ECMAScript 2015, also known as ES6 or ES2015, is a fundamental version of the ECMAScript standard.
The most important changes in ES2015 include:
Arrow FunctionsClassesEnhanced Object LiteralsTemplate StringsDestructuringDefault + Rest + SpreadLet + ConstIterators + For..OfGeneratorsUnicodeModulesModule LoadersMap + Set + Weakmap + WeaksetProxiesSymbolsSubclassable Built-insPromisesMath + Number + String + Array + Object APIsBinary and Octal LiteralsReflect APITail Calls
Arrow functions have changed how most JavaScript code looks (and works).
Visually, it’s a simple and welcome change, from:
const foo = function foo() {
// ...
}To:
const foo = () => {
// ...
}And if the function body is a one-liner, just:
const foo = () => doSomething()Also, if you have a single parameter, you could write:
const foo = param => doSomething(param)This is not a breaking change, as regular functions continue to work just as before.
A new this scope
The this scope with arrow functions is inherited from the context.
With regular functions, this always refers to the nearest function, while with arrow functions this problem is removed, and you won't need to write let that = this ever again.
ES6 classes are a simple sugar over the prototype-based OO pattern. Having a single convenient declarative form makes class patterns easier to use, and encourages interoperability. Classes support prototype-based inheritance, super calls, instance and static methods and constructors.
class SkinnedMesh extends THREE.Mesh {
constructor(geometry, materials) {
super(geometry, materials)
this.idMatrix = SkinnedMesh.defaultMatrix()
this.bones = []
this.boneMatrices = []
// ...
}
update(camera) {
// ...
super.update()
}
get boneCount() {
return this.bones.length
}
set matrixType(matrixType) {
this.idMatrix = SkinnedMesh[matrixType]()
}
static defaultMatrix() {
return new THREE.Matrix4()
}
}Object literals are extended to support setting the prototype at construction, shorthand for foo: foo assignments, defining methods, making super calls, and computing property names with expressions. Together, these also bring object literals and class declarations closer together, and let object-based design benefit from some of the same conveniences.
var obj = {
// __proto__
__proto__: theProtoObj,
// Shorthand for ‘handler: handler’
handler,
// Methods
toString() {
// Super calls
return 'd ' + super.toString()
},
// Computed (dynamic) property names
[ 'prop_' + (() => 42)() ]: 42
}Template strings provide syntactic sugar for constructing strings. This is similar to string interpolation features in Perl, Python and more. Optionally, a tag can be added to allow the string construction to be customized, avoiding injection attacks or constructing higher level data structures from string contents.
// Basic literal string creation
`In JavaScript '\n' is a line-feed.`// Multiline strings
`In JavaScript this is
not legal.`// String interpolation
var name = 'Bob', time = 'today'
console.log(`Hello ${name}, how are you ${time}?`)Destructuring allows binding using pattern matching, with support for matching arrays and objects. Destructuring is fail-soft, similar to standard object lookup foo['bar'], producing undefined values when not found.
// List matching
var [a, , b] = [1,2,3]// Object matching
var { op: a, lhs: { op: b }, rhs: c } = getASTNode()// Object matching shorthand
// Binds `op`, `lhs` and `rhs` in scope
var { op, lhs, rhs } = getASTNode()// Can be used in parameter position
function g({ name: x }) {
console.log(x)
}
g({name: 5})// Fail-soft destructuring
var [a] = []
a === undefined// Fail-soft destructuring with defaults
var [a = 1] = []
a === 1Callee-evaluated default parameter values. Turn an array into consecutive arguments in a function call. Bind trailing parameters to an array. Rest replaces the need for arguments and addresses common cases more directly.
function f(x, y = 12) {
// y is 12 if not passed (or passed as undefined)
return x + y
}
f(3) == 15function f(x, ...y) {
// y is an Array
return x * y.length
}
f(3, 'hello', true) == 6function f(x, y, z) {
return x + y + z
}
// Pass each elem of array as argument
f(...[1,2,3]) == 6Block-scoped binding constructs: let is the new var & const is single-assignment. Static restrictions prevent use before assignment.
