one that is even.
toFixed returns a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.
The following steps are performed:
Let x be ? thisNumberValue(this value).
Let f be ? ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0.)
If f < 0 or f > 20, throw a RangeError exception. However, an implementation is permitted to extend the behaviour of toFixed for values of f less than 0 or greater than 20. In this case toFixed would not necessarily throw RangeError for such values.
Let n be an integer for which the exact mathematical value of n ÷ 10f - x is as close to zero as possible. If there are two such n, pick the larger n.
If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let k be the number of elements in m.
If k ≤ f, then
Let z be the String consisting of f+1-k occurrences of the code unit 0x0030 (DIGIT ZERO).
Let m be the concatenation of Strings z and m.
Let k be f + 1.
Let a be the first k-f elements of m, and let b be the remaining f elements of m.
Let m be the concatenation of the three Strings a, ".", and b.
Return the concatenation of the Strings s and m.
If the toFixed method is called with more than one argument, then the behaviour is undefined (see clause 17).
Note 2
The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent number values. For example,
(1000000000000000128).toString() returns "1000000000000000100", while
(1000000000000000128).toFixed(0) returns "1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Produces a String value that represents this Number value formatted according to the conventions of the host environment's current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Return a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision-1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, call ToString instead. Specifically, perform the following steps:
Return the String that is the concatenation of s and "Infinity".
If p < 1 or p > 21, throw a RangeError exception. However, an implementation is permitted to extend the behaviour of toPrecision for values of p less than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for such values.
If x = 0, then
Let m be the String consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else x ≠ 0,
Let e and n be integers such that 10p-1 ≤ n < 10p and for which the exact mathematical value of n × 10e-p+1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e-p+1 is larger.
Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If e < -6 or e ≥ p, then
Assert: e ≠ 0.
Let a be the first element of m, and let b be the remaining p-1 elements of m.
Let m be the concatenation of a, ".", and b.
If e > 0, then
Let c be code unit 0x002B (PLUS SIGN).
Else e < 0,
Let c be code unit 0x002D (HYPHEN-MINUS).
Let e be -e.
Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
Return the concatenation of s, m, code unit 0x0065 (LATIN SMALL LETTER E), c, and d.
If e = p-1, return the concatenation of the Strings s and m.
If e ≥ 0, then
Let m be the concatenation of the first e+1 elements of m, the code unit 0x002E (FULL STOP), and the remaining p- (e+1) elements of m.
Else e < 0,
Let m be the String formed by the concatenation of code unit 0x0030 (DIGIT ZERO), code unit 0x002E (FULL STOP), -(e+1) occurrences of code unit 0x0030 (DIGIT ZERO), and the String m.
Return the String that is the concatenation of s and m.
If the toPrecision method is called with more than one argument, then the behaviour is undefined (see clause 17).
The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is undefined the Number 10 is used as the value of radix.
The following steps are performed:
Let x be ? thisNumberValue(this value).
If radix is not present, let radixNumber be 10.
Else if radix is undefined, let radixNumber be 10.
Return the String representation of this Number value using the radix specified by radixNumber. Letters a-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent, however the algorithm should be a generalization of that specified in 7.1.12.1.
The toString function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
Number instances are ordinary objects that inherit properties from the Number prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number object.
The Math object is the %Math% intrinsic object and the initial value of the Math property of the global object. The Math object is a single ordinary object.
The value of the [[Prototype]] internal slot of the Math object is the intrinsic object %ObjectPrototype%.
The Math object is not a function object. It does not have a [[Construct]] internal method; it is not possible to use the Math object as a constructor with the new operator. The Math object also does not have a [[Call]] internal method; it is not possible to invoke the Math object as a function.
Note
In this specification, the phrase “the Number value for x” has a technical meaning defined in 6.1.6.
Each of the following Math object functions applies the ToNumber abstract operation to each of its arguments (in left-to-right order if there is more than one). If ToNumber returns an abrupt completion, that Completion Record is immediately returned. Otherwise, the function performs a computation on the resulting Number value(s). The value returned by each function is a Number.
In the function descriptions below, the symbols NaN, -0, +0, -∞ and +∞ refer to the Number values described in 6.1.6.
Note
The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log,log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754-2008 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from -π to +π.
If either x or y is NaN, the result is NaN.
If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
If y>0 and x is -0, the result is an implementation-dependent approximation to +π/2.
If y is +0 and x>0, the result is +0.
If y is +0 and x is +0, the result is +0.
If y is +0 and x is -0, the result is an implementation-dependent approximation to +π.
If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
If y is -0 and x>0, the result is -0.
If y is -0 and x is +0, the result is -0.
If y is -0 and x is -0, the result is an implementation-dependent approximation to -π.
If y is -0 and x<0, the result is an implementation-dependent approximation to -π.
If y<0 and x is +0, the result is an implementation-dependent approximation to -π/2.
If y<0 and x is -0, the result is an implementation-dependent approximation to -π/2.
If y>0 and y is finite and x is +∞, the result is +0.
If y>0 and y is finite and x is -∞, the result is an implementation-dependent approximation to +π.
If y<0 and y is finite and x is +∞, the result is -0.
If y<0 and y is finite and x is -∞, the result is an implementation-dependent approximation to -π.
If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
If y is -∞ and x is finite, the result is an implementation-dependent approximation to -π/2.
If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
If y is +∞ and x is -∞, the result is an implementation-dependent approximation to +3π/4.
If y is -∞ and x is +∞, the result is an implementation-dependent approximation to -π/4.
If y is -∞ and x is -∞, the result is an implementation-dependent approximation to -3π/4.
Returns the smallest (closest to -∞) Number value that is not less than x and is equal to a mathematical integer. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is less than 0 but greater than -1, the result is -0.
The value of Math.ceil(x) is the same as the value of -Math.floor(-x).
Returns an implementation-dependent approximation to the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).
Returns an implementation-dependent approximation to subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close 0.
Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a mathematical integer. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 1, the result is +0.
Note
The value of Math.floor(x) is the same as the value of -Math.ceil(-x).
Math.hypot returns an implementation-dependent approximation of the square root of the sum of squares of its arguments.
If no arguments are passed, the result is +0.
If any argument is +∞, the result is +∞.
If any argument is -∞, the result is +∞.
If no argument is +∞ or -∞, and any argument is NaN, the result is NaN.
If all arguments are either +0 or -0, the result is +0.
Note
Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.
Returns an implementation-dependent approximation to the natural logarithm of 1 + x. The result is computed in a way that is accurate even when the value of x is close to zero.
Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.
If no arguments are given, the result is -∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the largest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.
Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.
If no arguments are given, the result is +∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the smallest value is done using the Abstract Relational Comparison algorithm except that +0 is considered to be larger than -0.
Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.
Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.
Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.
If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is -0, the result is -0.
If x is +∞, the result is +∞.
If x is -∞, the result is -∞.
If x is greater than 0 but less than 0.5, the result is +0.
If x is less than 0 but greater than or equal to -0.5, the result is -0.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of Math.floor(x+0.5). When x is -0 or is less than 0 but greater than or equal to -0.5, Math.round(x) returns -0, but Math.floor(x+0.5) returns +0. Math.round(x) may also differ from the value of Math.floor(x+0.5)because of internal rounding when computing x+0.5.
20.3.1Overview of Date Objects and Definitions of Abstract Operations#
The following functions are abstract operations that operate on time values (defined in 20.3.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.
A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is called a time value. A time value may also be NaN, indicating that the Date object does not represent a specific instant of time.
Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to measure times to millisecond precision for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly -100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.
ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and date within that year. In this system, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by
All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:
A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t) from a time valuet to a date number is defined by:
An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents the local standard time. Daylight saving time is not reflected by LocalTZA.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database http://www.iana.org/time-zones/.
An implementation dependent algorithm using best available information on time zones to determine the local daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds. An implementation of ECMAScript is expected to make its best effort to determine the local daylight saving time adjustment.
Note
It is recommended that implementations use the time zone information of the IANA Time Zone Database http://www.iana.org/time-zones/.
The abstract operation MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number values. This operator functions as follows:
If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
Let t be h*msPerHour+m*msPerMinute+s*msPerSecond+milli, performing the arithmetic according to IEEE 754-2008 rules (that is, as if using the ECMAScript operators * and +).
The abstract operation MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This operator functions as follows:
If year is not finite or month is not finite or date is not finite, return NaN.
Find a value t such that YearFromTime(t) is ym and MonthFromTime(t) is mn and DateFromTime(t) is 1; but if this is not possible (because some argument is out of range), return NaN.
The abstract operation MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number values. This operator functions as follows:
If day is not finite or time is not finite, return NaN.
The abstract operation TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value. This operator functions as follows:
The point of step 4 is that an implementation is permitted a choice of internal representations of time values, for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal representation may or may not distinguish -0 and +0.
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the fields are as follows:
YYYY
is the decimal digits of the year 0000 to 9999 in the Gregorian calendar.
-
"-" (hyphen) appears literally twice in the string.
MM
is the month of the year from 01 (January) to 12 (December).
DD
is the day of the month from 01 to 31.
T
"T" appears literally in the string, to indicate the beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
:
":" (colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
.
"." (dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
Z
is the time zone offset specified as "Z" (for UTC) or either "+" or "-" followed by a time expression HH:mm
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional time zone offset appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
All numbers must be base 10. If the MM or DD fields are absent "01" is used as the value. If the HH, mm, or ss fields are absent "00" is used as the value and the value of an absent sss field is "000". When the time zone offset is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00
Note 2
There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and this format specifies numeric representations of date and time.
ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,426 years, either forward or backward, from 01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601 permits the expansion of the year representation, but only by prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year representation shall have 2 extra year digits and is always prefixed with a + or - sign. The year 0 is considered positive and hence prefixed with a + sign.
The Date constructor is the %Date% intrinsic object and the initial value of the Date property of the global object. When called as a constructor it creates and initializes a new Date object. When Date is called as a function rather than as a constructor, it returns a String representing the current time (UTC).
The Date constructor is a single function whose behaviour is overloaded based upon the number and types of its arguments.
The Date constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Date constructor to create and initialize the subclass instance with a [[DateValue]] internal slot.
The length property of the Date constructor function is 7.
Let tv be the result of parsing v as a date, in exactly the same manner as for the parse method (20.3.3.2). If the parse resulted in an abrupt completion, tv is the Completion Record.
The parse function applies the ToString operator to its argument. If ToString results in an abrupt completion the Completion Record is immediately returned. Otherwise, parse interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the format of the String according to the rules (including extended years) called out in Date Time String Format (20.3.1.16). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognizable Strings or dates containing illegal element values in the format String shall cause Date.parse to return NaN.
If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:
is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (20.3.1.16) and that could not be produced in that implementation by the toString or toUTCString method.
When the UTC function is called with fewer than two arguments, the behaviour is implementation-dependent. When the UTC function is called with two to seven arguments, it computes the date from year, month and (optionally) date, hours, minutes, seconds and ms. The following steps are taken:
The UTC function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.
The Date prototype object is the intrinsic object %DatePrototype%. The Date prototype object is itself an ordinary object. It is not a Date instance and does not have a [[DateValue]] internal slot.
The value of the [[Prototype]] internal slot of the Date prototype object is the intrinsic object %ObjectPrototype%.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time value.
The abstract operation thisTimeValue(value) performs the following steps:
If Type(value) is Object and value has a [[DateValue]] internal slot, then
Return the value of value's [[DateValue]] internal slot.
Throw a TypeError exception.
In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is the this value for the invocation of the function. If the Type of the this value is not Object, a TypeError exception is thrown. The phrase “this time value” within the specification of a method refers to the result returned by calling the abstract operation thisTimeValue with the this value of the method invocation passed as the argument.
Set the [[DateValue]] internal slot of this Date object to u.
Return u.
The length property of the setFullYear method is 3.
Note
If month is not specified, this method behaves as if month were specified with the value getMonth(). If date is not specified, it behaves as if date were specified with the value getDate().
20.3.4.22Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )#
Set the [[DateValue]] internal slot of this Date object to u.
Return u.
The length property of the setHours method is 4.
Note
If min is not specified, this method behaves as if min were specified with the value getMinutes(). If sec is not specified, it behaves as if sec were specified with the value getSeconds(). If ms is not specified, it behaves as if ms were specified with the value getMilliseconds().
Set the [[DateValue]] internal slot of this Date object to u.
Return u.
The length property of the setMinutes method is 3.
Note
If sec is not specified, this method behaves as if sec were specified with the value getSeconds(). If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().
20.3.4.25Date.prototype.setMonth ( month [ , date ] )#
Set the [[DateValue]] internal slot of this Date object to v.
Return v.
The length property of the setUTCFullYear method is 3.
Note
If month is not specified, this method behaves as if month were specified with the value getUTCMonth(). If date is not specified, it behaves as if date were specified with the value getUTCDate().
20.3.4.30Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )#
Set the [[DateValue]] internal slot of this Date object to v.
Return v.
The length property of the setUTCHours method is 4.
Note
If min is not specified, this method behaves as if min were specified with the value getUTCMinutes(). If sec is not specified, it behaves as if sec were specified with the value getUTCSeconds(). If ms is not specified, it behaves as if ms were specified with the value getUTCMilliseconds().
20.3.4.31Date.prototype.setUTCMilliseconds ( ms )#
Set the [[DateValue]] internal slot of this Date object to v.
Return v.
The length property of the setUTCMinutes method is 3.
Note
If sec is not specified, this method behaves as if sec were specified with the value getUTCSeconds(). If ms is not specified, it function behaves as if ms were specified with the value return by getUTCMilliseconds().
20.3.4.33Date.prototype.setUTCMonth ( month [ , date ] )#
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form.
This function returns a String value representing the instance in time corresponding to this time value. The format of the String is the Date Time string format defined in 20.3.1.16. All fields are present in the String. The time zone is always UTC, denoted by the suffix Z. If this time value is not a finite Number or if the year is not a value that can be represented in that format (if necessary using extended year format), a RangeError exception is thrown.
The toJSON function is intentionally generic; it does not require that its this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleDateString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleTimeString method is used.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
For any Date object d whose milliseconds amount is zero, the result of Date.parse(d.toString()) is equal to d.valueOf(). See 20.3.3.2.
Note 2
The toString function is intentionally generic; it does not require that its this value be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method.
Return an implementation-dependent String value that represents tv as a date and time in the current time zone using a convenient, human-readable form.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form.
This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent this time value in a convenient, human-readable form in UTC.
Note
The intent is to produce a String representation of a date that is more readable than the format specified in 20.3.1.16. It is not essential that the chosen format be unambiguous or easily machine parsable. If an implementation does not have a preferred human-readable format it is recommended to use the format defined in 20.3.1.16 but with a space rather than a "T" used to separate the date and time elements.
