. var el = header; while (el && el.tagName !== 'HR') { var nextEl = el.nextElementSibling; container.append(el); el = nextEl; } // Add handler to open accordion on click. header.addEventListener('click', function() { header.parentNode.classList.toggle('open'); }); }); // Then remove all
s since everything's accordionized. document.querySelectorAll('hr').forEach(function(el) { el.parentNode.removeChild(el); }); }); </script>
[TOC]
This is a list of ECMAScript6 features allowed in Chromium code.
This is not a status list of v8's support for language features.
TBD: Do we need to differentiate per-project?
TBD: Cross-platform build support? As in: transpilers?
You can propose changing the status of a feature by sending an email to [email protected]. Include a short blurb on what the feature is and why you think it should or should not be allowed, along with links to any relevant previous discussion. If the list arrives at some consensus, send a codereview to change this file accordingly, linking to your discussion thread.
Some descriptions and Usage examples are from kangax and http://es6-features.org/
The following features are allowed in Chromium development.
Arrow functions provide a concise syntax to create a function, and fix a number
of difficulties with this (e.g. eliminating the need to write const self = this). Particularly useful for nested functions or callbacks.
Prefer arrow functions over .bind(this).
Arrow functions have an implicit return when used without a body block.
Usage Example:
// General usage, eliminating need for .bind(this).
setTimeout(() => {
this.doSomething();
}, 1000); // no need for .bind(this) or const self = this.
// Another example...
window.addEventListener('scroll', (event) => {
this.doSomething(event);
}); // no need for .bind(this) or const self = this.
// Implicit return: returns the value if expression not inside a body block.
() => 1 // returns 1.
() => {1} // returns undefined - body block does not implicitly return.
() => {return 1;} // returns 1.Documentation: link
Discussion Notes / Link to Thread:
Note: => does not work in iOS9. Don't use it in code that runs on Chrome for iOS. There's a presubmit that should warn you about this.
The Promise object is used for asynchronous computations. A Promise represents a value which may be available now, or in the future, or never.
Usage Example:
/** @type {!Promise} */
var fullyLoaded = new Promise(function(resolve) {
function isLoaded() { return document.readyState == 'complete'; }
if (isLoaded()) {
resolve();
} else {
document.onreadystatechange = function() {
if (isLoaded()) resolve();
};
}
});
// ... some time later ...
fullyLoaded.then(startTheApp).then(maybeShowFirstRun);Discussion Notes: Feature already extensively used prior to creation of this document.
OOP-style and boilerplate-free class syntax, including inheritance, super(),
static members, and getters and setters.
Usage Example:
class Shape {
constructor(x, y) {
this.x = x;
this.y = y;
}
}
// Note: let Shape = class {...}; is also valid.
class Rectangle extends Shape {
constructor(x, y, width, height) {
super(id, x, y);
this.width = width;
this.height = height;
}
static goldenRectangle() {
var PHI = (1 + Math.sqrt(5)) / 2;
return new Rectangle(0, 0, PHI, 1);
}
}Documentation: link
Discussion Notes / Link to Thread: https://groups.google.com/a/chromium.org/d/msg/chromium-dev/S1h-0m2ohOw/jyaiMGDlCwAJ
Note: Not fully supported in iOS9. Don't use it in code that runs on Chrome for iOS, unless you can verify it works. TODO: Remove this note once support for iOS9 is dropped.
A simple key/value map in which any value (both objects and primitive values) may be used as either a key or a value.
Usage Example:
var map = new Map();
map.size === 0; // true
map.get('foo'); // undefined
var key = 54;
map.set(key, 123);
map.size === 1; // true
map.has(key); // true
map.get(key); // 123
map.delete(key);
map.has(key); // false
map.size === 0; // trueDiscussion Notes: Feature already extensively used prior to creation of this document.
An object that lets you store unique values of any type, whether primitive values or object references.
Usage Example:
var set = new Set();
set.add(123);
set.size(); // 1
set.has(123); // true
set.add(123);
set.size(); // 1Discussion Notes: Feature already extensively used prior to creation of this document.
let declares a variable within the scope of a block. This differs from var,
which uses function level scope.
Usage Example:
// Scope.
function varTest() {
var x = 1;
if (true) {
var x = 2; // Same variable!
console.log(x); // 2
}
console.log(x); // 2
}
function letTest() {
let x = 1;
if (true) {
let x = 2; // Different variable.
console.log(x); // 2
}
console.log(x); // 1
}
// Redeclaration throws.
function f() {
var a = 'hello';
var a = 'hola'; // No error!
let b = 'world';
let b = 'mundo; // TypeError Identifier 'b' has already been declared.
}Documentation: link
Discussion Notes / Link to Thread: link
Note: let is not fully supported in iOS9.
Don't use it in code that runs on Chrome for iOS, until support for iOS9 is
dropped.
Constants (also known as "immutable variables") are variables which cannot be re-assigned new content. Note that if the value is an object, the object itself is still mutable.
Also note that in Chrome, const is block scoped like let.
