<!-- Feature template markdown:
### Header
**Usage Example:**
```js
```
**Documentation:** [link]()
**Discussion Notes / Link to Thread:**
hyphen-hyphen-hyphen (change to actual hyphen)
-->
# ECMAScript Features in Chromium
This doc extends the [style guide](web.md#JavaScript) by specifying which new
features of ES2015 and beyond are allowed in Chromium.
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.
[TOC]
# ES2015 Support In Chromium
This is a list of [ECMAScript 6 a.k.a.
ES2015](https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_in_JavaScript/ECMAScript_6_support_in_Mozilla)
features allowed in Chromium code.
This is **not** a status list of [V8](https://developers.google.com/v8/)'s
support for language features.
> Some descriptions and usage examples were taken from
> [kangax](https://kangax.github.io/compat-table/es6/)
> and [http://es6-features.org/](http://es6-features.org/)
## Allowed Features
The following features are allowed in Chromium development.
### => (Arrow Functions)
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:**
```js
// 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](https://tc39.github.io/ecma262/#sec-arrow-function-definitions)
**Discussion Notes / Link to Thread:**
[link](https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/iJrC4PVSfoU)
---
### Promise
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:**
```js
/** @type {!Promise} */
const 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);
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-promise-objects)
[link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)
**Discussion Notes:** Feature already extensively used prior to creation of
this document.
---
### Proxy
Hooking into runtime-level object meta-operations.
**Usage Example:**
```js
const 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](https://tc39.github.io/ecma262/#sec-proxy-object-internal-methods-and-internal-slots)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/g/chromium-dev/c/-vdPXELXCx4/m/gXfP5vpVBwAJ)
---
### Classes
OOP-style and boilerplate-free class syntax, including inheritance, `super()`,
static members, and getters and setters.
**Usage Example:**
```js
class Shape {
constructor(x, y) {
this.x = x;
this.y = y;
}
}
// Note: const 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() {
const PHI = (1 + Math.sqrt(5)) / 2;
return new Rectangle(0, 0, PHI, 1);
}
}
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-class-definitions)
**Discussion Notes / Link to Thread:**
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/S1h-0m2ohOw/jyaiMGDlCwAJ
---
### Map
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:**
```js
const map = new Map();
map.size === 0; // true
map.get('foo'); // undefined
const 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; // true
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-map-objects)
[link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map)
**Discussion Notes:** Feature already extensively used prior to creation of
this document.
---
### Set
An object that lets you store unique values of any type, whether primitive
values or object references.
**Usage Example:**
```js
const set = new Set();
set.add(123);
set.size(); // 1
set.has(123); // true
set.add(123);
set.size(); // 1
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-set-objects)
[link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set)
**Discussion Notes:** Feature already extensively used prior to creation of
this document.
---
### const (Block-Scoped Constants)
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.
`const` is block-scoped, just like `let`.
**Usage Example:**
```js
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](https://tc39.github.io/ecma262/#sec-let-and-const-declarations)
**See also:** [Object.freeze()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/MJhTok8Usr8/XCrkisaBBQAJ)
---
### let (Block-Scoped Variables)
`let` declares a variable within the scope of a block, like `const`.
This differs from `var`, which uses function-level scope.
**Usage Example:**
```js
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](https://tc39.github.io/ecma262/#sec-let-and-const-declarations)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/MJhTok8Usr8/XCrkisaBBQAJ)
---
### Array Static & Prototype Methods
**Usage Example:**
```js
// Static methods
const a1 = Array.from(document.querySelectorAll('div'));
const 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 iterator
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-properties-of-the-array-constructor)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/d_2zUYQZJTg/-_PSji_OAQAJ)
---
### Number Properties
**Usage Example:**
```js
// Number.isFinite
// Number.isInteger
// Number.isSafeInteger
// Number.isNaN
// Number.EPSILON
// Number.MIN_SAFE_INTEGER
// Number.MAX_SAFE_INTEGER
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-isfinite-number)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/d_2zUYQZJTg/-_PSji_OAQAJ)
---
### Object Static Methods
**Usage Example:**
```js
// 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 true
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-properties-of-the-object-constructor)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/d_2zUYQZJTg/-_PSji_OAQAJ)
---
### for...of Loops
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:**
```js
// Given an iterable collection of Fibonacci numbers...
for (const n of fibonacci) {
console.log(n); // 1, 1, 2, 3, ...