20.3.4.45Date.prototype [ @@toPrimitive ] ( hint )#
This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for hint are "default", "number", and "string". Date objects, are unique among built-in ECMAScript object in that they treat "default" as being equivalent to "string", All other built-in ECMAScript objects treat "default" as being equivalent to "number".
When the @@toPrimitive method is called with argument hint, the following steps are taken:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If hint is the String value "string" or the String value "default", then
Let tryFirst be "string".
Else if hint is the String value "number", then
Let tryFirst be "number".
Else, throw a TypeError exception.
Return ? OrdinaryToPrimitive(O, tryFirst).
The value of the name property of this function is "[Symbol.toPrimitive]".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Date instances are ordinary objects that inherit properties from the Date prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is the time value represented by this Date object.
The String constructor is the %String% intrinsic object and the initial value of the String property of the global object. When called as a constructor it creates and initializes a new String object. When String is called as a function rather than as a constructor, it performs a type conversion.
The String constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the String constructor to create and initialize the subclass instance with a [[StringData]] internal slot.
The String.raw function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form the Listsubstitutions. The following steps are taken:
Let substitutions be a List consisting of all of the arguments passed to this function, starting with the second argument. If fewer than two arguments were passed, the List is empty.
Let numberOfSubstitutions be the number of elements in substitutions.
Append in order the code unit elements of nextSeg to the end of stringElements.
If nextIndex + 1 = literalSegments, then
Return the String value whose code units are, in order, the elements in the ListstringElements. If stringElements has no elements, the empty string is returned.
If nextIndex < numberOfSubstitutions, let next be substitutions[nextIndex].
Append in order the code unit elements of nextSub to the end of stringElements.
Let nextIndex be nextIndex + 1.
Note
String.raw is intended for use as a tag function of a Tagged Template (12.3.7). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.
The String prototype object is the intrinsic object %StringPrototype%. The String prototype object is an ordinary object. The String prototype is itself a String object; it has a [[StringData]] internal slot with the value "".
The value of the [[Prototype]] internal slot of the String prototype object is the intrinsic object %ObjectPrototype%.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the this value passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.
The abstract operation thisStringValue(value) performs the following steps:
If Type(value) is Object and value has a [[StringData]] internal slot, then
Assert: value's [[StringData]] internal slot is a String value.
Return the value of value's [[StringData]] internal slot.
Throw a TypeError exception.
The phrase “this String value” within the specification of a method refers to the result returned by calling the abstract operation thisStringValue with the this value of the method invocation passed as the argument.
Returns a single element String containing the code unit at index pos in the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.
If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result of x.substring(pos, pos+1).
When the charAt method is called with one argument pos, the following steps are taken:
If position < 0 or position ≥ size, return the empty String.
Return a String of length 1, containing one code unit from S, namely the code unit at index position.
Note 2
The charAt function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Returns a Number (a nonnegative integer less than 216) that is the code unit value of the string element at index pos in the String resulting from converting this object to a String. If there is no element at that index, the result is NaN.
When the charCodeAt method is called with one argument pos, the following steps are taken:
Return a value of Number type, whose value is the code unit value of the element at index position in the String S.
Note 2
The charCodeAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Returns a nonnegative integer Number less than 1114112 (0x110000) that is the code point value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos in the String resulting from converting this object to a String. If there is no element at that index, the result is undefined. If a valid UTF-16 surrogate pair does not begin at pos, the result is the code unit at pos.
When the codePointAt method is called with one argument pos, the following steps are taken:
The codePointAt function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
When the concat method is called it returns a String consisting of the code units of the this object (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.
When the concat method is called with zero or more arguments, the following steps are taken:
Let R be the String value consisting of the code units of the previous value of R followed by the code units of nextString.
Return R.
The length property of the concat method is 1.
Note 2
The concat function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Let searchLength be the number of elements in searchStr.
Let start be end - searchLength.
If start is less than 0, return false.
If the sequence of elements of S starting at start of length searchLength is the same as the full element sequence of searchStr, return true.
Otherwise, return false.
Note 1
Returns true if the sequence of elements of searchString converted to a String is the same as the corresponding elements of this object (converted to a String) starting at endPosition - length(this). Otherwise returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The endsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.7String.prototype.includes ( searchString [ , position ] )#
The includes method takes two arguments, searchString and position, and performs the following steps:
Let searchLen be the number of elements in searchStr.
If there exists any integer k not smaller than start such that k + searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j of S is the same as the code unit at index j of searchStr, return true; but if there is no such integer k, return false.
Note 1
If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, return true; otherwise, returns false. If position is undefined, 0 is assumed, so as to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The includes function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.8String.prototype.indexOf ( searchString [ , position ] )#
Note 1
If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise, -1 is returned. If position is undefined, 0 is assumed, so as to search all of the String.
The indexOf method takes two arguments, searchString and position, and performs the following steps:
Let searchLen be the number of elements in searchStr.
Return the smallest possible integer k not smaller than start such that k+searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j of S is the same as the code unit at index j of searchStr; but if there is no such integer k, return the value -1.
Note 2
The indexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.9String.prototype.lastIndexOf ( searchString [ , position ] )#
Note 1
If searchString appears as a substring of the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise, -1 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.
The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:
Let searchLen be the number of elements in searchStr.
Return the largest possible nonnegative integer k not larger than start such that k+searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the code unit at index k+j of S is the same as the code unit at index j of searchStr; but if there is no such integer k, return the value -1.
Note 2
The lastIndexOf function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the localeCompare method is used.
When the localeCompare method is called with argument that, it returns a Number other than NaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order specified by a host default locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.
Before performing the comparisons, the following steps are performed to prepare the Strings:
The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.
The localeCompare method, if considered as a function of two arguments this and that, is a consistent comparison function (as defined in 22.1.3.25) on the set of all Strings.
The actual return values are implementation-defined to permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings. This function must treat Strings that are canonically equivalent according to the Unicode standard as identical and must return 0 when comparing Strings that are considered canonically equivalent.
Note 1
The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.
Note 2
This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment's current locale. However, regardless of the host provided comparison capabilities, this function must treat Strings that are canonically equivalent according to the Unicode standard as identical. It is recommended that this function should not honour Unicode compatibility equivalences or decompositions. For a definition and discussion of canonical equivalence see the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms (http://www.unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in Applications (http://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (http://www.unicode.org/reports/tr10/).
Note 3
The localeCompare function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The match function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The normalize function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Let T be a String value that is made from n copies of S appended together. If n is 0, T is the empty String.
Return T.
Note 1
This method creates a String consisting of the code units of the this object (converted to String) repeated count times.
Note 2
The repeat function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Search string for the first occurrence of searchString and let pos be the index within string of the first code unit of the matched substring and let matched be searchString. If no occurrences of searchString were found, return string.
If functionalReplace is true, then
Let replValue be ? Call(replaceValue, undefined, « matched, pos, string »).
Let replStr be GetSubstitution(matched, string, pos, captures, replaceValue).
Let tailPos be pos + the number of code units in matched.
Let newString be the String formed by concatenating the first pos code units of string, replStr, and the trailing substring of string starting at index tailPos. If pos is 0, the first element of the concatenation will be the empty String.
Return newString.
Note
The replace function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let result be a String value derived from replacement by copying code unit elements from replacement to result while performing replacements as specified in Table 46. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements.
Return result.
Table 46: Replacement Text Symbol Substitutions
Code units
Unicode Characters
Replacement text
0x0024, 0x0024
$$
$
0x0024, 0x0026
$&
matched
0x0024, 0x0060
$`
If position is 0, the replacement is the empty String. Otherwise the replacement is the substring of str that starts at index 0 and whose last code unit is at index position - 1.
0x0024, 0x0027
$'
If tailPos ≥ stringLength, the replacement is the empty String. Otherwise the replacement is the substring of str that starts at index tailPos and continues to the end of str.
0x0024, N
Where
0x0031 ≤ N ≤ 0x0039
$n where
n is one of 1 2 3 4 5 6 7 8 9 and $n is not followed by a decimal digit
The nth element of captures, where n is a single digit in the range 1 to 9. If n≤m and the nth element of captures is undefined, use the empty String instead. If n>m, the result is implementation-defined.
0x0024, N, N
Where
0x0030 ≤ N ≤ 0x0039
$nn where
n is one of 0 1 2 3 4 5 6 7 8 9
The nnth element of captures, where nn is a two-digit decimal number in the range 01 to 99. If nn≤m and the nnth element of captures is undefined, use the empty String instead. If nn is 00 or nn>m, the result is implementation-defined.
0x0024
$ in any context that does not match any of the above.
The search function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The slice method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength+end where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:
Return a String value containing span consecutive elements from S beginning with the element at index from.
Note
The slice function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as an RegExp, that has a @@split method.
When the split method is called, the following steps are taken:
The value of separator may be an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.
If the this object is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.
Note 2
The split function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.17.1Runtime Semantics: SplitMatch ( S, q, R )#
The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String R, and performs the following steps in order to return either false or the end index of a match:
If there exists an integer i between 0 (inclusive) and r (exclusive) such that the code unit at index q+i of S is different from the code unit at index i of R, return false.
Return q+r.
21.1.3.18String.prototype.startsWith ( searchString [ , position ] )#
Let searchLength be the number of elements in searchStr.
If searchLength+start is greater than len, return false.
If the sequence of elements of S starting at start of length searchLength is the same as the full element sequence of searchStr, return true.
Otherwise, return false.
Note 1
This method returns true if the sequence of elements of searchString converted to a String is the same as the corresponding elements of this object (converted to a String) starting at index position. Otherwise returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
The startsWith function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
21.1.3.19String.prototype.substring ( start, end )#
The substring method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end is undefined). The result is a String value, not a String object.
If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.
Return a String whose length is to - from, containing code units from S, namely the code units with indices from through to - 1, in ascending order.
Note
The substring function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleLowerCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The toLocaleLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleUpperCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the host environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
The toLocaleUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let cpList be a List containing in order the code points as defined in 6.1.4 of S, starting at the first element of S.
For each code point c in cpList, if the Unicode Character Database provides a language insensitive lower case equivalent of c, then replace c in cpList with that equivalent code point(s).
For each code point c in cpList, in order, append to cuList the elements of the UTF16Encoding of c.
Let L be a String whose elements are, in order, the elements of cuList.
Return L.
The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt file that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().
Note 2
The toLowerCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This function behaves in exactly the same way as String.prototype.toLowerCase, except that code points are mapped to their uppercase equivalents as specified in the Unicode Character Database.
Note
The toUpperCase function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Let T be a String value that is a copy of S with both leading and trailing white space removed. The definition of white space is the union of WhiteSpace and LineTerminator. When determining whether a Unicode code point is in Unicode general category “Zs”, code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in 6.1.4.
Return T.
Note
The trim function is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
When the @@iterator method is called it returns an Iterator object (25.1.1.2) that iterates over the code points of a String value, returning each code point as a String value. The following steps are taken:
String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from the String prototype object. String instances also have a [[StringData]] internal slot.
String instances have a length property, and a set of enumerable properties with integer indexed names.
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
An String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a named constructor for String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.
Several methods of String objects return Iterator objects. The abstract operation CreateStringIterator with argument string is used to create such iterator objects. It performs the following steps:
All String Iterator Objects inherit properties from the %StringIteratorPrototype% intrinsic object. The %StringIteratorPrototype% object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %StringIteratorPrototype% has the following properties:
String Iterator instances are ordinary objects that inherit properties from the %StringIteratorPrototype% intrinsic object. String Iterator instances are initially created with the internal slots listed in Table 47.
Table 47: Internal Slots of String Iterator Instances
Internal Slot
Description
[[IteratedString]]
The String value whose elements are being iterated.
[[StringIteratorNextIndex]]
The integer index of the next string index to be examined by this iteration.
The RegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion of Pattern.
A regular expression pattern is converted into an internal procedure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The internal procedure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.
A Pattern is either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a "u". A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of Pattern is defined as if the source code for the Pattern was a List of SourceCharacter values where each SourceCharacter corresponds to a Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of the List.
Note
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character) List consisting of the single code point 0x1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two element List consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode pattern UTF16Decode must be used in producing a List consisting of a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.
The descriptions below use the following variables:
Input is a List consisting of all of the characters, in order, of the String being matched by the regular expression pattern. Each character is either a code unit or a code point, depending upon the kind of pattern involved. The notation Input[n] means the nth character of Input, where n can range between 0 (inclusive) and InputLength (exclusive).
InputLength is the number of characters in Input.
NcapturingParens is the total number of left capturing parentheses (i.e. the total number of times the Atom::(Disjunction) production is expanded) in the pattern. A left capturing parenthesis is any ( pattern character that is matched by the ( terminal of the Atom::(Disjunction) production.
IgnoreCase is true if the RegExp object's [[OriginalFlags]] internal slot contains "i" and otherwise is false.
Multiline is true if the RegExp object's [[OriginalFlags]] internal slot contains "m" and otherwise is false.
Unicode is true if the RegExp object's [[OriginalFlags]] internal slot contains "u" and otherwise is false.
Furthermore, the descriptions below use the following internal data structures:
A CharSet is a mathematical set of characters, either code units or code points depending up the state of the Unicode flag. “All characters” means either all code unit values or all code point values also depending upon the state if Unicode.
A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is a List of NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either a List that represents the value obtained by the nth set of capturing parentheses or undefined if the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a State or the special token failure that indicates that the match failed.
A Continuation procedure is an internal closure (i.e. an internal procedure with some arguments already bound to values) that takes one State argument and returns a MatchResult result. If an internal closure references variables which are bound in the function that creates the closure, the closure uses the values that these variables had at the time the closure was created. The Continuation attempts to match the remaining portion (specified by the closure's already-bound arguments) of the pattern against Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returns failure.
A Matcher procedure is an internal closure that takes two arguments — a State and a Continuation — and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's already-bound arguments) of the pattern against Input, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
An AssertionTester procedure is an internal closure that takes a State argument and returns a Boolean result. The assertion tester tests a specific condition (specified by the closure's already-bound arguments) against the current place in Input and returns true if the condition matched or false if not.
An EscapeValue is either a character or an integer. An EscapeValue is used to denote the interpretation of a DecimalEscape escape sequence: a character ch means that the escape sequence is interpreted as the character ch, while an integer n means that the escape sequence is interpreted as a backreference to the nth set of capturing parentheses.
Return an internal closure that takes two arguments, a String str and an integer index, and performs the following steps:
Assert: index ≤ the number of elements in str.
