ECMAScript

There are eleven editions of ECMA-262 published. Work on version 11 of the standard was finalized in June 2020.[10]

In June 2004, Ecma International published ECMA-357 standard, defining an extension to ECMAScript, known as ECMAScript for XML (E4X). Ecma also defined a "Compact Profile" for ECMAScript – known as ES-CP, or ECMA 327 – that was designed for resource-constrained devices, which was withdrawn in 2015.[17]

The proposed fourth edition of ECMA-262 (ECMAScript 4 or ES4) would have been the first major update to ECMAScript since the third edition was published in 1999. The specification (along with a reference implementation) was originally targeted for completion by October 2008.[18] The first draft was dated February 1999.[19] An overview of the language was released by the working group on October 23, 2007.[20]

By August 2008, the ECMAScript 4th edition proposal had been scaled back into a project codenamed ECMAScript Harmony. Features under discussion for Harmony at the time included:

• classes,
• a module system,
• optional type annotations and static typing, probably using a structural type system,
• generators and iterators,
• destructuring assignment, and
• algebraic data types.

The intent of these features was partly to better support programming in the large, and to allow sacrificing some of the script's ability to be dynamic to improve performance. For example, Tamarin – the virtual machine for ActionScript, developed and open-sourced by Adobe – has just-in-time compilation (JIT) support for certain classes of scripts.

In addition to introducing new features, some ES3 bugs were proposed to be fixed in edition 4.[21][22] These fixes and others, and support for JSON encoding/decoding, have been folded into the ECMAScript, 5th Edition specification.[23]

Work started on Edition 4 after the ES-CP (Compact Profile) specification was completed, and continued for approximately 18 months where slow progress was made balancing the theory of Netscape's JavaScript 2 specification with the implementation experience of Microsoft's JScript .NET. After some time, the focus shifted to the ECMAScript for XML (E4X) standard. The update has not been without controversy. In late 2007, a debate between Eich, later the Mozilla Foundation's CTO, and Chris Wilson, Microsoft's platform architect for Internet Explorer, became public on a number of blogs. Wilson cautioned that because the proposed changes to ECMAScript made it backwards incompatible in some respects to earlier versions of the language, the update amounted to "breaking the Web,"[24] and that stakeholders who opposed the changes were being "hidden from view".[25] Eich responded by stating that Wilson seemed to be "repeating falsehoods in blogs" and denied that there was attempt to suppress dissent and challenged critics to give specific examples of incompatibility.[26] He pointed out that Microsoft Silverlight and Adobe AIR rely on C# and ActionScript 3 respectively, both of which are larger and more complex than ECMAScript Edition 3.[27]

Yahoo, Microsoft, Google, and other 4th edition dissenters formed their own subcommittee to design a less ambitious update of ECMAScript 3, tentatively named ECMAScript 3.1. This edition would focus on security and library updates, with a large emphasis on compatibility. After the aforementioned public sparring, the ECMAScript 3.1 and ECMAScript 4 teams agreed on a compromise: the two editions would be worked on, in parallel, with coordination between the teams to ensure that ECMAScript 3.1 remains a strict subset of ECMAScript 4 in both semantics and syntax.

However, the differing philosophies in each team resulted in repeated breakages of the subset rule, and it remained doubtful that the ECMAScript 4 dissenters would ever support or implement ECMAScript 4 in the future. After over a year since the disagreement over the future of ECMAScript within the Ecma Technical Committee 39, the two teams reached a new compromise in July 2008: Brendan Eich announced that Ecma TC39 would focus work on the ECMAScript 3.1 (later renamed to ECMAScript, 5th Edition) project with full collaboration of all parties, and vendors would target at least two interoperable implementations by early 2009.[28][29] In April 2009, Ecma TC39 published the "final" draft of the 5th edition and announced that testing of interoperable implementations was expected to be completed by mid-July.[30] On December 3, 2009, ECMA-262 5th edition was published.[31]

The 6th edition, initially known as ECMAScript 6 (ES6) then and later renamed to ECMAScript 2015, was finalized in June 2015.[12][32] This update adds significant new syntax for writing complex applications, including class declarations (class Foo { ... }), ES6 modules like import * as moduleName from "..."; export const Foo, but defines them semantically in the same terms as ECMAScript 5 strict mode. Other new features include iterators and for...of loops, Python-style generators, arrow function expression (() => {...}), let keyword for local declarations, const keyword for constant local declarations, binary data, typed arrays, new collections (maps, sets and WeakMap), promises, number and math enhancements, reflection, proxies (metaprogramming for virtual objects and wrappers) and template literals for strings.[33][34] The complete list is extensive.[35][36] As the first "ECMAScript Harmony" specification, it is also known as "ES6 Harmony."

The 7th edition, officially known as ECMAScript 2016, was finalized in June 2016.[13] Its features include block-scoping of variables and functions, destructuring patterns (of variables), proper tail calls, exponentiation operator ** for numbers, await, async keywords for asynchronous programming, and the Array.prototype.includes function.[13][37]

The exponentiation operator is equivalent to Math.pow, but provides a simpler syntax similar to languages like Python, F#, Perl, and Ruby.async /await was hailed as an easier way to use promises and develop asynchronous code.

The 8th edition, officially known as ECMAScript 2017, was finalized in June 2017.[14] Its features include the Object.values, Object.entries and Object.getOwnPropertyDescriptors functions for easy manipulation of Objects, async/await constructions which use generators and promises, and additional features for concurrency and atomics.[38][14]

The 9th edition, officially known as ECMAScript 2018, was finalized in June 2018.[15] New features include rest/spread operators for object literals (three dots: ...identifier), asynchronous iteration, Promise.prototype.finally and additions to RegExp.[15]

The spread operator allows for the easy copying of object properties, as shown below.

let object = {a: 1, b: 2}

let objectClone = Object.assign({}, object) // before ES9
let objectClone = {...object} // ES9 syntax

let otherObject = {c: 3, ...object}
console.log(otherObject) // -> {c: 3, a: 1, b: 2}

The 10th edition, officially known as ECMAScript 2019, was published in June 2019.[16] Added features include, but are not limited to, Array.prototype.flat, Array.prototype.flatMap, changes to Array.sort and Object.fromEntries.[16]

Array.sort is now guaranteed to be stable, meaning that elements with the same sorting precedence will appear in the same order in the sorted array. Array.prototype.flat(depth=1) flattens an array to a specified depth, meaning that all subarray elements (up to the specified depth) are concatenated recursively.

The 11th edition, officially known as ECMAScript 2020, was published in June 2020.[10] In addition to new functions, this version includes a BigInt primitive type for arbitrary-sized integers, the nullish coalescing operator, and the globalThis object.[10]

BigInts are created either with the BigInt constructor or with the syntax 10n, where "n" is placed after the number literal. BigInts allow the representation and manipulation of integers beyond Number.MAX_SAFE_INTEGER, while Numbers are represented by a double-precision 64-bit IEEE 754 value. The built-in functions in Math are not compatible with BigInts; for example, exponentiation of BigInts must be done with the ** operator instead of Math.pow.

The nullish coalescing operator, ??, returns its right-hand side operand when its left-hand side is null or undefined. This contrasts with the || operator, which would return "string" for all the examples.

false ?? "string" // -> false
NaN ?? "string" // -> NaN
undefined ?? "string" // -> "string"

Optional chaining allows you to access for the nested nodes in an object without having a AND check at each level.

An example is const zipcode = person?.address?.zipcode. If any of the properties are not present, zipcode will be undefined.

ES.Next is a dynamic name that refers to whatever the next version is at the time of writing. ES.Next features are finished proposals (aka "stage 4 proposals") as listed at finished proposal that are not part of a ratified specification. The language committee follows a "living spec" model, so these changes are part of the standard, and ratification is a formality.

ECMAScript JavaScript supports C style structured programming. However, there exist some dissimilarities between both languages implementation of scoping. Until ECMAScript 2015, JavaScript supported only function scoping using the keyword var. ECMAScript 2015 added the keywords let and const allowing JavaScript to support both block scoping as well as function scoping. JavaScript supports automatic semicolon insertion, meaning that semicolons that are normally used to terminate a statement in C may be omitted in JavaScript.[40]

Like C-style languages, control flow is done with the while, for, do/while, if/else, and switch statements. Functions are weakly typed and may accept and return any type. Arguments not provided default to undefined.

ECMAScript is weakly typed. This means that certain types are assigned implicitly based on the operation being performed. However, there are several quirks in JavaScript's implementation of the conversion of a variable from one type to another. These quirks have drawn criticism from many developers.

ECMAScript is dynamically typed. Thus, a type is associated with a value rather than an expression. ECMAScript supports various ways to test the type of objects, including duck typing.[41]

Since ES 2015, transpiling JavaScript has become very common. Transpilation is a source-to-source compilation in which the newer versions of JavaScript are used in the user's source code, and the transpiler rewrites them so that they are compliant with the current specification. Usually, transpilers transpile down to ES3 to maintain compatibility with all versions of browsers. The settings to transpiling to a specific version can be configured according to need. Transpiling adds an extra step to the build process and is sometimes done to avoid needing polyfills. Polyfills allow using functionalities from newer ECMA versions in older environments that lack them. Polyfills do this at runtime in the interpreter, such as the user's browser or on the server. Instead, transpiling rewrites the ECMA code itself during the build phase of development before it reaches the interpreter.

• ISO 16262

• ECMA-262
  • ECMA-262 ECMAScript Language Specification 3rd edition (December 1999)
  • ECMAScript Language Specification, Edition 3 Final, 24-Mar-00
  • 4th Edition (overview): PDF
  • 4th Edition (final draft): HTML, PDF
  • ECMA-262 ECMAScript Language Specification 5th edition (December 2009)
  • 5.1 Edition / June 2011: HTML, PDF
  • 6th Edition / June 2015 (ECMAScript 2015 Language Specification): HTML, PDF
  • 7th Edition / June 2016 (ECMAScript 2016 Language Specification): HTML, PDF
  • 8th edition, June 2017 (ECMAScript 2017 Language Specification): HTML, PDF
  • 9th edition, June 2018 (ECMAScript 2018 Language Specification): HTML, PDF
  • 10th edition, June 2019 (ECMAScript 2019 Language Specification): HTML, PDF
• ECMA-290 ECMAScript Components Specification (June 1999)
• ECMA-327 ECMAScript 3rd Edition Compact Profile (June 2001)
• ECMA-357 ECMAScript for XML (E4X) Specification (June 2004)