Usage Example:
const gravity = 9.81;
gravity = 0; // TypeError: Assignment to constant variable.
gravity === 9.81; // true
const frobber = {isFrobbing: true};
frobber = {isFrobbing: false}; // TypeError: Assignment to constant variable.
frobber.isFrobbing = false; // Works.Documentation: link
See also: Object.freeze()
Discussion Notes / Link to Thread: link
Note: const not fully supported in iOS9.
Don't use it in code that runs on Chrome for iOS, until support for iOS9 is
dropped.
Usage Example:
// Static methods
let a1 = Array.from(document.querySelectorAll('div'));
let a2 = Array.of(7);
// Prototype methods
['a', 'b', 'c', 'd'].copyWithin(2, 0); // Returns ['a', 'b', 'a', 'b']
[2, 4, 6, 8].find(i => i == 6); // Returns 6
[2, 4, 6, 8].findIndex(i => i == 6); // Returns 2
[2, 4, 6, 8].fill(1); // Returns [1, 1, 1, 1]
[2, 4, 6, 8].keys(); // Returns an Array iterator
[2, 4, 6, 8].entries(); // Returns an Array iteratorDocumentation: link
Discussion Notes / Link to Thread: link
Note: Array.prototype.values is not implemented in Chrome and should not be used. If the code in question is Closure compiled, a compile-time error will be thrown.
Usage Example:
// Number.isFinite
// Number.isInteger
// Number.isSafeInteger
// Number.isNaN
// Number.EPSILON
// Number.MIN_SAFE_INTEGER
// Number.MAX_SAFE_INTEGERDocumentation: link
Discussion Notes / Link to Thread: link
Usage Example:
// Object.assign
var o = Object.assign({a:true}, {b:true}, {c:true}); // {a: true, b: true, c: true}
'a' in o && 'b' in o && 'c' in o; // true
// Object.setPrototypeOf
Object.setPrototypeOf({}, Array.prototype) instanceof Array; // true
// Object.is
Object.is(null, null) // true
Object.is(NaN, NaN) // true
Object.is(-0, +0) // false, btw: -0 === +0 is trueDocumentation: link
Discussion Notes / Link to Thread: link
Convenient operator to iterate over all values in an iterable collection. This
differs from for ...in, which iterates over all enumerable properties of an
object.
Usage Example:
// Given an iterable collection of Fibonacci numbers...
for (let n of fibonacci) {
console.log(n); // 1, 1, 2, 3, ...
}Discussion Notes / Link to Thread: link
Expression interpolation for Strings, with the ability to access raw template pieces.
Usage Example:
// Simple example
let greeting = 'hello';
let myName = {first: 'Foo', last: 'Bar'};
let from = 1900;
let to = 2000;
var message = `${greeting}, I am ${myName.first}${myName.last},
and I am ${to - from} years old`;
// message == 'hello,\nI am FooBar,\nand I am 100 years old'Documentation: link
Discussion Notes / Link to Thread:
The following features are banned for Chromium development.
The following features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.
Usage Example:
{
function foo() {
return 1;
}
// foo() === 1
{
function foo() {
return 2;
}
// foo() === 2
}
// foo() === 1
}Documentation: link
Discussion Notes / Link to Thread:
Initialize parameters with default values if no value or undefined is passed.
Usage Example:
/**
* @param {!Element} element An element to hide.
* @param {boolean=} animate Whether to animatedly hide |element|.
*/
function hide(element, animate=true) {
function setHidden() { element.hidden = true; }
if (animate)
element.animate({...}).then(setHidden);
else
setHidden();
}
hide(document.body); // Animated, animate=true by default.
hide(document.body, false); // Not animated.Discussion Notes / Link to Thread:
Aggregation of function arguments into one Array variable.
Usage Example:
function usesRestParams(a, b, ...theRest) {
console.log(a); // 'a'
console.log(b); // 'b'
console.log(theRest); // [1, 2, 3]
}
usesRestParams('a', 'b', 1, 2, 3);Documentation: link
Discussion Notes / Link to Thread:
Spreading the elements from an iterable collection into individual literals as function parameters.
Usage Example:
// Spreading an Array
var params = ['hello', true, 7];
var other = [1, 2, ...params]; // [1, 2, 'hello', true, 7]
// Spreading a String
var str = 'foo';
var chars = [...str]; // ['f', 'o', 'o']Documentation: link
Discussion Notes / Link to Thread:
Convenient new ways for object property definition.
Usage Example:
// Computed property name
var prop = 'foo';
var o = {
[prop]: 'hey',
['b' + 'ar']: 'there',
};
console.log(o); // {foo: 'hey', bar: 'there'}
// Shorthand property
var foo = 1;
var bar = 2;
var o = {foo, bar};
console.log(o); // {foo: 1, bar: 2}
// Method property
var clearSky = {
// Basically the same as clouds: function() { return 0; }.
clouds() { return 0; },
color() { return 'blue'; },
};
console.log(clearSky.color()); // 'blue'
console.log(clearSky.clouds()); // 0Discussion Notes / Link to Thread:
Usage Example:
0b111110111 === 503;
0o767 === 503;Documentation: link
Discussion Notes / Link to Thread:
Usage Example:
'𠮷'.match(/./u)[0].length === 2;Documentation: link
Discussion Notes / Link to Thread:
Usage Example:
'\u{1d306}' == '\ud834\udf06'; // trueDocumentation: link
Discussion Notes / Link to Thread:
Keep the matching position sticky between matches and this way support efficient parsing of arbitrarily long input strings, even with an arbitrary number of distinct regular expressions.