If Unicode is true, let Input be a List consisting of the sequence of code points of str interpreted as a UTF-16 encoded (6.1.4) Unicode string. Otherwise, let Input be a List consisting of the sequence of code units that are the elements of str. Input will be used throughout the algorithms in 21.2.2. Each element of Input is considered to be a character.
Let InputLength be the number of characters contained in Input. This variable will be used throughout the algorithms in 21.2.2.
Let listIndex be the index into Input of the character that was obtained from element index of str.
Let c be a Continuation that always returns its State argument as a successful MatchResult.
Let cap be a List of NcapturingParensundefined values, indexed 1 through NcapturingParens.
Let x be the State (listIndex, cap).
Call m(x, c) and return its result.
Note
A Pattern evaluates (“compiles”) to an internal procedure value. RegExpBuiltinExec can then apply this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms in 21.2.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying the resulting internal procedure to find a match in a String cannot throw an exception (except for any host-defined exceptions that can occur anywhere such as out-of-memory).
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Call m1(x, c) and let r be its result.
If r is not failure, return r.
Call m2(x, c) and return its result.
Note
The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead of Strings. Thus, for example,
The production Alternative::[empty] evaluates by returning a Matcher that takes two arguments, a State x and a Continuation c, and returns the result of calling c(x).
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Create a Continuation d that takes a State argument y and returns the result of calling m2(y, c).
Call m1(x, d) and return its result.
Note
Consecutive Terms try to simultaneously match consecutive portions of Input. If the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative.
The production Term::Assertion evaluates by returning an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Evaluate Assertion to obtain an AssertionTester t.
Call t(x) and let r be the resulting Boolean value.
Evaluate Quantifier to obtain the three results: an integer min, an integer (or ∞) max, and Boolean greedy.
If max is finite and less than min, throw a SyntaxError exception.
Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion's Term. This is the total number of times the Atom::(Disjunction) production is expanded prior to this production's Term plus the total number of Atom::(Disjunction) productions enclosing this Term.
Let parenCount be the number of left capturing parentheses in the expansion of this production's Atom. This is the total number of Atom::(Disjunction) productions enclosed by this production's Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps when evaluated:
Call RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount) and return its result.
The abstract operation RepeatMatcher takes eight parameters, a Matcher m, an integer min, an integer (or ∞) max, a Boolean greedy, a State x, a Continuation c, an integer parenIndex, and an integer parenCount, and performs the following steps:
If max is zero, return c(x).
Create an internal Continuation closure d that takes one State argument y and performs the following steps when evaluated:
If min is zero and y's endIndex is equal to x's endIndex, return failure.
If min is zero, let min2 be zero; otherwise let min2 be min-1.
If max is ∞, let max2 be ∞; otherwise let max2 be max-1.
Call RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount) and return its result.
For every integer k that satisfies parenIndex < k and k ≤ parenIndex+parenCount, set cap[k] to undefined.
Let e be x's endIndex.
Let xr be the State (e, cap).
If min is not zero, return m(xr, d).
If greedy is false, then
Call c(x) and let z be its result.
If z is not failure, return z.
Call m(xr, d) and return its result.
Call m(xr, d) and let z be its result.
If z is not failure, return z.
Call c(x) and return its result.
Note 1
An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input character sequence that it matches, so different repetitions of the Atom can match different input substrings.
Note 2
If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (nth) repetition of Atom are tried before moving on to the next choice in the next-to-last (n-1)st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n-1)st repetition of Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:
Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured Strings contained in the quantified Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.
Note 4
Step 1 of the RepeatMatcher's d closure states that, once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:
The production Assertion::^ evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
If e is zero, return true.
If Multiline is false, return false.
If the character Input[e-1] is one of LineTerminator, return true.
Return false.
Note
Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if Multiline is true) at the beginning of a line.
The production Assertion::$ evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
If e is equal to InputLength, return true.
If Multiline is false, return false.
If the character Input[e] is one of LineTerminator, return true.
Return false.
The production Assertion::\b evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
Call IsWordChar(e-1) and let a be the Boolean result.
Call IsWordChar(e) and let b be the Boolean result.
If a is true and b is false, return true.
If a is false and b is true, return true.
Return false.
The production Assertion::\B evaluates by returning an internal AssertionTester closure that takes a State argument x and performs the following steps when evaluated:
Let e be x's endIndex.
Call IsWordChar(e-1) and let a be the Boolean result.
Call IsWordChar(e) and let b be the Boolean result.
Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to the left of this production expansion's initial left parenthesis. This is the total number of times the Atom::(Disjunction) production is expanded prior to this production's Atom plus the total number of Atom::(Disjunction) productions enclosing this Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following steps:
Create an internal Continuation closure d that takes one State argument y and performs the following steps:
The abstract operation Canonicalize takes a character parameter ch and performs the following steps:
If IgnoreCase is false, return ch.
If Unicode is true, then
If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
Else, return ch.
Else,
Assert: ch is a UTF-16 code unit.
Let s be the ECMAScript String value consisting of the single code unit ch.
Let u be the same result produced as if by performing the algorithm for String.prototype.toUpperCase using s as the this value.
Assert: u is a String value.
If u does not consist of a single code unit, return ch.
Let cu be u's single code unit element.
If ch's code unit value ≥ 128 and cu's code unit value < 128, return ch.
Return cu.
Note 1
Parentheses of the form (Disjunction) serve both to group the components of the Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a nonzero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching internal procedure. To inhibit the capturing behaviour of parentheses, use the form (?:Disjunction) instead.
Note 2
The form (?=Disjunction) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\1/.exec("baaabac")
This expression returns
["aba", "a"]
and not:
["aaaba", "a"]
Note 3
The form (?!Disjunction) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined because the negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole expression returns the array:
["baaabaac", "ba", undefined, "abaac"]
Note 4
In case-insignificant matches when Unicode is true, all characters are implicitly case-folded using the simple mapping provided by the Unicode standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, "ß" (U+00DF) to "SS". It may however map a code point outside the Basic Latin range to a character within, for example, "ſ" (U+017F) to "s". Such characters are not mapped if Unicode is false. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such as /[a-z]/i, but they will match /[a-z]/ui.
If there exists an integer i between 0 (inclusive) and len (exclusive) such that Canonicalize(s[i]) is not the same character value as Canonicalize(Input[e+i]), return failure.
An escape sequence of the form \ followed by a nonzero decimal number n matches the result of the nth set of capturing parentheses (see 0). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one is undefined because it has not captured anything, then the backreference always succeeds.
If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left capturing parentheses in the entire regular expression. \0 represents the <NUL> character and cannot be followed by a decimal digit.
The production CharacterClassEscape::s evaluates by returning the set of characters containing the characters that are on the right-hand side of the WhiteSpace or LineTerminator productions.
Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.
Note 1
ClassRanges can expand into a single ClassAtom and/or ranges of two ClassAtom separated by dashes. In the latter case the ClassRanges includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if the first ClassAtom's character value is greater than the second ClassAtom's character value.
Note 2
Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.
Note 3
A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.
A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside a ClassAtom causes an error.
The RegExp constructor is the %RegExp% intrinsic object and the initial value of the RegExp property of the global object. When RegExp is called as a function rather than as a constructor, it creates and initializes a new RegExp object. Thus the function call RegExp(…) is equivalent to the object creation expression new RegExp(…) with the same arguments.
The RegExp constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a super call to the RegExp constructor to create and initialize subclass instances with the necessary internal slots.
If pattern is supplied using a StringLiteral, the usual escape sequence substitutions are performed before the String is processed by RegExp. If pattern must contain an escape sequence to be recognized by RegExp, any U+005C (REVERSE SOLIDUS) code points must be escaped within the StringLiteral to prevent them being removed when the contents of the StringLiteral are formed.
21.2.3.2Abstract Operations for the RegExp Constructor#
If F contains any code unit other than "g", "i", "m", "u", or "y" or if it contains the same code unit more than once, throw a SyntaxError exception.
If F contains "u", let BMP be false; else let BMP be true.
If BMP is true, then
Parse P using the grammars in 21.2.1 and interpreting each of its 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the elements. The goal symbol for the parse is Pattern. Throw a SyntaxError exception if P did not conform to the grammar, if any elements of P were not matched by the parse, or if any Early Error conditions exist.
Let patternCharacters be a List whose elements are the code unit elements of P.
Else,
Parse P using the grammars in 21.2.1 and interpreting P as UTF-16 encoded Unicode code points (6.1.4). The goal symbol for the parse is Pattern[U]. Throw a SyntaxError exception if P did not conform to the grammar, if any elements of P were not matched by the parse, or if any Early Error conditions exist.
Let patternCharacters be a List whose elements are the code points resulting from applying UTF-16 decoding to P's sequence of elements.
Set the value of obj's [[OriginalSource]] internal slot to P.
Set the value of obj's [[OriginalFlags]] internal slot to F.
Set obj's [[RegExpMatcher]] internal slot to the internal procedure that evaluates the above parse of P by applying the semantics provided in 21.2.2 using patternCharacters as the pattern's List of SourceCharacter values and F as the flag parameters.
21.2.3.2.4Runtime Semantics: EscapeRegExpPattern ( P, F )#
When the abstract operation EscapeRegExpPattern with arguments P and F is called, the following occurs:
Let S be a String in the form of a Pattern (Pattern[U] if F contains "u") equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code points are escaped as described below. S may or may not be identical to P; however, the internal procedure that would result from evaluating S as a Pattern (Pattern[U] if F contains "u") must behave identically to the internal procedure given by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results.
The code points / or any LineTerminator occurring in the pattern shall be escaped in S as necessary to ensure that the String value formed by concatenating the Strings "/", S, "/", and F can be parsed (in an appropriate lexical context) as a RegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if P is "/", then S could be "\/" or "\u002F", among other possibilities, but not "/", because /// followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be "(?:)".
RegExp[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
RegExp prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
The RegExp prototype object is the intrinsic object %RegExpPrototype%. The RegExp prototype object is an ordinary object. It is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects.
The value of the [[Prototype]] internal slot of the RegExp prototype object is the intrinsic object %ObjectPrototype%.
Note
The RegExp prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the Object prototype object.
Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, or null if string did not match.
The String ToString(string) is searched for an occurrence of the regular expression pattern as follows:
Let R be the this value.
If Type(R) is not Object, throw a TypeError exception.
If R does not have a [[RegExpMatcher]] internal slot, throw a TypeError exception.
If a callable exec property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular expressions did not perform a dynamic property lookup of exec.
21.2.5.2.2Runtime Semantics: RegExpBuiltinExec ( R, S )#
The abstract operation RegExpBuiltinExec with arguments R and S performs the following steps:
e is an index into the Input character list, derived from S, matched by matcher. Let eUTF be the smallest index into S that corresponds to the character at element e of Input. If e is greater than or equal to the length of Input, then eUTF is the number of code units in S.
The value of the name property of this function is "[Symbol.match]".
Note
The @@match property is used by the IsRegExp abstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a @@match property or the existence of such a property whose value does not Boolean coerce to true indicates that the object is not intended to be used as a regular expression object.
Let replacement be GetSubstitution(matched, S, position, captures, replaceValue).
If position ≥ nextSourcePosition, then
NOTE position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
Let accumulatedResult be the String formed by concatenating the code units of the current value of accumulatedResult with the substring of S consisting of the code units from nextSourcePosition (inclusive) up to position (exclusive) and with the code units of replacement.
Let nextSourcePosition be position + matchLength.
If nextSourcePosition ≥ lengthS, return accumulatedResult.
Return the String formed by concatenating the code units of accumulatedResult with the substring of S consisting of the code units from nextSourcePosition (inclusive) up through the final code unit of S (inclusive).
The value of the name property of this function is "[Symbol.replace]".
Returns an Array object into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of the this value regular expression; these occurrences are not part of any substring in the returned array, but serve to divide up the String value.
The this value may be an empty regular expression or a regular expression that can match an empty String. In this case, the regular expression does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if the regular expression matches the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.) Only the first match at a given index of the String is considered, even if backtracking could yield a non-empty-substring match at that index. (For example, /a*?/[Symbol.split]("ab") evaluates to the array ["a","b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If the string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If the regular expression contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,
RegExp instances are ordinary objects that inherit properties from the RegExp prototype object. RegExp instances have internal slots [[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The value of the [[RegExpMatcher]] internal slot is an implementation dependent representation of the Pattern of the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties source, global, ignoreCase, and multiline. Those properties are now specified as accessor properties of RegExp.prototype.
RegExp instances also have the following property:
The value of the lastIndex property specifies the String index at which to start the next match. It is coerced to an integer when used (see 21.2.5.2.2). This property shall have the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Array objects are exotic objects that give special treatment to a certain class of property names. See 9.4.2 for a definition of this special treatment.
The Array constructor is the %Array% intrinsic object and the initial value of the Array property of the global object. When called as a constructor it creates and initializes a new exotic Array object. When Array is called as a function rather than as a constructor, it also creates and initializes a new Array object. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.
The Array constructor is a single function whose behaviour is overloaded based upon the number and types of its arguments.
Array构造函数是一个函数，它的行为根据其参数的数量和类型而被重载。
The Array constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Array constructor to initialize subclass instances that are exotic Array objects. However, most of the Array.prototype methods are generic methods that are not dependent upon their this value being an exotic Array object.
The length property of the Array constructor function is 1.
The from function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.
The items argument is assumed to be a well-formed rest argument value.
Note 2
The of function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.
Array[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
Array prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
The Array prototype object is the intrinsic object %ArrayPrototype%. The Array prototype object is an Array exotic objects and has the internal methods specified for such objects. It has a length property whose initial value is 0 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
The value of the [[Prototype]] internal slot of the Array prototype object is the intrinsic object %ObjectPrototype%.
Note
The Array prototype object is specified to be an Array exotic object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
When the concat method is called with zero or more arguments, it returns an array containing the array elements of the object followed by the array elements of each argument in order.
Let items be a List whose first element is O and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
Repeat, while items is not empty
Remove the first element from items and let E be the value of the element.
The explicit setting of the length property in step 6 is necessary to ensure that its value is correct in situations where the trailing elements of the result Array are not present.
Note 2
The concat function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.1.1Runtime Semantics: IsConcatSpreadable ( O )#
The abstract operation IsConcatSpreadable with argument O performs the following steps:
The initial value of Array.prototype.constructor is the intrinsic object %Array%.
22.1.3.3Array.prototype.copyWithin (target, start [ , end ] )#
The copyWithin method takes up to three arguments target, start and end.
Note 1
The end argument is optional with the length of the this object as its default value. If target is negative, it is treated as length+target where length is the length of the array. If start is negative, it is treated as length+start. If end is negative, it is treated as length+end.
The copyWithin function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns false. If such an element is found, every immediately returns false. Otherwise, if callbackfn returned true for all elements, every will return true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.