}
```
**Documentation:** [link1](https://tc39.github.io/ecma262/#sec-for-in-and-for-of-statements)
[link2](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/d_2zUYQZJTg/-_PSji_OAQAJ)
---
### Template Literals
Expression interpolation for Strings, with the ability to access raw template
pieces.
**Usage Example:**
```js
// Simple example
const greeting = 'hello';
const myName = {first: 'Foo', last: 'Bar'};
const from = 1900;
const 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](https://tc39.github.io/ecma262/#sec-template-literals)
**Discussion Notes / Link to Thread:**
---
### Spread Operators
Spreading the elements from an iterable collection into individual literals as
function parameters.
This only applies to arrays and not objects.
**Usage Example:**
```js
// 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](http://www.ecma-international.org/ecma-262/6.0/#sec-argument-lists-runtime-semantics-argumentlistevaluation)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/LqP4AniIs8c)
---
### Rest Parameters
Aggregation of function arguments into one Array variable.
This only applies to arrays and function parameters, and not objects.
**Usage Example:**
```js
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](http://www.ecma-international.org/ecma-262/6.0/#sec-function-definitions)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/LqP4AniIs8c)
---
### Destructuring Assignment
Flexible destructuring of collections or parameters.
**Usage Example:**
```js
// Array
const [a, , b] = [1, 2, 3]; // a = 1, b = 3
// Object
const {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});
```
**Mixing with [Rest Parameters](#rest-parameters)**
Using rest parameters while destructuring objects is not supported by iOS 10 and requires setting the closure arg `language_in` to `ECMASCRIPT_2018`.
```js
const {one, ...rest} = {one: 1, two: 2, three: 3};
```
Using rest parameters while destructuring arrays, on the other hand, is supported by iOS 10 and `ECMASCRIPT_2017`.
```js
const [one, ...rest] = [1, 2, 3];
```
**Documentation:** [link](http://www.ecma-international.org/ecma-262/6.0/#sec-destructuring-assignment)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/d/topic/chromium-dev/mwFnj7MTzgU)
---
### Modules
Support for exporting/importing values from/to modules without global
namespace pollution.
**Usage Example:**
```js
// lib/rect.js
export function getArea() {...};
export {width, height, unimportant};
// app.js
import {getArea, width, height} from './lib/rect.js';
```
**Documentation:** [link](https://developers.google.com/web/fundamentals/primers/modules)
**Discussion Notes / Link to Thread:**
Dynamic Import [link](https://v8.dev/features/dynamic-import) are not allowed
yet, see separate entry in the [Features To Be Discussed](##es2015-support-in-chromium-features-to-be-discussed)
section.
---
### Object Literal Extensions
Convenient new ways for object property definition.
**Usage Example:**
```js
// Computed property name
const prop = 'foo';
const o = {
[prop]: 'hey',
['b' + 'ar']: 'there',
};
console.log(o); // {foo: 'hey', bar: 'there'}
// Shorthand property
const foo = 1;
const bar = 2;
const o = {foo, bar};
console.log(o); // {foo: 1, bar: 2}
// Method property
const clearSky = {
// Basically the same as clouds: function() { return 0; }.
clouds() { return 0; },
color() { return 'blue'; },
};
console.log(clearSky.color()); // 'blue'
console.log(clearSky.clouds()); // 0
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-object-initialiser)
[link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer)
**Discussion Notes / Link to Thread:**
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/RqOdTlxuGVg/M7I0CTryDQAJ
Note: clang-format has some issues formatting complex computed property names.
---
## Banned Features
The following features are banned for Chromium development.
## Features To Be Discussed
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.
### Block Scope Functions
**Usage Example:**
```js
{
function foo() {
return 1;
}
// foo() === 1
{
function foo() {
return 2;
}
// foo() === 2
}
// foo() === 1
}
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-functiondeclarationinstantiation)
**Discussion Notes / Link to Thread:**
---
### Default Function Parameters
Initialize parameters with default values if no value or `undefined` is passed.
**Usage Example:**
```js
/**
* @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.