function f() {
{
let x
{
// Okay, block scoped name
const x = 'sneaky'
// Error, const
x = 'foo'
}
// Error, already declared in block
let x = 'inner'
}
}Iterator objects enable custom iteration like CLR IEnumerable or Java Iterable. Generalize for..in to custom iterator-based iteration with for..of. Don’t require realizing an array, enabling lazy design patterns like LINQ.
let fibonacci = {
[Symbol.iterator]() {
let pre = 0, cur = 1
return {
next() {
[pre, cur] = [cur, pre + cur]
return { done: false, value: cur }
}
}
}
}for (var n of fibonacci) {
// Truncate the sequence at 1000
if (n > 1000) {
break
}
console.log(n)
}Iteration is based on these duck-typed interfaces (using TypeScript type syntax for exposition only):
interface IteratorResult {
done: boolean
value: any
}
interface Iterator {
next(): IteratorResult
}
interface Iterable {
[Symbol.iterator](): Iterator
}Generators are a special kind of function with the ability to pause themselves, and resume later, allowing other code to run in the meantime.
The code decides that it has to wait, so it lets other code "in the queue" run, and keeps the right to resume its operations "when the thing it's waiting for" is done.
All this is done with a single, simple keyword: yield. When a generator contains that keyword, the execution is halted. A generator can contain many yield keywords, thus halting itself multiple items, and it's identified by the *function keyword, which is not to be confused with the pointer dereference operator used in lower level programming languages such as C, C++ or Go.
Generators enable whole new paradigms of programming in JavaScript, allowing:
- 2-way communication while a generator is running
- long-lived while loops which do not freeze your program
Here is an example of a generator which explains how it all works.
function *calculator(input) {
var doubleThat = 2 * (yield (input / 2))
var another = yield (doubleThat)
return (input * doubleThat * another)
}We initialize it with:
const calc = calculator(10)Then we start the iterator on our generator:
calc.next()This first iteration starts the iterator. The code returns this object:
{
done: false
value: 5
}What happens is: the code runs the function, with input = 10 as it was passed in the generator constructor. It runs until it reaches the yield, and returns the content of yield: input / 2 = 5. So we get a value of 5, and the indication that the iteration is not done (the function is just paused).
In the second iteration we pass the value 7:
calc.next(7)And what we get back is:
{
done: false
value: 14
}7 was placed as the value of doubleThat.
Important: you might think that input / 2 is the argument, but that's just the return value of the first iteration. We now skip that, and use the new input value, 7, and multiply it by 2.
We then reach the second yield, and that returns doubleThat, so the returned value is 14.
In the next, and last, iteration, we pass in 100
calc.next(100)And in return we get
{
done: true
value: 14000
}As the iteration is done (no more yield keywords found), we just return (input * doubleThat * another) which amounts to 10 * 14 * 100.
Non-breaking additions to support full Unicode, including new Unicode literal form in strings and new RegExp u mode to handle code points, as well as new APIs to process strings at the 21bit code points level. These additions support building global apps in JavaScript.
// Same as ES5.1
'𠮷'.length == 2// New RegExp behaviour, opt-in ‘u’
'𠮷'.match(/./u)[0].length == 2// New form
'\u{20BB7}' == '𠮷' == '\uD842\uDFB7'// New String ops
'𠮷'.codePointAt(0) == 0x20BB7// For-of iterates code points
for(var c of '𠮷') {
console.log(c)
}Language-level support for modules for component definition. Codifies patterns from popular JavaScript module loaders (AMD, CommonJS). Runtime behaviour defined by a host-defined default loader. Implicitly async model – no code executes until requested modules are available and processed.