When the every method is called with one or two arguments, the following steps are taken:
Let testResult be ToBoolean(? Call(callbackfn, T, « kValue, k, O »)).
If testResult is false, return false.
Increase k by 1.
Return true.
Note 2
The every function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The fill method takes up to three arguments value, start and end.
Note 1
The start and end arguments are optional with default values of 0 and the length of the this object. If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end.
The fill function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.
When the filter method is called with one or two arguments, the following steps are taken:
The filter function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The find method is called with one or two arguments, predicate and thisArg.
Note 1
predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. find calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns true. If such an element is found, find immediately returns that element value. Otherwise, find returns undefined.
If a thisArg parameter is provided, it will be used as the this value for each invocation of predicate. If it is not provided, undefined is used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
find does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by find is set before the first call to callbackfn. Elements that are appended to the array after the call to find begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that find visits them.
When the find method is called, the following steps are taken:
Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
If testResult is true, return kValue.
Increase k by 1.
Return undefined.
Note 2
The find function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
predicate should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. findIndex calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns true. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.
If a thisArg parameter is provided, it will be used as the this value for each invocation of predicate. If it is not provided, undefined is used instead.
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
findIndex does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by findIndex is set before the first call to callbackfn. Elements that are appended to the array after the call to findIndex begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findIndex visits them.
When the findIndex method is called with one or two arguments, the following steps are taken:
Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
If testResult is true, return k.
Increase k by 1.
Return -1.
Note 2
The findIndex function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
When the forEach method is called with one or two arguments, the following steps are taken:
The forEach function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
includes compares searchElement to the elements of the array, in ascending order, using the SameValueZero algorithm, and if found at any position, returns true; otherwise, false is returned.
The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, false is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.
When the includes method is called, the following steps are taken:
Let n be ? ToInteger(fromIndex). (If fromIndex is undefined, this step produces the value 0.)
If n ≥ 0, then
Let k be n.
Else n < 0,
Let k be len + n.
If k < 0, let k be 0.
Repeat, while k < len
Let elementK be the result of ? Get(O, ! ToString(k)).
If SameValueZero(searchElement, elementK) is true, return true.
Increase k by 1.
Return false.
Note 2
The includes function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
The includes method intentionally differs from the similar indexOf method in two ways. First, it uses the SameValueZero algorithm, instead of Strict Equality Comparison, allowing it to detect NaN array elements. Second, it does not skip missing array elements, instead treating them as undefined.
indexOf compares searchElement to the elements of the array, in ascending order, using the Strict Equality Comparison algorithm, and if found at one or more indices, returns the smallest such index; otherwise, -1 is returned.
The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.
When the indexOf method is called with one or two arguments, the following steps are taken:
The indexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.
The join method takes one argument, separator, and performs the following steps:
If element is undefined or null, let next be the empty String; otherwise, let next be ? ToString(element).
Let R be a String value produced by concatenating S and next.
Increase k by 1.
Return R.
Note 2
The join function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
lastIndexOf compares searchElement to the elements of the array in descending order using the Strict Equality Comparison algorithm, and if found at one or more indices, returns the largest such index; otherwise, -1 is returned.
The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, -1 is returned.
When the lastIndexOf method is called with one or two arguments, the following steps are taken:
The lastIndexOf function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.
When the map method is called with one or two arguments, the following steps are taken:
The map function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The pop function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.
When the push method is called with zero or more arguments, the following steps are taken:
The push function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element present in the array, in ascending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is a TypeError if the array contains no elements and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.
Let accumulator be ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).
Increase k by 1.
Return accumulator.
let arr = [1,2,3,4,5];
arr.reduce((a, b) => a + b, 100);
// 115
Note 2
The reduce function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element present in the array, in descending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was provided, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is a TypeError if the array contains no elements and initialValue is not provided.
reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.
When the reduceRight method is called with one or two arguments, the following steps are taken:
Let accumulator be ? Call(callbackfn, undefined, « accumulator, kValue, k, O »).
Decrease k by 1.
Return accumulator.
Note 2
The reduceRight function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The reverse function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
The shift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The slice method takes two arguments, start and end, and returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end where length is the length of the array.
The explicit setting of the length property of the result Array in step 11 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting length became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.
Note 3
The slice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns true. If such an element is found, some immediately returns true. Otherwise, some returns false. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns false.
When the some method is called with one or two arguments, the following steps are taken:
Let testResult be ToBoolean(? Call(callbackfn, T, « kValue, k, O »)).
If testResult is true, return true.
Increase k by 1.
Return false.
Note 2
The some function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not necessarily remain in their original order). If comparefn is not undefined, it should be a function that accepts two arguments x and y and returns a negative value if x < y, zero if x = y, or a positive value if x > y.
Upon entry, the following steps are performed to initialize evaluation of the sort function:
Within this specification of the sort method, an object, obj, is said to be sparse if the following algorithm returns true:
For each integer i in the range 0≤i< len
Let elem be obj.[[GetOwnProperty]](! ToString(i)).
If elem is undefined, return true.
Return false.
The sort order is the ordering, after completion of this function, of the integer indexed property values of obj whose integer indexes are less than len. The result of the sort function is then determined as follows:
If comparefn is not undefined and is not a consistent comparison function for the elements of this array (see below), the sort order is implementation-defined. The sort order is also implementation-defined if comparefn is undefined and SortCompare does not act as a consistent comparison function.
Let proto be obj.[[GetPrototypeOf]](). If proto is not null and there exists an integer j such that all of the conditions below are satisfied then the sort order is implementation-defined:
Any integer index property of obj whose name is a nonnegative integer less than len is a data property whose [[Configurable]] attribute is false.
The sort order is also implementation defined if any of the following conditions are true:
If obj is an exotic object (including Proxy exotic objects) whose behaviour for [[Get]], [[Set]], [[Delete]], and [[GetOwnProperty]] is not the ordinary object implementation of these internal methods.
If any index property of obj whose name is a nonnegative integer less than len is an accessor property or is a data property whose [[Writable]] attribute is false.
If comparefn is undefined and the application of ToString to any value passed as an argument to SortCompare modifies obj or any object on obj's prototype chain.
If comparefn is undefined and all applications of ToString, to any specific value passed as an argument to SortCompare, do not produce the same result.
The following steps are taken:
Perform an implementation-dependent sequence of calls to the [[Get]] and [[Set]] internal methods of obj, to the DeletePropertyOrThrow and HasOwnProperty abstract operation with obj as the first argument, and to SortCompare (described below), such that:
The property key argument for each call to [[Get]], [[Set]], HasOwnProperty, or DeletePropertyOrThrow is the string representation of a nonnegative integer less than len.
The arguments for calls to SortCompare are values returned by a previous call to the [[Get]] internal method, unless the properties accessed by those previous calls did not exist according to HasOwnProperty. If both perspective arguments to SortCompare correspond to non-existent properties, use +0 instead of calling SortCompare. If only the first perspective argument is non-existent use +1. If only the second perspective argument is non-existent use -1.
If any [[Set]] call returns false a TypeError exception is thrown.
If an abrupt completion is returned from any of these operations, it is immediately returned as the value of this function.
Return obj.
Unless the sort order is specified above to be implementation-defined, the returned object must have the following two characteristics:
There must be some mathematical permutation π of the nonnegative integers less than len, such that for every nonnegative integer j less than len, if property old[j] existed, then new[π(j)] is exactly the same value as old[j]. But if property old[j] did not exist, then new[π(j)] does not exist.
Then for all nonnegative integers j and k, each less than len, if SortCompare(old[j], old[k]) < 0 (see SortCompare below), then new[π(j)] < new[π(k)].
Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j before this function is executed, and the notation new[j] to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j after this function has been executed.
A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <CFb means comparefn(a, b) < 0; a =CFb means comparefn(a, b) = 0 (of either sign); and a >CFb means comparefn(a, b) > 0.
Calling comparefn(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this implies that exactly one of a <CFb, a =CFb, and a >CFb will be true for a given pair of a and b.
Calling comparefn(a, b) does not modify obj or any object on obj's prototype chain.
a =CFa (reflexivity)
If a =CFb, then b =CFa (symmetry)
If a =CFb and b =CFc, then a =CFc (transitivity of =CF)
If a <CFb and b <CFc, then a <CFc (transitivity of <CF)
If a >CFb and b >CFc, then a >CFc (transitivity of >CF)
Note 1
The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.
Note 2
The sort function is intentionally generic; it does not require that its this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.25.1Runtime Semantics: SortCompare( x, y )#
The SortCompare abstract operation is called with two arguments x and y. It also has access to the comparefn argument passed to the current invocation of the sort method. The following steps are taken:
If x and y are both undefined, return +0.
If x is undefined, return 1.
If y is undefined, return -1.
If the argument comparefn is not undefined, then
Let v be ? ToNumber(? Call(comparefn, undefined, « x, y »)).
Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value, undefined property values always sort to the end of the result, followed by non-existent property values.
Note 2
Method calls performed by the ToString abstract operations in steps 5 and 7 have the potential to cause SortCompare to not behave as a consistent comparison function.
When the splice method is called with two or more arguments start, deleteCount and zero or more items, the deleteCount elements of the array starting at integer index start are replaced by the arguments items. An Array object containing the deleted elements (if any) is returned.
Let items be a List whose elements are, in left to right order, the portion of the actual argument list starting with the third argument. The list is empty if fewer than three arguments were passed.
Perform ? Set(O, "length", len - actualDeleteCount + itemCount, true).
Return A.
Note 2
The explicit setting of the length property of the result Array in step 19 was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting length became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.
Note 3
The splice function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Array.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for the Array.prototype.toLocaleString method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Let separator be the String value for the list-separator String appropriate for the host environment's current locale (this is derived in an implementation-defined way).
Let R be ? ToString(? Invoke(nextElement, "toLocaleString")).
Let R be a String value produced by concatenating S and R.
Increase k by 1.
Return R.
Note 2
The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.
Note 3
The toLocaleString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The toString function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.
When the unshift method is called with zero or more arguments item1, item2, etc., the following steps are taken:
The unshift function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Note
The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array object.
Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit properties from the Array prototype object.
Array instances have a length property, and a set of enumerable properties with array index names.
The length property of an Array instance is a data property whose value is always numerically greater than the name of every configurable own property whose name is an array index.
The length property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Reducing the value of the length property has the side-effect of deleting own array elements whose array index is between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the length property of an Array object to a value that is numerically less than or equal to the largest numeric own property name of an existing non-configurable array indexed property of the array will result in the length being set to a numeric value that is one greater than that non-configurable numeric own property name. See 9.4.2.1.
An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a named constructor for Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.
Several methods of Array objects return Iterator objects. The abstract operation CreateArrayIterator with arguments array and kind is used to create such iterator objects. It performs the following steps:
All Array Iterator Objects inherit properties from the %ArrayIteratorPrototype% intrinsic object. The %ArrayIteratorPrototype% object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %ArrayIteratorPrototype% has the following properties:
Array Iterator instances are ordinary objects that inherit properties from the %ArrayIteratorPrototype% intrinsic object. Array Iterator instances are initially created with the internal slots listed in Table 49.
Table 49: Internal Slots of Array Iterator Instances
Internal Slot
Description
[[IteratedObject]]
The object whose array elements are being iterated.
[[ArrayIteratorNextIndex]]
The integer index of the next integer index to be examined by this iteration.
[[ArrayIterationKind]]
A String value that identifies what is returned for each element of the iteration. The possible values are: "key", "value", "key+value".
TypedArray objects present an array-like view of an underlying binary data buffer (24.1). Each element of a TypedArray instance has the same underlying binary scalar data type. There is a distinct TypedArray constructor, listed in Table 50, for each of the nine supported element types. Each constructor in Table 50 has a corresponding distinct prototype object.
In the definitions below, references to TypedArray should be replaced with the appropriate constructor name from the above table. The phrase “the element size in bytes” refers to the value in the Element Size column of the table in the row corresponding to the constructor. The phrase “element Type” refers to the value in the Element Type column for that row.
The %TypedArray% intrinsic object is a constructor function object that all of the TypedArray constructor object inherit from. %TypedArray% and its corresponding prototype object provide common properties that are inherited by all TypedArray constructors and their instances. The %TypedArray% intrinsic does not have a global name or appear as a property of the global object.
The %TypedArray% intrinsic function object acts as the abstract superclass of the various TypedArray constructors. Because it is an abstract class constructor it will throw an error when invoked. The TypeArray constructors do not perform a super call to it.
%TypedArray%[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
%TypedArrayPrototype% methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
22.2.3Properties of the %TypedArrayPrototype% Object#
The value of the [[Prototype]] internal slot of the %TypedArrayPrototype% object is the intrinsic object %ObjectPrototype%. The %TypedArrayPrototype% object is an ordinary object. It does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
%TypedArray%.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
22.2.3.5%TypedArray%.prototype.copyWithin (target, start [ , end ] )#
%TypedArray%.prototype.copyWithin is a distinct function that implements the same algorithm as Array.prototype.copyWithin as defined in 22.1.3.3 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length" and the actual copying of values in step 12 must be performed in a manner that preserves the bit-level encoding of the source data
The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
22.2.3.5.1Runtime Semantics: ValidateTypedArray ( O )#
When called with argument O, the following steps are taken:
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
%TypedArray%.prototype.every is a distinct function that implements the same algorithm as Array.prototype.every as defined in 22.1.3.5 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.fill is a distinct function that implements the same algorithm as Array.prototype.fill as defined in 22.1.3.6 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.find is a distinct function that implements the same algorithm as Array.prototype.find as defined in 22.1.3.8 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to predicate may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.findIndex is a distinct function that implements the same algorithm as Array.prototype.findIndex as defined in 22.1.3.9 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to predicate may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.forEach is a distinct function that implements the same algorithm as Array.prototype.forEach as defined in 22.1.3.10 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.indexOf is a distinct function that implements the same algorithm as Array.prototype.indexOf as defined in 22.1.3.12 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.includes is a distinct function that implements the same algorithm as Array.prototype.includes as defined in 22.1.3.11 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.join is a distinct function that implements the same algorithm as Array.prototype.join as defined in 22.1.3.13 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.lastIndexOf is a distinct function that implements the same algorithm as Array.prototype.lastIndexOf as defined in 22.1.3.15 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.length is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
%TypedArray%.prototype.reduce is a distinct function that implements the same algorithm as Array.prototype.reduce as defined in 22.1.3.19 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.reduceRight is a distinct function that implements the same algorithm as Array.prototype.reduceRight as defined in 22.1.3.20 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.reverse is a distinct function that implements the same algorithm as Array.prototype.reverse as defined in 22.1.3.21 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
Sets multiple values in this TypedArray, reading the values from the object array. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.
