Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this edition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplementary characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to `Array.prototype` called `includes`.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a new memory model that allows multi-agent programs to communicate using atomic operations that ensure a well-defined execution order even on parallel CPUs. It also included new static methods on Object: `Object.values`, `Object.entries`, and `Object.getOwnPropertyDescriptors`.
-ECMAScript 2018 introduced support for asynchronous iteration via the AsyncIterator protocol and async generators. It also included four new regular expression features: the `dotAll` flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.
+ECMAScript 2018 introduced support for asynchronous iteration via the async iterator protocol and async generators. It also included four new regular expression features: the `dotAll` flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.
ECMAScript 2019 introduced a few new built-in functions: `flat` and `flatMap` on `Array.prototype` for flattening arrays, `Object.fromEntries` for directly turning the return value of `Object.entries` into a new Object, and `trimStart` and `trimEnd` on `String.prototype` as better-named alternatives to the widely implemented but non-standard `String.prototype.trimLeft` and `trimRight` built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other updates included requiring that `Array.prototype.sort` be a stable sort, requiring that `JSON.stringify` return well-formed UTF-8 regardless of input, and clarifying `Function.prototype.toString` by requiring that it either return the corresponding original source text or a standard placeholder.
ECMAScript 2020, the 11th edition, introduced the `matchAll` method for Strings, to produce an iterator for all match objects generated by a global regular expression; `import()`, a syntax to asynchronously import Modules with a dynamic specifier; `BigInt`, a new number primitive for working with arbitrary precision integers; `Promise.allSettled`, a new Promise combinator that does not short-circuit; `globalThis`, a universal way to access the global `this` value; dedicated `export * as ns from 'module'` syntax for use within modules; increased standardization of `for-in` enumeration order; `import.meta`, a host-populated object available in Modules that may contain contextual information about the Module; as well as adding two new syntax features to improve working with “nullish” values (*undefined* or *null*): nullish coalescing, a value selection operator; and optional chaining, a property access and function invocation operator that short-circuits if the value to access/invoke is nullish.
ECMAScript 2021, the 12th edition, introduced the `replaceAll` method for Strings; `Promise.any`, a Promise combinator that short-circuits when an input value is fulfilled; `AggregateError`, a new Error type to represent multiple errors at once; logical assignment operators (`??=`, `&&=`, `||=`); `WeakRef`, for referring to a target object without preserving it from garbage collection, and `FinalizationRegistry`, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (`1_000`); and `Array.prototype.sort` was made more precise, reducing the amount of cases that result in an implementation-defined sort order.
@@ -1230,7 +1230,7 @@Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (
Within this specification a well-known symbol is referred to using the standard
| - A method that returns the default AsyncIterator for an object. Called by the semantics of the `for`-`await`-`of` statement. + A method that returns the default async iterator for an object. Called by the semantics of the `for`-`await`-`of` statement. | |
| - A method that returns the default Iterator for an object. Called by the semantics of the for-of statement. + A method that returns the default iterator for an object. Called by the semantics of the for-of statement. | |
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- A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by the |
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- The prototype of Array iterator objects ( |
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- The prototype of Iterator Helper Objects ( |
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- The prototype of Map iterator objects ( |
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- The prototype of Set iterator objects ( |
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- The prototype of String iterator objects ( |
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- The prototype of wrapped Iterator objects returned by Iterator.from ( |
An Iterator Record is a Record value used to encapsulate an Iterator or AsyncIterator along with the `next` method.
+An Iterator Record is a Record value used to encapsulate an iterator or async iterator along with the `next` method.
Iterator Records have the fields listed in
| - An object that conforms to the Iterator or AsyncIterator interface. + An object that conforms to the iterator interface or the async iterator interface. | ||||||||||||||||||||||||
| - a function that returns an Iterator object + a function that returns an iterator object | - The returned object must conform to the Iterator interface. + The returned object must conform to the iterator interface. |
An object that implements the Iterator interface must include the property in
An object that implements the iterator interface must include the property in
| - a function that returns an IteratorResult object + a function that returns an IteratorResult object | - 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. + 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. |
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.