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-functiondeclarationinstantiation)
[link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters)
**Discussion Notes / Link to Thread:**
---
### Binary & Octal Literals
**Usage Example:**
```js
0b111110111 === 503;
0o767 === 503;
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-literals-numeric-literals)
**Discussion Notes / Link to Thread:**
---
### `/u` Unicode Regex Literal
**Usage Example:**
```js
'𠮷'.match(/./u)[0].length === 2;
```
**Documentation:** [link](https://mathiasbynens.be/notes/es6-unicode-regex)
**Discussion Notes / Link to Thread:**
---
### `\u{}` Unicode String
**Usage Example:**
```js
'\u{1d306}' == '\ud834\udf06'; // true
```
**Documentation:** [link](https://mathiasbynens.be/notes/javascript-unicode#escape-sequences)
**Discussion Notes / Link to Thread:**
---
### `/y` Regex Sticky Matching
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:**
```js
var re = new RegExp('yy', 'y');
re.lastIndex = 3;
var result = re.exec('xxxyyxx')[0];
result === 'yy' && re.lastIndex === 5; // true
```
**Documentation:** [link](http://es6-features.org/#RegularExpressionStickyMatching)
**Discussion Notes / Link to Thread:**
### Dynamic Import
Dynamic import() introduces a new function-like form of import that returns a
promise for the module namespace object of the requested module.
**Usage Example:**
```js
// lib/rect.js
export function getArea() {...};
export {width, height, unimportant};
// app.js
if (calculateArea) {
import('./lib/rect.js').then(rect => {
rect.getArea(...);
});
}
```
**Documentation:** [link](https://v8.dev/features/dynamic-import)
**Discussion Notes / Link to Thread:**
---
### Symbol Type
Unique and immutable data type to be used as an identifier for object
properties.
**Usage Example:**
```js
const foo = Symbol();
const bar = Symbol();
typeof foo === 'symbol'; // true
typeof bar === 'symbol'; // true
const obj = {};
obj[foo] = 'foo';
obj[bar] = 'bar';
JSON.stringify(obj); // {}
Object.keys(obj); // []
Object.getOwnPropertyNames(obj); // []
Object.getOwnPropertySymbols(obj); // [foo, bar]
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-symbol-constructor)
**Discussion Notes / Link to Thread:**
---
### String Static & Prototype methods
**Usage Example:**
```js
// String.raw
// String.fromCodePoint
// String.prototype.codePointAt
// String.prototype.normalize
// String.prototype.repeat
// String.prototype.startsWith
// String.prototype.endsWith
// String.prototype.includes
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-properties-of-the-string-constructor)
**Discussion Notes / Link to Thread:**
---
### Iterators
**Usage Example:**
```js
const 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:**
---
### Generators
Special iterators with specified pausing points.
**Usage Example:**
```js
function* range(start, end, step) {
while (start < end) {
yield start;
start += step;
}
}
for (const i of range(0, 10, 2)) {
console.log(i); // 0, 2, 4, 6, 8
}
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-generator-function-definitions)
**Discussion Notes / Link to Thread:**
---
### WeakMap
WeakMap does not prevent garbage collection if nothing else refers to an object
within the collection.
**Usage Example:**
```js
const key = {};
const weakmap = new WeakMap();
weakmap.set(key, 123);
weakmap.has(key) && weakmap.get(key) === 123; // true
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-weakmap-objects)
**Discussion Notes / Link to Thread:**
---
### WeakSet
WeakSet does not prevent garbage collection if nothing else refers to an object
within the collection.
**Usage Example:**
```js
const obj1 = {};
const weakset = new WeakSet();
weakset.add(obj1);
weakset.add(obj1);
weakset.has(obj1); // true
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-weakset-objects)
**Discussion Notes / Link to Thread:**
---
### Typed Arrays
A lot of new typed Arrays...
**Usage Example:**
```js
new Int8Array();
new UInt8Array();
new UInt8ClampedArray();
// ... You get the idea. Click on the Documentation link below to see all.
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-typedarray-objects)
**Discussion Notes / Link to Thread:**
---
### Reflection
Make calls corresponding to the object meta-operations.