Assert: array is any ECMAScript language value other than an Object with a [[TypedArrayName]] internal slot. If it is such an Object, the definition in 22.2.3.23.2 applies.
Let target be the this value.
If Type(target) is not Object, throw a TypeError exception.
If target does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: target has a [[ViewedArrayBuffer]] internal slot.
Sets multiple values in this TypedArray, reading the values from the typedArray argument object. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.
Assert: typedArray has a [[TypedArrayName]] internal slot. If it does not, the definition in 22.2.3.23.1 applies.
Let target be the this value.
If Type(target) is not Object, throw a TypeError exception.
If target does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: target has a [[ViewedArrayBuffer]] internal slot.
22.2.3.24%TypedArray%.prototype.slice ( start, end )#
The interpretation and use of the arguments of %TypedArray%.prototype.slice are the same as for Array.prototype.slice as defined in 22.1.3.23. The following steps are taken:
%TypedArray%.prototype.some is a distinct function that implements the same algorithm as Array.prototype.some as defined in 22.1.3.24 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm and must take into account the possibility that calls to callbackfn may cause the this value to become detached.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
%TypedArray%.prototype.sort is a distinct function that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in 22.1.3.25. The implementation of the %TypedArray%.prototype.sort specification may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. The only internal methods of the this object that the algorithm may call are [[Get]] and [[Set]].
This function is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
Upon entry, the following steps are performed to initialize evaluation of the sort function. These steps are used instead of the entry steps in 22.1.3.25:
Let len be the value of obj's [[ArrayLength]] internal slot.
The implementation defined sort order condition for exotic objects is not applied by %TypedArray%.prototype.sort.
The following version of SortCompare is used by %TypedArray%.prototype.sort. It performs a numeric comparison rather than the string comparison used in 22.1.3.25. SortCompare has access to the comparefn and buffer values of the current invocation of the sort method.
When the TypedArray SortCompare abstract operation is called with two arguments x and y, the following steps are taken:
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
If v is NaN, return +0.
Return v.
If x and y are both NaN, return +0.
If x is NaN, return 1.
If y is NaN, return -1.
If x < y, return -1.
If x > y, return 1.
If x is -0 and y is +0, return -1.
If x is +0 and y is -0, return 1.
Return +0.
Note
Because NaN always compares greater than any other value, NaN property values always sort to the end of the result when comparefn is not provided.
22.2.3.27%TypedArray%.prototype.subarray( begin, end )#
Returns a new TypedArray object whose element type is the same as this TypedArray and whose ArrayBuffer is the same as the ArrayBuffer of this TypedArray, referencing the elements at begin, inclusive, up to end, exclusive. If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[TypedArrayName]] internal slot, throw a TypeError exception.
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
Let srcLength be the value of O's [[ArrayLength]] internal slot.
%TypedArray%.prototype.toLocaleString is a distinct function that implements the same algorithm as Array.prototype.toLocaleString as defined in 22.1.3.27 except that the this object's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This function is not generic. ValidateTypedArray is applied to the this value prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.
The initial value of the %TypedArray%.prototype.toString data property is the same built-in function object as the Array.prototype.toString method defined in 22.1.3.28.
%TypedArray%.prototype[@@toStringTag] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Each of the TypedArray constructor objects is an intrinsic object that has the structure described below, differing only in the name used as the constructor name instead of TypedArray, in Table 50.
The TypedArray intrinsic constructor functions are single functions whose behaviour is overloaded based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
The TypedArray constructors are not intended to be called as a function and will throw an exception when called in that manner.
The TypedArray constructors are designed to be subclassable. They may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include a super call to the TypedArray constructor to create and initialize the subclass instance with the internal state necessary to support the %TypedArray%.prototype built-in methods.
The length property of the TypedArray constructor function is 3.
The abstract operation AllocateTypedArray with arguments constructorName, newTarget, defaultProto and optional argument length is used to validate and create an instance of a TypedArray constructor. constructorName is required to be the name of a TypedArray constructor in Table 50. If the length argument is passed an ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides common semantics that is used by all of the TypedArray overloads. AllocateTypedArray performs the following steps:
22.2.4.2.2Runtime Semantics: AllocateTypedArrayBuffer ( O, length )#
The abstract operation AllocateTypedArrayBuffer with arguments O and length allocates and associates an ArrayBuffer with the TypedArray instance O. It performs the following steps:
Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
Assert: The [[ViewedArrayBuffer]] internal slot of O is undefined.
Assert: length ≥ 0.
Let constructorName be the String value of O's [[TypedArrayName]] internal slot.
Let elementSize be the Element Size value in Table 50 for constructorName.
This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object has a [[TypedArrayName]] internal slot.
TypedArray called with argument typedArray performs the following steps:
Assert: Type(typedArray) is Object and typedArray has a [[TypedArrayName]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 50 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArrayPrototype%").
Let srcArray be typedArray.
Let srcData be the value of srcArray's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(srcData) is true, throw a TypeError exception.
Let constructorName be the String value of O's [[TypedArrayName]] internal slot.
Let elementType be the String value of the Element Type value in Table 50 for constructorName.
Let elementLength be the value of srcArray's [[ArrayLength]] internal slot.
Let srcName be the String value of srcArray's [[TypedArrayName]] internal slot.
Let srcType be the String value of the Element Type value in Table 50 for srcName.
Let srcElementSize be the Element Size value in Table 50 for srcName.
Let srcByteOffset be the value of srcArray's [[ByteOffset]] internal slot.
Let elementSize be the Element Size value in Table 50 for constructorName.
This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
TypedArray called with argument object performs the following steps:
Assert: Type(object) is Object and object does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 50 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArrayPrototype%").
This description applies only if the TypedArray function is called with at least one argument and the Type of the first argument is Object and that object has an [[ArrayBufferData]] internal slot.
TypedArray called with arguments buffer, byteOffset, and length performs the following steps:
Assert: Type(buffer) is Object and buffer has an [[ArrayBufferData]] internal slot.
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 50 for this TypedArray constructor.
Let O be ? AllocateTypedArray(constructorName, NewTarget, "%TypedArrayPrototype%").
Let constructorName be the String value of O's [[TypedArrayName]] internal slot.
Let elementSize be the Number value of the Element Size value in Table 50 for constructorName.
The abstract operation TypedArrayCreate with arguments constructor and argumentList is used to specify the creation of a new TypedArray object using a constructor function. It performs the following steps:
Let newTypedArray be ? Construct(constructor, argumentList).
The abstract operation TypedArraySpeciesCreate with arguments exemplar and argumentList is used to specify the creation of a new TypedArray object using a constructor function that is derived from exemplar. It performs the following steps:
Assert: exemplar is an Object that has a [[TypedArrayName]] internal slot.
Let defaultConstructor be the intrinsic object listed in column one of Table 50 for the value of exemplar's [[TypedArrayName]] internal slot.
The value of the [[Prototype]] internal slot of a TypedArray prototype object is the intrinsic object %TypedArrayPrototype%. A TypedArray prototype object is an ordinary object. It does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
TypedArray instances are Integer Indexed exotic objects. Each TypedArray instance inherits properties from the corresponding TypedArray prototype object. Each TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using the SameValueZero comparison algorithm.
Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.
The Map constructor is the %Map% intrinsic object and the initial value of the Map property of the global object. When called as a constructor it creates and initializes a new Map object. Map is not intended to be called as a function and will throw an exception when called in that manner.
The Map constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include a super call to the Map constructor to create and initialize the subclass instance with the internal state necessary to support the Map.prototype built-in methods.
If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.
Map[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to determine the constructor to use to create the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.
The Map prototype object is the intrinsic object %MapPrototype%. The value of the [[Prototype]] internal slot of the Map prototype object is the intrinsic object %ObjectPrototype%. The Map prototype object is an ordinary object. It does not have a [[MapData]] internal slot.
If Type(M) is not Object, throw a TypeError exception.
If M does not have a [[MapData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of M's [[MapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} p that is an element of entries,
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return undefined.
Note
The existing [[MapData]] List is preserved because there may be existing Map Iterator objects that are suspended midway through iterating over that List.
If Type(M) is not Object, throw a TypeError exception.
If M does not have a [[MapData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of M's [[MapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} p that is an element of entries,
If p.[[Key]] is not empty and SameValueZero(p.[[Key]], key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
When the forEach method is called with one or two arguments, the following steps are taken:
Let M be the this value.
If Type(M) is not Object, throw a TypeError exception.
If M does not have a [[MapData]] internal slot, throw a TypeError exception.
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let entries be the List that is the value of M's [[MapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} e that is an element of entries, in original key insertion order
If e.[[Key]] is not empty, then
Perform ? Call(callbackfn, T, « e.[[Value]], e.[[Key]], M »).
Return undefined.
Note
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each key/value pair present in the map object, in key insertion order. callbackfn is called only for keys of the map which actually exist; it is not called for keys that have been deleted from the map.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the value of the item, the key of the item, and the Map object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a named constructor for Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.
Several methods of Map objects return Iterator objects. The abstract operation CreateMapIterator with arguments map and kind is used to create such iterator objects. It performs the following steps:
If Type(map) is not Object, throw a TypeError exception.
If map does not have a [[MapData]] internal slot, throw a TypeError exception.
All Map Iterator Objects inherit properties from the %MapIteratorPrototype% intrinsic object. The %MapIteratorPrototype% intrinsic object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %MapIteratorPrototype% has the following properties:
Let entries be the List that is the value of the [[MapData]] internal slot of m.
Repeat while index is less than the total number of elements of entries. The number of elements must be redetermined each time this method is evaluated.
Let e be the Record {[[Key]], [[Value]]} that is the value of entries[index].
Set index to index+1.
Set the [[MapNextIndex]] internal slot of O to index.
If e.[[Key]] is not empty, then
If itemKind is "key", let result be e.[[Key]].
Else if itemKind is "value", let result be e.[[Value]].
Map Iterator instances are ordinary objects that inherit properties from the %MapIteratorPrototype% intrinsic object. Map Iterator instances are initially created with the internal slots described in Table 51.
Table 51: Internal Slots of Map Iterator Instances
Internal Slot
Description
[[Map]]
The Map object that is being iterated.
[[MapNextIndex]]
The integer index of the next Map data element to be examined by this iterator.
[[MapIterationKind]]
A String value that identifies what is to be returned for each element of the iteration. The possible values are: "key", "value", "key+value".
Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using the SameValueZero comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Set objects specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.
The Set constructor is the %Set% intrinsic object and the initial value of the Set property of the global object. When called as a constructor it creates and initializes a new Set object. Set is not intended to be called as a function and will throw an exception when called in that manner.
The Set constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include a super call to the Set constructor to create and initialize the subclass instance with the internal state necessary to support the Set.prototype built-in methods.
Set[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
Methods that create derived collection objects should call @@species to determine the constructor to use to create the derived objects. Subclass constructor may over-ride @@species to change the default constructor assignment.
The Set prototype object is the intrinsic object %SetPrototype%. The value of the [[Prototype]] internal slot of the Set prototype object is the intrinsic object %ObjectPrototype%. The Set prototype object is an ordinary object. It does not have a [[SetData]] internal slot.
If Type(S) is not Object, throw a TypeError exception.
If S does not have a [[SetData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of S's [[SetData]] internal slot.
Repeat for each e that is an element of entries,
Replace the element of entries whose value is e with an element whose value is empty.
Return undefined.
Note
The existing [[SetData]] List is preserved because there may be existing Set Iterator objects that are suspended midway through iterating over that List.
If Type(S) is not Object, throw a TypeError exception.
If S does not have a [[SetData]] internal slot, throw a TypeError exception.
Let entries be the List that is the value of S's [[SetData]] internal slot.
Repeat for each e that is an element of entries,
If e is not empty and SameValueZero(e, value) is true, then
Replace the element of entries whose value is e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each value present in the set object, in value insertion order. callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.
callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.
The callbackfn is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.
A Set Iterator is an ordinary object, with the structure defined below, that represents a specific iteration over some specific Set instance object. There is not a named constructor for Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.
Several methods of Set objects return Iterator objects. The abstract operation CreateSetIterator with arguments set and kind is used to create such iterator objects. It performs the following steps:
If Type(set) is not Object, throw a TypeError exception.
If set does not have a [[SetData]] internal slot, throw a TypeError exception.
All Set Iterator Objects inherit properties from the %SetIteratorPrototype% intrinsic object. The %SetIteratorPrototype% intrinsic object is an ordinary object and its [[Prototype]] internal slot is the %IteratorPrototype% intrinsic object. In addition, %SetIteratorPrototype% has the following properties:
Let entries be the List that is the value of the [[SetData]] internal slot of s.
Repeat while index is less than the total number of elements of entries. The number of elements must be redetermined each time this method is evaluated.
Let e be entries[index].
Set index to index+1.
Set the [[SetNextIndex]] internal slot of O to index.
Set Iterator instances are ordinary objects that inherit properties from the %SetIteratorPrototype% intrinsic object. Set Iterator instances are initially created with the internal slots specified in Table 52.
Table 52: Internal Slots of Set Iterator Instances
Internal Slot
Description
[[IteratedSet]]
The Set object that is being iterated.
[[SetNextIndex]]
The integer index of the next Set data element to be examined by this iterator
[[SetIterationKind]]
A String value that identifies what is to be returned for each element of the iteration. The possible values are: "key", "value", "key+value". "key" and "value" have the same meaning.
WeakMap objects are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. If an object that is being used as the key of a WeakMap key/value pair is only reachable by following a chain of references that start within that WeakMap, then that key/value pair is inaccessible and is automatically removed from the WeakMap. WeakMap implementations must detect and remove such key/value pairs and any associated resources.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.
WeakMap objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this WeakMap objects specification are only intended to describe the required observable semantics of WeakMap objects. It is not intended to be a viable implementation model.
Note
WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSets:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
The WeakMap constructor is the %WeakMap% intrinsic object and the initial value of the WeakMap property of the global object. When called as a constructor it creates and initializes a new WeakMap object. WeakMap is not intended to be called as a function and will throw an exception when called in that manner.
The WeakMap constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include a super call to the WeakMap constructor to create and initialize the subclass instance with the internal state necessary to support the WeakMap.prototype built-in methods.
If the parameter iterable is present, it is expected to be an object that implements an @@iterator method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key.
The WeakMap prototype object is the intrinsic object %WeakMapPrototype%. The value of the [[Prototype]] internal slot of the WeakMap prototype object is the intrinsic object %ObjectPrototype%. The WeakMap prototype object is an ordinary object. It does not have a [[WeakMapData]] internal slot.
Repeat for each Record {[[Key]], [[Value]]} p that is an element of entries,
If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.
WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects it holds. If an object that is contained by a WeakSet is only reachable by following a chain of references that start within that WeakSet, then that object is inaccessible and is automatically removed from the WeakSet. WeakSet implementations must detect and remove such objects and any associated resources.
An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object that does not require the observer to present the observed object.
WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects specification is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be a viable implementation model.
The WeakSet constructor is the %WeakSet% intrinsic object and the initial value of the WeakSet property of the global object. When called as a constructor it creates and initializes a new WeakSet object. WeakSet is not intended to be called as a function and will throw an exception when called in that manner.
The WeakSet constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include a super call to the WeakSet constructor to create and initialize the subclass instance with the internal state necessary to support the WeakSet.prototype built-in methods.
The WeakSet prototype object is the intrinsic object %WeakSetPrototype%. The value of the [[Prototype]] internal slot of the WeakSet prototype object is the intrinsic object %ObjectPrototype%. The WeakSet prototype object is an ordinary object. It does not have a [[WeakSetData]] internal slot.
Let entries be the List that is the value of S's [[WeakSetData]] internal slot.
Repeat for each e that is an element of entries,
If e is not empty and SameValue(e, value) is true, then
Replace the element of entries whose value is e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.
The abstract operation AllocateArrayBuffer with arguments constructor and byteLength is used to create an ArrayBuffer object. It performs the following steps:
Let obj be ? OrdinaryCreateFromConstructor(constructor, "%ArrayBufferPrototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
Detaching an ArrayBuffer instance disassociates the Data Block used as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by this specification use the DetachArrayBuffer abstract operation. However, an ECMAScript implementation or host environment may define such operations.
The abstract operation CloneArrayBuffer takes three parameters, an ArrayBuffer srcBuffer, an integer srcByteOffset and optionally a constructor function cloneConstructor. It creates a new ArrayBuffer whose data is a copy of srcBuffer's data starting at srcByteOffset. This operation performs the following steps:
Assert: Type(srcBuffer) is Object and it has an [[ArrayBufferData]] internal slot.
24.1.1.5GetValueFromBuffer ( arrayBuffer, byteIndex, type [ , isLittleEndian ] )#
The abstract operation GetValueFromBuffer takes four parameters, an ArrayBuffer arrayBuffer, an integer byteIndex, a String type, and optionally a Boolean isLittleEndian. This operation performs the following steps:
Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
Assert: byteIndex is an integer value ≥ 0.
Let block be arrayBuffer's [[ArrayBufferData]] internal slot.
Let elementSize be the Number value of the Element Size value specified in Table 50 for Element Type type.
Let rawValue be a List of elementSize containing, in order, the elementSize sequence of bytes starting with block[byteIndex].
If isLittleEndian is not present, set isLittleEndian to either true or false. The choice is implementation dependent and should be the alternative that is most efficient for the implementation. An implementation must use the same value each time this step is executed and the same value must be used for the corresponding step in the SetValueInBuffer abstract operation.
If isLittleEndian is false, reverse the order of the elements of rawValue.
If type is "Float32", then
Let value be the byte elements of rawValue concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary32 value.
If value is an IEEE 754-2008 binary32 NaN value, return the NaN Number value.
Return the Number value that corresponds to value.
If type is "Float64", then
Let value be the byte elements of rawValue concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2008 binary64 value.
If value is an IEEE 754-2008 binary64 NaN value, return the NaN Number value.
Return the Number value that corresponds to value.
If the first code unit of type is "U", then
Let intValue be the byte elements of rawValue concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
Else,
Let intValue be the byte elements of rawValue concatenated and interpreted as a bit string encoding of a binary little-endian 2's complement number of bit length elementSize × 8.
Return the Number value that corresponds to intValue.
The abstract operation SetValueInBuffer takes five parameters, an ArrayBuffer arrayBuffer, an integer byteIndex, a String type, a Number value, and optionally a Boolean isLittleEndian. This operation performs the following steps:
Let block be arrayBuffer's [[ArrayBufferData]] internal slot.
Assert: block is not undefined.
If isLittleEndian is not present, set isLittleEndian to either true or false. The choice is implementation dependent and should be the alternative that is most efficient for the implementation. An implementation must use the same value each time this step is executed and the same value must be used for the corresponding step in the GetValueFromBuffer abstract operation.
If type is "Float32", then
Set rawBytes to a List containing the 4 bytes that are the result of converting value to IEEE 754-2008 binary32 format using “Round to nearest, ties to even” rounding mode. If isLittleEndian is false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value is NaN, rawValue may be set to any implementation chosen IEEE 754-2008 binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else, if type is "Float64", then
Set rawBytes to a List containing the 8 bytes that are the IEEE 754-2008 binary64 format encoding of value. If isLittleEndian is false, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value is NaN, rawValue may be set to any implementation chosen IEEE 754-2008 binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else,
Let n be the Number value of the Element Size specified in Table 50 for Element Type type.
Let convOp be the abstract operation named in the Conversion Operation column in Table 50 for Element Type type.
Let intValue be convOp(value).
If intValue ≥ 0, then
Let rawBytes be a List containing the n-byte binary encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Else,
Let rawBytes be a List containing the n-byte binary 2's complement encoding of intValue. If isLittleEndian is false, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
Store the individual bytes of rawBytes into block, in order, starting at block[byteIndex].
The ArrayBuffer constructor is the %ArrayBuffer% intrinsic object and the initial value of the ArrayBuffer property of the global object. When called as a constructor it creates and initializes a new ArrayBuffer object. ArrayBuffer is not intended to be called as a function and will throw an exception when called in that manner.
The ArrayBuffer constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.
ArrayBuffer[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Return the this value.
The value of the name property of this function is "get [Symbol.species]".
Note
ArrayBuffer prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
24.1.4Properties of the ArrayBuffer Prototype Object#
The ArrayBuffer prototype object is the intrinsic object %ArrayBufferPrototype%. The value of the [[Prototype]] internal slot of the ArrayBuffer prototype object is the intrinsic object %ObjectPrototype%. The ArrayBuffer prototype object is an ordinary object. It does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have an [[ArrayBufferData]] internal slot, throw a TypeError exception.
ArrayBuffer instances inherit properties from the ArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot and an [[ArrayBufferByteLength]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is null are considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.
24.2.1.1GetViewValue ( view, requestIndex, isLittleEndian, type )#
The abstract operation GetViewValue with arguments view, requestIndex, isLittleEndian, and type is used by functions on DataView instances is to retrieve values from the view's buffer. It performs the following steps:
If Type(view) is not Object, throw a TypeError exception.
If view does not have a [[DataView]] internal slot, throw a TypeError exception.
24.2.1.2SetViewValue ( view, requestIndex, isLittleEndian, type, value )#
The abstract operation SetViewValue with arguments view, requestIndex, isLittleEndian, type, and value is used by functions on DataView instances to store values into the view's buffer. It performs the following steps:
If Type(view) is not Object, throw a TypeError exception.
If view does not have a [[DataView]] internal slot, throw a TypeError exception.
The DataView constructor is the %DataView% intrinsic object and the initial value of the DataView property of the global object. When called as a constructor it creates and initializes a new DataView object. DataView is not intended to be called as a function and will throw an exception when called in that manner.
The DataView constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include a super call to the DataView constructor to create and initialize subclass instances with the internal state necessary to support the DataView.prototype built-in methods.
The initial value of DataView.prototype is the intrinsic object %DataViewPrototype%.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
24.2.4Properties of the DataView Prototype Object#
The DataView prototype object is the intrinsic object %DataViewPrototype%. The value of the [[Prototype]] internal slot of the DataView prototype object is the intrinsic object %ObjectPrototype%. The DataView prototype object is an ordinary object. It does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.
DataView.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let size be the value of O's [[ByteLength]] internal slot.
DataView.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps:
Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
If O does not have a [[ViewedArrayBuffer]] internal slot, throw a TypeError exception.
Let buffer be the value of O's [[ViewedArrayBuffer]] internal slot.
If IsDetachedBuffer(buffer) is true, throw a TypeError exception.
Let offset be the value of O's [[ByteOffset]] internal slot.
DataView instances are ordinary objects that inherit properties from the DataView prototype object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataView constructor.
The JSON object is the %JSON% intrinsic object and the initial value of the JSON property of the global object. The JSON object is a single ordinary object that contains two functions, parse and stringify, that are used to parse and construct JSON texts. The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404.
Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.
The value of the [[Prototype]] internal slot of the JSON object is the intrinsic object %ObjectPrototype%. The value of the [[Extensible]] internal slot of the JSON object is set to true.
The JSON object does not have a [[Construct]] internal method; it is not possible to use the JSON object as a constructor with the new operator.
The JSON object does not have a [[Call]] internal method; it is not possible to invoke the JSON object as a function.
The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format is a subset of the syntax for ECMAScript literals, Array Initializers and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and null.
The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns undefined then the property is deleted from the result.
Parse JText interpreted as UTF-16 encoded Unicode points (6.1.4) as a JSON text as specified in ECMA-404. Throw a SyntaxError exception if JText is not a valid JSON text as defined in that specification.
Let scriptText be the result of concatenating "(", JText, and ");".
Let completion be the result of parsing and evaluating scriptText as if it was the source text of an ECMAScript Script, but using the alternative definition of DoubleStringCharacter provided below. The extended PropertyDefinitionEvaluation semantics defined in B.3.1 must not be used during the evaluation.
Let unfiltered be completion.[[Value]].
Assert: unfiltered will be either a primitive value or an object that is defined by either an ArrayLiteral or an ObjectLiteral.
JSON allows Unicode code units 0x2028 (LINE SEPARATOR) and 0x2029 (PARAGRAPH SEPARATOR) to directly appear in String literals without using an escape sequence. This is enabled by using the following alternative definition of DoubleStringCharacter when parsing scriptText in step 4:
The syntax of a valid JSON text is a subset of the ECMAScript PrimaryExpression syntax. Hence a valid JSON text is also a valid PrimaryExpression. Step 2 above verifies that JText conforms to that subset. When scriptText is parsed and evaluated as a Script the result will be either a String, Number, Boolean, or Null primitive value or an Object defined as if by an ArrayLiteral or ObjectLiteral.
The abstract operation InternalizeJSONProperty is a recursive abstract operation that takes two parameters: a holder object and the String name of a property in that object. InternalizeJSONProperty uses the value of reviver that was originally passed to the above parse function.
It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.
Note
In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.
24.3.2JSON.stringify ( value [ , replacer [ , space ] ] )#
The stringify function returns a String in UTF-16 encoded JSON format representing an ECMAScript value. It can take three parameters. The value parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number or null. The optional replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as a white list for selecting the object properties that will be stringified. The optional space parameter is a String or Number that allows the result to have white space injected into it to improve human readability.
The length property of the stringify function is 3.
Note 1
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then the stringify function must throw a TypeError exception. This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as the String null.
The undefined value is not rendered.
The true value is rendered in JSON text as the String true.
The false value is rendered in JSON text as the String false.
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String null.
Note 5
Values that do not have a JSON representation (such as undefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the String null. In objects an unrepresentable value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the key or property name, a U+003A (COLON), and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
The abstract operation SerializeJSONProperty with arguments key, and holder has access to ReplacerFunction from the invocation of the stringify method. Its algorithm is as follows:
24.3.2.2Runtime Semantics: QuoteJSONString ( value )#
The abstract operation QuoteJSONString with argument value wraps a String value in QUOTATION MARK code units and escapes certain other code units within it.
Let product be code unit 0x0022 (QUOTATION MARK).
For each code unit C in value
If C is 0x0022 (QUOTATION MARK) or 0x005C (REVERSE SOLIDUS), then
Let product be the concatenation of product and code unit 0x005C (REVERSE SOLIDUS).
Let product be the concatenation of product and C.
Else if C is 0x0008 (BACKSPACE), 0x000C (FORM FEED), 0x000A (LINE FEED), 0x000D (CARRIAGE RETURN), or 0x0009 (CHARACTER TABULATION), then
Let product be the concatenation of product and code unit 0x005C (REVERSE SOLIDUS).
Let abbrev be the String value corresponding to the value of C as follows:
BACKSPACE
"b"
FORM FEED (FF)
"f"
LINE FEED (LF)
"n"
CARRIAGE RETURN (CR)
"r"
CHARACTER TABULATION
"t"
Let product be the concatenation of product and abbrev.
Else if C has a code unit value less than 0x0020 (SPACE), then
Let product be the concatenation of product and code unit 0x005C (REVERSE SOLIDUS).
Let product be the concatenation of product and "u".
Let hex be the string result of converting the numeric code unit value of C to a String of four hexadecimal digits. Alphabetic hexadecimal digits are presented as lowercase Latin letters.
Let product be the concatenation of product and hex.
Else,
Let product be the concatenation of product and C.
Let product be the concatenation of product and code unit 0x0022 (QUOTATION MARK).
Return product.
24.3.2.3Runtime Semantics: SerializeJSONObject ( value )#
The abstract operation SerializeJSONObject with argument value serializes an object. It has access to the stack, indent, gap, and PropertyList values of the current invocation of the stringify method.
If stack contains value, throw a TypeError exception because the structure is cyclical.
Append value to stack.
Let stepback be indent.
Let indent be the concatenation of indent and gap.
Let member be the concatenation of member and the string ":".
If gap is not the empty String, then
Let member be the concatenation of member and code unit 0x0020 (SPACE).
Let member be the concatenation of member and strP.
Append member to partial.
If partial is empty, then
Let final be "{}".
Else,
If gap is the empty String, then
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let final be the result of concatenating "{", properties, and "}".
Else gap is not the empty String
Let separator be the result of concatenating code unit 0x002C (COMMA), code unit 0x000A (LINE FEED), and indent.
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be the result of concatenating "{", code unit 0x000A (LINE FEED), indent, properties, code unit 0x000A (LINE FEED), stepback, and "}".
Remove the last element of stack.
Let indent be stepback.
Return final.
24.3.2.4Runtime Semantics: SerializeJSONArray ( value )#
The abstract operation SerializeJSONArray with argument value serializes an array. It has access to the stack, indent, and gap values of the current invocation of the stringify method.
If stack contains value, throw a TypeError exception because the structure is cyclical.
Append value to stack.
Let stepback be indent.
Let indent be the concatenation of indent and gap.
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let final be the result of concatenating "[", properties, and "]".
Else,
Let separator be the result of concatenating code unit 0x002C (COMMA), code unit 0x000A (LINE FEED), and indent.
Let properties be a String formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be the result of concatenating "[", code unit 0x000A (LINE FEED), indent, properties, code unit 0x000A (LINE FEED), stepback, and "]".
Remove the last element of stack.
Let indent be stepback.
Return final.
Note
The representation of arrays includes only the elements between zero and array.length - 1 inclusive. Properties whose keys are not array indexes are excluded from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.
An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.
A function that returns an IteratorResult object.
一个返回IteratorResult对象的函数。
The returned object must conform to the IteratorResult interface. If a previous call to the next method of an Iterator has returned an IteratorResult object whose done property is true, then all subsequent calls to the next method of that object should also return an IteratorResult object whose done property is true. However, this requirement is not enforced.
返回的对象必须符合IteratorResult接口。 如果先前对Iterator的下一个方法的调用返回了其done属性为true的IteratorResult对象，则对该对象的下一个方法的所有后续调用也应返回其done属性为true的IteratorResult对象。 但是，此要求未得到强制执行。
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
A function that returns an IteratorResult object.
一个返回IteratorResult对象的函数。
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more next method calls to the Iterator. The returned IteratorResult object will typically have a done property whose value is true, and a value property with the value passed as the argument of the return method. However, this requirement is not enforced.
返回的对象必须符合IteratorResult接口。 调用此方法通知Iterator对象调用者不打算再对Iterator进行下一次方法调用。 返回的IteratorResult对象通常具有一个值为true的done属性，以及一个value属性，其值作为return方法的参数传递。 但是，此要求未得到强制执行。
throw
A function that returns an IteratorResult object.
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a done property whose value is true.
返回的对象必须符合IteratorResult接口。 调用此方法会通知Iterator对象调用方已检测到错误情况。 该参数可用于识别错误条件，并且通常是异常对象。 典型的响应是抛出作为参数传递的值。 如果方法没有抛出，则返回的IteratorResult对象通常会有一个值为true的done属性。
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.
The IteratorResult interface includes the properties listed in Table 56(IteratorResult接口包括表56中列出的属性：):
Table 56: IteratorResult Interface Properties
Property
Value
Requirements
done
Either true or false.
This is the result status of an iteratornext method call. If the end of the iterator was reached done is true. If the end was not reached done is false and a value is available. If a done property (either own or inherited) does not exist, it is consider to have the value false.
这是迭代器next方法调用的结果状态。 如果到达迭代器的末尾，则为true。 如果未达到结束，则为false并且值可用。 如果不存在done属性（自己的或继承的），则认为值为false。
If done is false, this is the current iteration element value. If done is true, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value, value is undefined. In that case, the value property may be absent from the conforming object if it does not inherit an explicit value property.
如果done为false，则这是当前迭代元素值。 如果done为true，则这是迭代器的返回值，如果它提供了一个。 如果迭代器没有返回值，则值为undefined。 在这种情况下，如果不继承显式值属性，则可以在符合对象中不存在value属性。
The value of the [[Prototype]] internal slot of the %IteratorPrototype% object is the intrinsic object %ObjectPrototype%. The %IteratorPrototype% object is an ordinary object. The initial value of the [[Extensible]] internal slot of the %IteratorPrototype% object is true.
All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%.The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.
The GeneratorFunction constructor is the %GeneratorFunction% intrinsic. When GeneratorFunction is called as a function rather than as a constructor, it creates and initializes a new GeneratorFunction object. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
GeneratorFunction is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.
The last argument specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.
When the GeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:
25.2.2Properties of the GeneratorFunction Constructor#
The GeneratorFunction constructor is a standard built-in function object that inherits from the Function constructor. The value of the [[Prototype]] internal slot of the GeneratorFunction constructor is the intrinsic object %Function%.
The value of the [[Extensible]] internal slot of the GeneratorFunction constructor is true.
The value of the name property of the GeneratorFunction is "GeneratorFunction".
The GeneratorFunction constructor has the following properties:
The initial value of GeneratorFunction.prototype is the intrinsic object %Generator%.
GeneratorFunction.prototype的初始值是内部对象％Generator％。
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
25.2.3Properties of the GeneratorFunction Prototype Object（GeneratorFunction原型对象的属性）#
The GeneratorFunction prototype object is an ordinary object. It is not a function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed in Table 27 or Table 57. In addition to being the value of the prototype property of the %GeneratorFunction% intrinsic, it is the %Generator% intrinsic (see Figure 2).
The value of the [[Prototype]] internal slot of the GeneratorFunction prototype object is the %FunctionPrototype% intrinsic object. The initial value of the [[Extensible]] internal slot of the GeneratorFunction prototype object is true.
Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 27. The value of the [[FunctionKind]] internal slot for all such instances is "generator".
Each GeneratorFunction instance has the following own properties:
The value of the length property is an integer that indicates the typical number of arguments expected by the GeneratorFunction. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a GeneratorFunction when invoked on a number of arguments other than the number specified by its length property depends on the function.
Whenever a GeneratorFunction instance is created another ordinary object is also created and is the initial value of the generator function's prototype property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator object when the generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Unlike function instances, the object that is the value of the a GeneratorFunction's prototype property does not have a constructor property whose value is the GeneratorFunction instance.
A Generator object is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.
Generator instances directly inherit properties from the object that is the value of the prototype property of the Generator function that created the instance. Generator instances indirectly inherit properties from the Generator Prototype intrinsic, %GeneratorPrototype%.
The Generator prototype object is the %GeneratorPrototype% intrinsic. It is also the initial value of the prototype property of the %Generator% intrinsic (the GeneratorFunction.prototype).
The Generator prototype is an ordinary object. It is not a Generator instance and does not have a [[GeneratorState]] internal slot.
The value of the [[Prototype]] internal slot of the Generator prototype object is the intrinsic object %IteratorPrototype%. The initial value of the [[Extensible]] internal slot of the Generator prototype object is true.
All Generator instances indirectly inherit properties of the Generator prototype object.
Generator instances are initially created with the internal slots described in Table 57.
最初使用表57中描述的内部插槽创建生成器实例。
Table 57: Internal Slots of Generator Instances
Internal Slot
Description
[[GeneratorState]]
The current execution state of the generator. The possible values are: undefined, "suspendedStart", "suspendedYield", "executing", and "completed".
生成器的当前执行状态。 可能的值包括：undefined，“suspendedStart”，“suspendedYield”，“execution”和“completed”。
[[GeneratorContext]]
The execution context that is used when executing the code of this generator.
执行此生成器代码时使用的执行上下文。
Set generator's [[GeneratorState]] internal slot to "completed".
Once a generator enters the "completed" state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
If result is a normal completion, let resultValue be undefined.
Else,
If result.[[Type]] is return, let resultValue be result.[[Value]].
Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
Set generator's [[GeneratorState]] internal slot to "completed".
Once a generator enters the "completed" state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
Set the code evaluation state of genContext such that when evaluation is resumed with a CompletionresumptionValue the following steps will be performed:
Return resumptionValue.
NOTE: This returns to the evaluation of the YieldExpression production that originally called this abstract operation.
A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
A PromiseCapability is a Record value used to encapsulate a promise object along with the functions that are capable of resolving or rejecting that promise object. PromiseCapability records are produced by the NewPromiseCapability abstract operation.
PromiseCapability Records have the fields listed in Table 58.
The PromiseReaction is a Record value used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction records are created by the PerformPromiseThen abstract operation, and are used by a PromiseReactionJob.
PromiseReaction records have the fields listed in Table 59.
The capabilities of the promise for which this record provides a reaction handler.
此记录提供反应处理器的promise的功能。
[[Handler]]
A function object or a String
函数对象或String
The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] is "Identity" it is equivalent to a function that simply returns its first argument. If [[Handler]] is "Thrower" it is equivalent to a function that throws its first argument as an exception.
应用于传入值的函数，其返回值将控制派生的promise所发生的情况。 如果[[Handler]]是“Identity”，它等同于一个只返回其第一个参数的函数。 如果[[Handler]]是“Thrower”，它等同于将其第一个参数作为异常抛出的函数。
The abstract operation NewPromiseCapability takes a constructor function, and attempts to use that constructor function in the fashion of the built-in Promise constructor to create a Promise object and extract its resolve and reject functions. The promise plus the resolve and reject functions are used to initialize a new PromiseCapability record which is returned as the value of this abstract operation.
If IsCallable(promiseCapability.[[Resolve]]) is false, throw a TypeError exception.
If IsCallable(promiseCapability.[[Reject]]) is false, throw a TypeError exception.
Set promiseCapability.[[Promise]] to promise.
Return promiseCapability.
Note
This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed executor function argument in the same way as the Promise constructor. It is used to generalize static methods of the Promise constructor to any subclass.
The abstract operation TriggerPromiseReactions takes a collection of PromiseReactionRecords and enqueues a new Job for each record. Each such Job processes the [[Handler]] of the PromiseReactionRecord, and if the [[Handler]] is a function calls it passing the given argument.
HostPromiseRejectionTracker is an implementation-defined abstract operation that allows host environments to track promise rejections.
An implementation of HostPromiseRejectionTracker must complete normally in all cases. The default implementation of HostPromiseRejectionTracker is to do nothing.
HostPromiseRejectionTracker is called in two scenarios:
在两种情况下调用HostPromiseRejectionTracker：
When a promise is rejected without any handlers, it is called with its operation argument set to "reject".
When a handler is added to a rejected promise for the first time, it is called with its operation argument set to "handle".
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.
If operation is "handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is "reject", since it is expected that rejections will be rare and not on hot code paths.
The job PromiseReactionJob with parameters reaction and argument applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler.
This Job uses the supplied thenable and its then method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.
The Promise constructor is the %Promise% intrinsic object and the initial value of the Promise property of the global object. When called as a constructor it creates and initializes a new Promise object. Promise is not intended to be called as a function and will throw an exception when called in that manner.
The Promise constructor is designed to be subclassable. It may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promise constructor to create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.
The executor argument must be a function object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise object. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.
The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise object. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another Promise object which will provide the value if it is fulfilled.
The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.
The resolve and reject functions passed to an executor function by the Promise constructor have the capability to actually resolve and reject the associated promise. Subclasses may have different constructor behaviour that passes in customized values for resolve and reject.
The all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.
When the PerformPromiseAll abstract operation is called with arguments iteratorRecord, constructor, and resultCapability, the following steps are taken:
Assert: constructor is a constructor function.
Assert: resultCapability is a PromiseCapability record.
A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capabilities]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function F is called with argument x, the following steps are taken:
Let alreadyCalled be the value of F's [[AlreadyCalled]] internal slot.
If alreadyCalled.[[Value]] is true, return undefined.
Set alreadyCalled.[[Value]] to true.
Let index be the value of F's [[Index]] internal slot.
Let values be the value of F's [[Values]] internal slot.
Let promiseCapability be the value of F's [[Capabilities]] internal slot.
Let remainingElementsCount be the value of F's [[RemainingElements]] internal slot.
Set values[index] to x.
Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
The race function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.
If the iterable argument is empty or if none of the promises in iterable ever settle then the pending promise returned by this method will never be settled.
Note 2
The race function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor. It also expects that its this value provides a resolve method.
25.4.4.3.1Runtime Semantics: PerformPromiseRace ( iteratorRecord, promiseCapability, C )#
When the PerformPromiseRace abstract operation is called with arguments iteratorRecord, promiseCapability, and C, the following steps are taken:
Repeat
Let next be IteratorStep(iteratorRecord.[[Iterator]]).
If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
The resolve function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by this constructor.
The value of the name property of this function is "get [Symbol.species]".
此函数的name属性的值为“get [Symbol.species]”。
Note
Promise prototype methods normally use their this object's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its @@species property.
class Ps extends Promise {
static get [Symbol.species]() {
return Promise;
}
}
let ps1 = new Ps(() => {}, null);
ps1 instanceof Ps;
// true
ps1.then(() => {}) instanceof Ps
// false
// 调用then的衍生对象构造函数指向了Promise
25.4.5Properties of the Promise Prototype Object（Promise原型对象的属性）#
The Promise prototype object is the intrinsic object %PromisePrototype%. The value of the [[Prototype]] internal slot of the Promise prototype object is the intrinsic object %ObjectPrototype%. The Promise prototype object is an ordinary object. It does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.
The abstract operation PerformPromiseThen performs the “then” operation on promise using onFulfilled and onRejected as its settlement actions. The result is resultCapability's promise.
Promise instances are ordinary objects that inherit properties from the Promise prototype object (the intrinsic, %PromisePrototype%). Promise instances are initially created with the internal slots described in Table 60.
A String value that governs how a promise will react to incoming calls to its then method. The possible values are: "pending", "fulfilled", and "rejected".
一个String值，用于控制promise对其then方法的传入调用的响应方式。 可能的值包括： "pending", "fulfilled", "rejected".
[[PromiseResult]]
The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is not "pending".
promise fulfilled或rejected的值（如果有）。 只有在[[PromiseState]]不是“pending”时才有意义。
[[PromiseFulfillReactions]]
A List of PromiseReaction records to be processed when/if the promise transitions from the "pending" state to the "fulfilled" state.
当/如果promise从“pending”状态转换到“fulfilled”状态时要处理的PromiseReaction记录列表。
[[PromiseRejectReactions]]
A List of PromiseReaction records to be processed when/if the promise transitions from the "pending" state to the "rejected" state.
当/如果promise从“pending”状态转换为“reject”状态时要处理的PromiseReaction记录列表。
[[PromiseIsHandled]]
A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.
一个布尔值，表示promise是否曾经有过fulfilled或reject处理程序; 用于未处理的rejection跟踪。
The Reflect object is the %Reflect% intrinsic object and the initial value of the Reflect property of the global object. The Reflect object is an ordinary object.
The value of the [[Prototype]] internal slot of the Reflect object is the intrinsic object %ObjectPrototype%.
The Reflect object is not a function object. It does not have a [[Construct]] internal method; it is not possible to use the Reflect object as a constructor with the new operator. The Reflect object also does not have a [[Call]] internal method; it is not possible to invoke the Reflect object as a function.
The Proxy constructor is the %Proxy% intrinsic object and the initial value of the Proxy property of the global object. When called as a constructor it creates and initializes a new proxy exotic object. Proxy is not intended to be called as a function and will throw an exception when called in that manner.
The value of the [[Prototype]] internal slot of the Proxy constructor is the intrinsic object %FunctionPrototype%.
The Proxy constructor does not have a prototype property because proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization.
The Proxy constructor has the following properties:
The Proxy.revocable function is used to create a revocable Proxy object. When Proxy.revocable is called with arguments target and handler, the following steps are taken:
A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an ImportDeclaration that includes a NameSpaceImport.
In addition to the properties specified in 9.4.6 each Module Namespace Object has the following own properties:
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser based ECMAScript implementations. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.
The syntax and semantics of 11.4 is extended as follows except that this extension is not allowed when parsing source code using the goal symbol Module:
The syntax of 21.2.1 is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [U] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [U] parameter present on the goal symbol.
