Specifying JavaScript.

TC39

Ecma International's TC39 is a group of JavaScript developers, implementers, academics, and more, collaborating with the community to maintain and evolve the definition of JavaScript.

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TC39 welcomes contributions from the JavaScript community, whether it is feedback on existing proposals, improved documentation, testing, implementations, or even language feature ideas. See our contributor guide for details.

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Specs

We develop the JavaScript (formally, ECMAScript) specification on GitHub and meet every two months to discuss proposals. To learn more about the process, please take a look at the five stages for new language feature proposals. See our meeting agendas and minutes to learn more.

State of Proposals

Current Candidates for the Specification

This section features proposals that are in Stage 3 of our process, which means they are close to completion.

How to read the proposals list

Each proposal has links out to other resources. The tags identify the following attributes. Below you will find a table with example tags and their meanings. All tags have hover text if you need help getting oriented.

Types of tags and what they mean

Last Presented:
December 2018
Available Tests:
Tests
Specification Text:
Specification

Legacy RegExp features in JavaScript

Author: Claude Pache | Champions: Mark Miller, Claude Pache

This is a specification draft for the legacy (deprecated) RegExp features in JavaScript, i.e., static properties of the constructor like RegExp.$1 as well as the RegExp.prototype.compile method.

Private instance methods and accessors

Author: Daniel Ehrenberg | Champions: Daniel Ehrenberg, Kevin Gibbons

Keeping state and behavior private to a class lets library authors present a clear, stable interface, while changing their code over time behind the scenes.

Show Example

class Counter extends HTMLElement {
  #xValue = 0;
  get #x() { return #xValue; }
  set #x(value) {
    this.#xValue = value;
    window.requestAnimationFrame(this.#render.bind(this));
  }
  #clicked() {
    this.#x++;
  }
  constructor() {
    super();
    this.onclick = this.#clicked.bind(this);
  }
  connectedCallback() { this.#render(); }
  #render() {
    this.textContent = this.#x.toString();
  }
}
window.customElements.define('num-counter', Counter);

Class Public Instance Fields & Private Instance Fields

Authors: Daniel Ehrenberg, Kevin Gibbons | Champions: Daniel Ehrenberg, Jeff Morrison, Kevin Smith, Kevin Gibbons

This proposes a combined vision for public fields and private fields, drawing on the earlier Orthogonal Classes and Class Evaluation Order proposals.

Show Example

class X {
  #foo;
  method() {
    console.log(this.#foo)
  }
}

Static class fields and private static methods

Authors: Daniel Ehrenberg, Kevin Gibbons, Jeff Morrison, Kevin Smith | Champions: Shu-Yu Guo, Daniel Ehrenberg

A proposal to add three features to JavaScript classes, building on the previous class fields and private methods proposals.

Show Example

class ColorFinder {
  static #red = "#ff0000";
  static #green = "#00ff00";
  static #blue = "#0000ff";

  static colorName(name) {
    switch (name) {
      case "red": return ColorFinder.#red;
      case "blue": return ColorFinder.#blue;
      case "green": return ColorFinder.#green;
      default: throw new RangeError("unknown color");
    }
  }

  // Somehow use colorName
}

Hashbang Grammar

Authors and Champions: Bradley Farias

This proposal is to match de-facto usage in some CLI JS hosts that allow for Shebangs / Hashbang. Such hosts strip the hashbang in order to generate valid JS source texts before passing to JS engines currently. This would unify and standardize how that is done.

Show Example

#!/usr/bin/env node
// in the Module Goal
export {};
console.log(1);

Top-level `await`

Authors and Champions: Myles Borins

Top-level await enables modules to act as big async functions. With top-level await, ECMAScript Modules (ESM) can await resources, causing other modules who import them to wait before they start evaluating their body.

Show Example

// file.html
<script type=module src="a.mjs"></script>
// a.mjs
await import("./b.mjs");
// b.mjs
await import("./a.mjs");

RegExp Match Indices

Authors and Champions: Ron Buckton

ECMAScript RegExp Match Indicies provide additional information about the start and end indices of captured substrings relative to the start of the input string.

Show Example

const re1 = /a+(?<Z>z)?/;
// indices are relative to start of the input string:
const s1 = "xaaaz";
const m1 = re1.exec(s1);
m1.indices[0][0] === 1;
m1.indices[0][1] === 5;
s1.slice(...m1.indices[0]) === "aaaz";
m1.indices[1][0] === 4;
m1.indices[1][1] === 5;
s1.slice(...m1.indices[1]) === "z";
m1.indices.groups["Z"][0] === 4;
m1.indices.groups["Z"][1] === 5;
s1.slice(...m1.indices.groups["Z"]) === "z";
// capture groups that are not matched return `undefined`:
const m2 = re1.exec("xaaay");
m2.indices[1] === undefined;
m2.indices.groups["Z"] === undefined;

`Atomics.waitAsync`

Author: Lars Hansen | Champions: Shu-yu Guo, Lars Hansen

A proposal for an “asynchronous atomic wait” for ECMAScript, primarily for use in agents that are not allowed to block.

`.item()`

Authors and Champions: Shu-yu Guo, Tab Atkins

A proposal to add a .item() method to all the basic indexable classes (Array, String, TypedArray).

Show Example

function item(n) {
	// ToInteger() abstract op
	n = Math.trunc(n) || 0;
	// Allow negative indexing from the end
	if(n < 0) n += this.length;
	// OOB access is guaranteed to return undefined
	if(n < 0 || n >= this.length) return undefined;
	// Otherwise, this is just normal property access
	return this[n];
}
// Other TypedArray constructors omitted for brevity.
for (let C of [Array, String, Uint8Array]) {
    Object.defineProperty(C.prototype, "item",
                          { value: item,
                            writable: true,
                            enumerable: false,
                            configurable: true });
}

Import Assertions

Authors and Champions: Myles Borins, Sven Sauleau, Dan Clark, Daniel Ehrenberg

A proposal for syntax to import ES modules with assertions.

Show Example

new Worker("foo.wasm", { type: "module", assert: { type: "webassembly" } });

See proposals in all stages