Usage Example:
var re = new RegExp('yy', 'y');
re.lastIndex = 3;
var result = re.exec('xxxyyxx')[0];
result === 'yy' && re.lastIndex === 5; // trueDocumentation: link
Discussion Notes / Link to Thread:
Flexible destructuring of collections or parameters.
Usage Example:
// Array
var [a, , b] = [1, 2, 3]; // a = 1, b = 3
// Object
var {width, height} = document.body.getBoundingClientRect();
// width = rect.width, height = rect.height
// Parameters
function f([name, val]) {
console.log(name, val); // 'bar', 42
}
f(['bar', 42, 'extra 1', 'extra 2']); // 'extra 1' and 'extra 2' are ignored.
function g({name: n, val: v}) {
console.log(n, v); // 'foo', 7
}
g({name: 'foo', val: 7});
function h({name, val}) {
console.log(name, val); // 'bar', 42
}
h({name: 'bar', val: 42});Documentation: link
Discussion Notes / Link to Thread:
Support for exporting/importing values from/to modules without global namespace pollution.
Usage Example:
// lib/rect.js
export function getArea() {...};
export {width, height, unimportant};
// app.js
import {getArea, width, height} from 'lib/rect';Documentation: link
Discussion Notes / Link to Thread:
Unique and immutable data type to be used as an identifier for object properties.
Usage Example:
const foo = Symbol();
const bar = Symbol();
typeof foo === 'symbol'; // true
typeof bar === 'symbol'; // true
let obj = {};
obj[foo] = 'foo';
obj[bar] = 'bar';
JSON.stringify(obj); // {}
Object.keys(obj); // []
Object.getOwnPropertyNames(obj); // []
Object.getOwnPropertySymbols(obj); // [foo, bar]Documentation: link
Discussion Notes / Link to Thread:
Usage Example:
// String.raw
// String.fromCodePoint
// String.prototype.codePointAt
// String.prototype.normalize
// String.prototype.repeat
// String.prototype.startsWith
// String.prototype.endsWith
// String.prototype.includesDocumentation: link
Discussion Notes / Link to Thread:
Usage Example:
let fibonacci = {
[Symbol.iterator]() {
let pre = 0, cur = 1;
return {
next () {
[pre, cur] = [cur, pre + cur];
return {done: false, value: cur};
}
};
}
};Documentation: link
Discussion Notes / Link to Thread:
Special iterators with specified pausing points.
Usage Example:
function* range(start, end, step) {
while (start < end) {
yield start;
start += step;
}
}
for (let i of range(0, 10, 2)) {
console.log(i); // 0, 2, 4, 6, 8
}Documentation: link
Discussion Notes / Link to Thread:
WeakMap does not prevent garbage collection if nothing else refers to an object within the collection.
Usage Example:
var key = {};
var weakmap = new WeakMap();
weakmap.set(key, 123);
weakmap.has(key) && weakmap.get(key) === 123; // trueDocumentation: link
Discussion Notes / Link to Thread:
WeakSet does not prevent garbage collection if nothing else refers to an object within the collection.
Usage Example:
var obj1 = {};
var weakset = new WeakSet();
weakset.add(obj1);
weakset.add(obj1);
weakset.has(obj1); // trueDocumentation: link
Discussion Notes / Link to Thread:
A lot of new typed Arrays...
Usage Example:
new Int8Array();
new UInt8Array();
new UInt8ClampedArray();
// ... You get the idea. Click on the Documentation link below to see all.Documentation: link
Discussion Notes / Link to Thread:
Hooking into runtime-level object meta-operations.
Usage Example:
var keyTracker = new Proxy({}, {
keysCreated: 0,
get (receiver, key) {
if (key in receiver) {
console.log('key already exists');
} else {
++this.keysCreated;
console.log(this.keysCreated + ' keys created!');
receiver[key] = true;
}
},
});
keyTracker.key1; // '1 keys created!'
keyTracker.key1; // 'key already exists'
keyTracker.key2; // '2 keys created!'Documentation: link
Discussion Notes / Link to Thread:
Make calls corresponding to the object meta-operations.
Usage Example:
let obj = {a: 1};
Object.defineProperty(obj, 'b', {value: 2});
obj[Symbol('c')] = 3;
Reflect.ownKeys(obj); // ['a', 'b', Symbol(c)]Documentation: link
Discussion Notes / Link to Thread:
A lot of new Math methods.
Usage Example:
// See DocDocumentation: link
Discussion Notes / Link to Thread: