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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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:
Test
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 Foo {
    instancePropertyBar = 0;
    static staticPropertyBar = 0;
    #privatePropertyBar = 0;
    static #privatePropertyBar = 0;

    set foo(value) { console.log(value); }
}

Static class fields and private static methods

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

This proposal adds Static public fields, Static private methods and Static private fields

Show Example

class ColorFinder {
  static #red = "#ff0000";

  static #blue = "#00ff00";

  static #green = "#0000ff";

  // ^ static class fields

  static white = "white";

  // ^ static public field.

  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");
    }
  }

  // Static method ^

  static #setColor(name) {

  }
  // ^ Static private method
}

Hashbang Grammar

Author and Champion: 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 Script Goal
'use strict';
console.log(1);

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

Top level await

Author and Champion: Myles Borins

Top-level await lets us rely on the module system itself to handle all of these promises, and make sure that things are well-coordinated.

Show Example

// awaiting.mjs
import { process } from "./some-module.mjs";
const dynamic = import(computedModuleSpecifier);
const data = fetch(url);
export const output = process((await dynamic).default, await data);
// usage.mjs
import { output } from "./awaiting.mjs";
export function outputPlusValue(value) { return output + value }
console.log(outputPlusValue(100));
setTimeout(() => console.log(outputPlusValue(100), 1000);

RegExp Match Indices

Author and Champion: 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)?/;
// 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.

Show Example

var sab = new SharedArrayBuffer(4096);
var ia = new Int32Array(sab);
ia[37] = 0x1337;
test1();

function test1() {

  Atomics.waitAsync(ia, 37, 0x1337, 1000).then(function (r) { log("Resolved: " + r); test2(); });

}

var code = `
var ia = null;
onmessage = function (ev) {

  if (!ia) {

    postMessage("Aux worker is running");

    ia = new Int32Array(ev.data);

  }

  postMessage("Aux worker is sleeping for a little bit");

  setTimeout(function () { postMessage("Aux worker is waking"); Atomics.notify(ia, 37); }, 1000);

}`;

function test2() {

  var w = new Worker("data:application/javascript," + encodeURIComponent(code));

  w.onmessage = function (ev) { log(ev.data) };

  w.postMessage(sab);

  Atomics.waitAsync(ia, 37, 0x1337).then(function (r) { log("Resolved: " + r); test3(w); });

}

function test3(w) {

  w.postMessage(false);

  Atomics.waitAsync(ia, 37, 0x1337).then(function (r) { log("Resolved 1: " + r); });

  Atomics.waitAsync(ia, 37, 0x1337).then(function (r) { log("Resolved 2: " + r); });

  Atomics.waitAsync(ia, 37, 0x1337).then(function (r) { log("Resolved 3: " + r); });

}

function log(msg) {

  document.getElementById("scrool").innerHTML += String(msg) + "\n";

}

See proposals in all stages

Aug	SEP	Oct
	28
2019	2020	2021