The escape function is a property of the global object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.
For those code units being replaced whose value is 0x00FF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code unit value is greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.
The escape function is the %escape% intrinsic object. When the escape function is called with one argument string, the following steps are taken:
Let char be the code unit (represented as a 16-bit unsigned integer) at index k within string.
If char is one of the code units in "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./", then
Let S be a String containing the single code unit char.
Else if char ≥ 256, then
Let S be a String containing six code units "%uwxyz" where wxyz are the code units of the four hexadecimal digits encoding the value of char.
Else, char < 256
Let S be a String containing three code units "%xy" where xy are the code units of two hexadecimal digits encoding the value of char.
Let R be a new String value computed by concatenating the previous value of R and S.
Increase k by 1.
Return R.
Note
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.
The unescape function is a property of the global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.
The unescape function is the %unescape% intrinsic object. When the unescape function is called with one argument string, the following steps are taken:
If k ≤ length-6 and the code unit at index k+1 within string is u and the four code units at indices k+2, k+3, k+4, and k+5 within string are all hexadecimal digits, then
Let c be the code unit whose value is the integer represented by the four hexadecimal digits at indices k+2, k+3, k+4, and k+5 within string.
Increase k by 5.
Else if k ≤ length-3 and the two code units at indices k+1 and k+2 within string are both hexadecimal digits, then
Let c be the code unit whose value is the integer represented by two zeroes plus the two hexadecimal digits at indices k+1 and k+2 within string.
Increase k by 2.
Let R be a new String value computed by concatenating the previous value of R and c.
Increase k by 1.
Return R.
B.2.2Additional Properties of the Object.prototype Object#
Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined as follows:
The substr method takes two arguments, start and length, and returns a substring of the result of converting the this object to a String, starting from index start and running for length code units (or through the end of the String if length is undefined). If start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:
If length is undefined, let end be +∞; otherwise let end be ? ToInteger(length).
Let size be the number of code units in S.
If intStart < 0, let intStart be max(size + intStart, 0).
Let resultLength be min(max(end, 0), size - intStart).
If resultLength ≤ 0, return the empty String "".
Return a String containing resultLength consecutive code units from S beginning with the code unit at index intStart.
Note
The substr function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.3.2.1Runtime Semantics: CreateHTML ( string, tag, attribute, value )#
The abstract operation CreateHTML is called with arguments string, tag, attribute, and value. The arguments tag and attribute must be String values. The following steps are taken:
Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence """.
Let p1 be the String value that is the concatenation of the following String values:
The String value of p1
Code unit 0x0020 (SPACE)
The String value of attribute
Code unit 0x003D (EQUALS SIGN)
Code unit 0x0022 (QUOTATION MARK)
The String value of escapedV
Code unit 0x0022 (QUOTATION MARK)
Let p2 be the String value that is the concatenation of p1 and ">".
Let p3 be the String value that is the concatenation of p2 and S.
Let p4 be the String value that is the concatenation of p3, "</", tag, and ">".
The property toUTCString is preferred. The toGMTString property is provided principally for compatibility with old code. It is recommended that the toUTCString property be used in new ECMAScript code.
The function object that is the initial value of Date.prototype.toGMTString is the same function object that is the initial value of Date.prototype.toUTCString.
B.2.5Additional Properties of the RegExp.prototype Object#
The compile method completely reinitializes the this object RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.
Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015, the grammar productions for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 13.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. For web browser compatibility, that rule is modified with the addition of the highlighted text:
It is a Syntax Error if any strict mode source code matches this rule.
B.3.3Block-Level Function Declarations Web Legacy Compatibility Semantics#
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an element of a Block statement's StatementList. However, support for that form of FunctionDeclaration was an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript code that uses Block level function declarations is only portable among browser implementation if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:
A function is declared and only referenced within a single block
A FunctionDeclaration whose BindingIdentifier is the name f occurs exactly once within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g
A function is declared and possibly used within a single Block but also referenced by an inner function definition that is not contained within that same Block.
A FunctionDeclaration whose BindingIdentifier is the name f occurs exactly once within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g
There is at least one occurrence of f as an IdentifierReference within another function h that is nested within g and no other declaration of f shadows the references to f from within h.
All invocations of h occur after the declaration of f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
A FunctionDeclaration whose BindingIdentifier is the name f occurs exactly once within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g
There is at least one occurrence of f as an IdentifierReference within the function code of g that lexically follows the Block containing the declaration of f.
The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript code that employs that use case will operate using the Block level function declarations semantics defined by clauses 9, 13, and 14 of this specification.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause 9, clause 14, clause 18.2.1 and clause 15.1.11 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains a FunctionDeclaration for which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create an early error, a warning message should not be produced.
B.3.3.1Changes to FunctionDeclarationInstantiation#
The above rules are only applied when parsing code that is not strict mode code. If any such code is match by one of these rules subsequent processing of that code takes places as if each matching occurrence of FunctionDeclaration[?Yield] was the sole StatementListItem of a BlockStatement occupying that position in the source code. The semantics of such a synthetic BlockStatement includes the web legacy compatibility semantics specified in B.3.3.
The Block of a Catch clause may contain var declarations that bind a name that is also bound by the CatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by the CatchParameter and hence the Initializer for such var declarations will assign to the corresponding catch parameter rather than the var binding. The relaxation of the normal static semantic rule does not apply to names only bound by for-of statements.
This modified behaviour also applies to var and function declarations introduced by direct eval calls contained within the Block of a Catch clause. This change is accomplished by modify the algorithm of 18.2.1.2 as follows:
Step 5.d.ii.2.a.i is replaced by:
If thisEnvRec is not the Environment Record for a Catch clause, throw a SyntaxError exception.
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the global object. When a simple assignment occurs within strict mode code, its LeftHandSideExpression must not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown (6.2.3.2). The LeftHandSideExpression also may not be a reference to a data property with the attribute value {[[Writable]]: false}, to an accessor property with the attribute value {[[Set]]: undefined}, nor to a non-existent property of an object whose [[Extensible]] internal slot has the value false. In these cases a TypeError exception is thrown (12.15).
Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw a TypeError exception on access (9.2.7).
Arguments objects for strict mode functions do not dynamically share their array indexed property values with the corresponding formal parameter bindings of their functions. (9.4.4).
For strict mode functions, if an arguments object is created the binding of the local identifier arguments to the arguments object is immutable and hence may not be the target of an assignment expression. (9.2.12).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval. Instead, a new variable environment is created and that environment is used for declaration binding instantiation for the eval code (18.2.1).
If this is evaluated within strict mode code, then the this value is not coerced to an object. A this value of null or undefined is not converted to the global object and primitive values are not converted to wrapper objects. The this value passed via a function call (including calls made using Function.prototype.apply and Function.prototype.call) do not coerce the passed this value to an object (9.2.1.2, 19.2.3.1, 19.2.3.3).
When a delete operator occurs within strict mode code, a SyntaxError is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name (12.5.3.1).
When a delete operator occurs within strict mode code, a TypeError is thrown if the property to be deleted has the attribute { [[Configurable]]: false } (12.5.3.2).
It is a SyntaxError if the same BindingIdentifier appears more than once in the FormalParameters of a strict mode function. An attempt to create such a function using a Function or Generator constructor is a SyntaxError (14.1.2, 19.2.1.1.1).
An implementation may not extend, beyond that defined in this specification, the meanings within strict mode functions of properties named caller or arguments of function instances. ECMAScript code may not create or modify properties with these names on function objects that correspond to strict mode functions (16.2).
DCorrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact#
8.1.1.4.15-8.1.1.4.18 Edition 5 and 5.1 used a property existence test to determine whether a global object property corresponding to a new global declaration already existed. ECMAScript 2015 uses an own property existence test. This corresponds to what has been most commonly implemented by web browsers.
9.4.2.1: The 5th Edition moved the capture of the current array length prior to the integer conversion of the array index or new length value. However, the captured length value could become invalid if the conversion process has the side-effect of changing the array length. ECMAScript 2015 specifies that the current array length must be captured after the possible occurrence of such side-effects.
20.3.1.15: Previous editions permitted the TimeClip abstract operation to return either +0 or -0 as the representation of a 0 time value. ECMAScript 2015 specifies that +0 always returned. This means that for ECMAScript 2015 the time value of a Date object is never observably -0 and methods that return time values never return -0.
20.3.1.16: If a time zone offset is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should be interpreted as "z".
20.3.4.36: If the year cannot be represented using the Date Time String Format specified in 20.3.1.16 a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.
20.3.4.41: Previous editions did not specify the value returned by Date.prototype.toString when this time value is NaN. ECMAScript 2015 specifies the result to be the String value is "Invalid Date".
21.2.3.1, 21.2.3.2.4: Any LineTerminator code points in the value of the source property of an RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the escaping of "/".
21.2.5.6, 21.2.5.8: In previous editions, the specifications for String.prototype.match and String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose global is flag set. The previous specifications stated that for each attempt to match the pattern, if lastIndex did not change it should be incremented by 1. The correct behaviour is that lastIndex should be incremented by one only if the pattern matched the empty string.
22.1.3.25, 22.1.3.25.1: Previous editions did not specify how a NaN value returned by a comparefn was interpreted by Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if +0 was returned from the comparefn. ECMAScript 2015 also specifies that ToNumber is applied to the result returned by a comparefn. In previous editions, the effect of a comparefn result that is not a Number value was implementation dependent. In practice, implementations call ToNumber.
EAdditions and Changes That Introduce Incompatibilities with Prior Editions#
6.2.3: In ECMAScript 2015, Function calls are not allowed to return a Reference value.
11.6: In ECMAScript 2015, the valid code points for an IdentifierName are specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid IdentifierName or Identifier code points were specified by enumerating various Unicode code point categories.
11.9.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is missing. This change aligns the specification with the actual behaviour of most existing implementations.
12.2.6.1: In ECMAScript 2015, it is no longer an early error to have duplicate property names in Object Initializers.
12.15.1: In ECMAScript 2015, strict mode code containing an assignment to an immutable binding such as the function name of a FunctionExpression does not produce an early error. Instead it produces a runtime error.
13.2: In ECMAScript 2015, a StatementList beginning with the token let followed by the input elements LineTerminator then Identifier is the start of a LexicalDeclaration. In previous editions, automatic semicolon insertion would always insert a semicolon before the Identifier input element.
13.6.7: In ECMAScript 2015, the normal completion value of an IfStatement is never the value empty. If no Statement part is evaluated or if the evaluated Statement part produces a normal completion whose value is empty, the completion value of the IfStatement is undefined.
13.7: In ECMAScript 2015, if the ( token of a for statement is immediately followed by the token sequence let [ then the let is treated as the start of a LexicalDeclaration. In previous editions such a token sequence would be the start of an Expression.
13.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence let [ then the let is treated as the start of a ForDeclaration. In previous editions such a token sequence would be the start of an LeftHandSideExpression.
13.7: Prior to ECMAScript 2015, an initialization expression could appear as part of the VariableDeclaration that precedes the in keyword. The value of that expression was always discarded. In ECMAScript 2015, the ForBinding in that same position does not allow the occurrence of such an initializer.
13.7: In ECMAScript 2015, the completion value of an IterationStatement is never the value empty. If the Statement part of an IterationStatement is not evaluated or if the final evaluation of the Statement part produces a completion whose value is empty, the completion value of the IterationStatement is undefined.
13.11.7: In ECMAScript 2015, the normal completion value of a WithStatement is never the value empty. If evaluation of the Statement part of a WithStatement produces a normal completion whose value is empty, the completion value of the WithStatement is undefined.
13.15: In ECMAScript 2015, it is an early error for a Catch clause to contain a var declaration for the same Identifier that appears as the Catch clause parameter. In previous editions, such a variable declaration would be instantiated in the enclosing variable environment but the declaration's Initializer value would be assigned to the Catch parameter.
13.15, 18.2.1.2: In ECMAScript 2015, a runtime SyntaxError is thrown if a Catch clause evaluates a non-strict direct eval whose eval code includes a var or FunctionDeclaration declaration that binds the same Identifier that appears as the Catch clause parameter.
13.15.8: In ECMAScript 2015, the completion value of a TryStatement is never the value empty. If the Block part of a TryStatement evaluates to a normal completion whose value is empty, the completion value of the TryStatement is undefined. If the Block part of a TryStatement evaluates to a throw completion and it has a Catch part that evaluates to a normal completion whose value is empty, the completion value of the TryStatement is undefined if there is no Finally clause or if its Finally clause evalulates to an empty normal completion.
14.3.9 In ECMAScript 2015, the function objects that are created as the values of the [[Get]] or [[Set]] attribute of accessor properties in an ObjectLiteral are not constructor functions and they do not have a prototype own property. In the previous edition, they were constructors and had a prototype property.
19.1.2.5: In ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.6: In ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.7: In ECMAScript 2015, if the argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.9: In ECMAScript 2015, if the argument to Object.getPrototypeOf is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.11: In ECMAScript 2015, if the argument to Object.isExtensible is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.12: In ECMAScript 2015, if the argument to Object.isFrozen is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.13: In ECMAScript 2015, if the argument to Object.isSealed is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.14: In ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.15: In ECMAScript 2015, if the argument to Object.preventExtensions is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.1.2.17: In ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
19.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to %FunctionPrototype%.
19.2.4.1: In ECMAScript 2015, the length property of function instances is configurable. In previous editions it was non-configurable.
19.5.6.2: In ECMAScript 2015, the [[Prototype]] internal slot of a NativeError constructor is the Error constructor. In previous editions it was the Function prototype object.
20.3.4 In ECMAScript 2015, the Date prototype object is not a Date instance. In previous editions it was a Date instance whose TimeValue was NaN.
21.1.3.10 In ECMAScript 2015, the String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.
21.1.3.22 and 21.1.3.24 In ECMAScript 2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion processing was only applied to individual code units. The only affected code points are those in the Deseret block of Unicode
21.1.3.25 In ECMAScript 2015, the String.prototype.trim method is defined to recognize white space code points that may exists outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been recognized as white space.
21.2.3.1 In ECMAScript 2015, If the pattern argument is a RegExp instance and the flags argument is not undefined, a new RegExp instance is created just like pattern except that pattern's flags are replaced by the argument flags. In previous editions a TypeError exception was thrown when pattern was a RegExp instance and flags was not undefined.
21.2.5 In ECMAScript 2015, the RegExp prototype object is not a RegExp instance. In previous editions it was a RegExp instance whose pattern is the empty string.
21.2.5 In ECMAScript 2015, source, global, ignoreCase, and multiline are accessor properties defined on the RegExp prototype object. In previous editions they were data properties defined on RegExp instances
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