+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 + 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 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. + 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. |
| - a function that returns an IteratorResult object + 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*. + 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*. |
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.
+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 AsyncIterable interface includes the properties described in
The async iterable interface includes the properties described in
| `%Symbol.asyncIterator%` | -a function that returns an AsyncIterator object | -The returned object must conform to the AsyncIterator interface. | +a function that returns an async iterator object | +The returned object must conform to the async iterator interface. |
An object that implements the AsyncIterator interface must include the properties in
An object that implements the async iterator interface must include the properties in
| *"next"* | -a function that returns a promise for an IteratorResult object | +a function that returns a promise for an IteratorResult object |
- The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. If a previous call to the `next` method of an AsyncIterator has returned a promise for an IteratorResult object whose *"done"* property is *true*, then all subsequent calls to the `next` method of that object should also return a promise for an IteratorResult object whose *"done"* property is *true*. However, this requirement is not enforced. +The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. If a previous call to the `next` method of an async iterator has returned a promise for an IteratorResult object whose *"done"* property is *true*, then all subsequent calls to the `next` method of that object should also return a promise for an IteratorResult object whose *"done"* property is *true*. However, this requirement is not enforced. -Additionally, the IteratorResult object that serves as a fulfillment value should have a *"value"* property whose value is not a promise (or "thenable"). However, this requirement is also not enforced. +Additionally, the IteratorResult object that serves as a fulfillment value should have a *"value"* property whose value is not a promise (or "thenable"). However, this requirement is also not enforced. |
Arguments may be passed to the `next` function but their interpretation and validity is dependent upon the target AsyncIterator. The `for`-`await`-`of` statement and other common users of AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
+Arguments may be passed to the `next` function but their interpretation and validity is dependent upon the target async iterator. The `for`-`await`-`of` statement and other common users of AsyncIterators do not pass any arguments, so async iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
| *"return"* | -a function that returns a promise for an IteratorResult object | +a function that returns a promise for an IteratorResult object |
- The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller does not intend to make any more `next` method calls to the AsyncIterator. The returned promise will fulfill with an IteratorResult object which 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. +The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the async iterator object that the caller does not intend to make any more `next` method calls to the async iterator. The returned promise will fulfill with an IteratorResult object which 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. -Additionally, the IteratorResult object that serves as a fulfillment value should have a *"value"* property whose value is not a promise (or "thenable"). If the argument value is used in the typical manner, then if it is a rejected promise, a promise rejected with the same reason should be returned; if it is a fulfilled promise, then its fulfillment value should be used as the *"value"* property of the returned promise's IteratorResult object fulfillment value. However, these requirements are also not enforced. +Additionally, the IteratorResult object that serves as a fulfillment value should have a *"value"* property whose value is not a promise (or "thenable"). If the argument value is used in the typical manner, then if it is a rejected promise, a promise rejected with the same reason should be returned; if it is a fulfilled promise, then its fulfillment value should be used as the *"value"* property of the returned promise's IteratorResult object fulfillment value. However, these requirements are also not enforced. |
| *"throw"* | -a function that returns a promise for an IteratorResult object | +a function that returns a promise for an IteratorResult object |
- The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator 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 return a rejected promise which rejects with the value passed as the argument. +The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the async 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 return a rejected promise which rejects with the value passed as the argument. -If the returned promise is fulfilled, the IteratorResult fulfillment value will typically have a *"done"* property whose value is *true*. Additionally, it should have a *"value"* property whose value is not a promise (or "thenable"), but this requirement is not enforced. +If the returned promise is fulfilled, the IteratorResult object fulfillment value will typically have a *"done"* property whose value is *true*. Additionally, it should have a *"value"* property whose value is not a promise (or "thenable"), but this requirement is not enforced. |
The IteratorResult interface includes the properties listed in
The IteratorResult interface includes the properties listed in