**Usage Example:**
```js
const obj = {a: 1};
Object.defineProperty(obj, 'b', {value: 2});
obj[Symbol('c')] = 3;
Reflect.ownKeys(obj); // ['a', 'b', Symbol(c)]
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-reflection)
**Discussion Notes / Link to Thread:**
---
### Math Methods
A lot of new Math methods.
**Usage Example:**
```js
// See Doc
```
**Documentation:** [link](https://tc39.github.io/ecma262/#sec-math)
**Discussion Notes / Link to Thread:**
---
# ES2016 Support In Chromium
This is a list of [ECMAScript 7 a.k.a.
ES2016](https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_in_JavaScript/ECMAScript_Next_support_in_Mozilla#ECMAScript_2016)
features allowed in Chromium code.
## Allowed Features
The following features are allowed in Chromium development.
## Banned Features
The following features are banned for Chromium development.
## Features To Be Discussed
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.
### Array.prototype.includes()
The includes() method determines whether an array includes a certain element, returning true or false as appropriate.
**Usage Example:**
```js
var array1 = [1, 2, 3];
console.log(array1.includes(2));
// expected output: true
var pets = ['cat', 'dog', 'bat'];
console.log(pets.includes('cat'));
// expected output: true
console.log(pets.includes('at'));
// expected output: false
```
**Documentation:** [link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes)
**Discussion Notes / Link to Thread:**
---
# ES2017 Support In Chromium
This is a list of [ECMAScript 8 a.k.a.
ES2017](https://developer.mozilla.org/en-US/docs/Web/JavaScript/New_in_JavaScript/ECMAScript_Next_support_in_Mozilla#ECMAScript_2017)
features allowed in Chromium code.
## Allowed Features
The following features are allowed in Chromium development.
### Async Functions (aka async/await)
The await expression causes async function execution to pause until a Promise is resolved, that is fulfilled or rejected, and to resume execution of the async function after fulfillment. When resumed, the value of the await expression is that of the fulfilled Promise.
**Usage Example:**
```js
function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
}
async function f1() {
var x = await resolveAfter2Seconds(10);
console.log(x); // 10
}
f1();
```
**Documentation:** [link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/forum/?utm_medium=email&utm_source=footer#!msg/chromium-dev/KCGiv5vGeEQ/u-gl1pRGAAAJ)
---
## Banned Features
The following features are banned for Chromium development.
## Features To Be Discussed
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.
### String.prototype.padEnd()/padStart()
The padStart() and padEnd() methods pad the current string with another string (multiple times, if needed) until the resulting string reaches the given length.
**Usage Example:**
```js
const str1 = '5';
console.log(str1.padStart(2, '0'));
// expected output: "05"
const fullNumber = '2034399002125581';
const last4Digits = fullNumber.slice(-4);
const maskedNumber = last4Digits.padStart(fullNumber.length, '*');
console.log(maskedNumber);
// expected output: "************5581"
```
**Documentation:** [link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart)
**Discussion Notes / Link to Thread:**
---
### Object.values() and Object.entries()
The Object.values() method returns an array of a given object's own enumerable property values, and Object.entries() method returns an array of a given object's own enumerable property [key, value] pairs.
**Usage Example:**
```js
const object1 = {
a: 'somestring',
b: 42,
c: false
};
console.log(Object.values(object1));
// expected output: Array ["somestring", 42, false]
const object2 = { foo: 'bar', baz: 42 };
console.log(Object.entries(object2)[1]);
// expected output: Array ["baz", 42]
```
**Documentation:** [link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/values) [link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries)
**Discussion Notes / Link to Thread:**
---
# ES2020 Support in Chromium
## Allowed features
### Optional Chaining (?.)
The optional chaining operator makes it easy to chain multiple functions /
property accesses that may return null or undefined.
**Usage Example:**
```js
// Before:
const currentKeyboard = keyboards.getCurrentKeyboard();
const keys = currentKeyboard && currentKeyboard.getKeys();
const enterKey = keys && keys.getEnterKey();
// After:
const enterKey = keyboards.getCurrentKeyboard()?.getKeys()?.getEnterKey();
```
**Documentation:** [link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining)
**Discussion Notes / Link to Thread:** [link](https://groups.google.com/a/chromium.org/g/chromium-dev/c/DHLSm05HHlo)
Codebase Browser
chromium