// lib/math.js
export function sum(x, y) {
return x + y
}
export var pi = 3.141593// app.js
import * as math from 'lib/math'
alert('2π = ' + math.sum(math.pi, math.pi))// otherApp.js
import {sum, pi} from 'lib/math'
alert('2π = ' + sum(pi, pi))Some additional features include export default and export *:
// lib/mathplusplus.js
export * from 'lib/math'
export var e = 2.71828182846
export default function(x) {
return Math.log(x)
}// app.js
import ln, {pi, e} from 'lib/mathplusplus'
alert('2π = ' + ln(e)*pi*2)Module loaders support:
- Dynamic loading.
- State isolation.
- Global namespace isolation.
- Compilation hooks.
- Nested virtualization.
The default module loader can be configured, and new loaders can be constructed to evaluate and load code in isolated or constrained contexts.
// Dynamic loading – ‘System’ is default loader
System.import('lib/math').then(function(m) {
alert('2π = ' + m.sum(m.pi, m.pi))
})// Create execution sandboxes – new Loaders
var loader = new Loader({
global: fixup(window) // replace ‘console.log’
})
loader.eval("console.log('hello world!')")// Directly manipulate module cache
System.get('jquery')
System.set('jquery', Module({$: $})) // WARNING: not yet finalizedEfficient data structures for common algorithms. WeakMaps provides leak-free object-key’d side tables.
// Sets
var s = new Set()
s.add('hello').add('goodbye').add('hello')
s.size === 2
s.has('hello') === true// Maps
var m = new Map()
m.set('hello', 42)
m.set(s, 34)
m.get(s) == 34// Weak Maps
var wm = new WeakMap()
wm.set(s, { extra: 42 })
wm.size === undefined// Weak Sets
var ws = new WeakSet()
ws.add({ data: 42 })
// Because the added object has no other references, it will not be held in the setProxies enable creation of objects with the full range of behaviors available to host objects. Can be used for interception, object virtualization, logging/profiling, etc.
// Proxying a normal object
var target = {}
var handler = {
get: function (receiver, name) {
return `Hello, ${name}!`
}
}var p = new Proxy(target, handler)
p.world === 'Hello, world!'
// Proxying a function object
var target = function () { return 'I am the target' }
var handler = {
apply: function (receiver, ...args) {
return 'I am the proxy'
}
}var p = new Proxy(target, handler)
p() === 'I am the proxy'There are traps available for all of the runtime-level meta-operations:
var handler =
{
get:...,
set:...,
has:...,
deleteProperty:...,
apply:...,
construct:...,
getOwnPropertyDescriptor:...,
defineProperty:...,
getPrototypeOf:...,
setPrototypeOf:...,
enumerate:...,
ownKeys:...,
preventExtensions:...,
isExtensible:...
}Symbols enable access control for object state. Symbols allow properties to be keyed by either string (as in ES5) or symbol. Symbols are a new primitive type. Optional description parameter used in debugging - but is not part of identity. Symbols are unique (like gensym), but not private since they are exposed via reflection features like Object.getOwnPropertySymbols.
var MyClass = (function() {
// module scoped symbol
var key = Symbol('key')
function MyClass(privateData) {
this[key] = privateData
}
MyClass.prototype = {
doStuff: function() {
... this[key] ...
}
}
return MyClass
})()var c = new MyClass('hello')
c['key'] === undefinedIn ES6, built-ins like Array, Date and DOM Elements can be subclassed.
Object construction for a function named Ctor now uses two-phases (both virtually dispatched):
Call Ctor[@@create] to allocate the object, installing any special behavior Invoke constructor on new instance to initialize The known @@create symbol is available via Symbol.create. Built-ins now expose their @@create explicitly.
// Pseudo-code of Array
class Array {
constructor(...args) { /* ... */ }
static [Symbol.create]() {
// Install special [[DefineOwnProperty]]
// to magically update 'length'
}
}// User code of Array subclass
class MyArray extends Array {
constructor(...args) { super(...args) }
}// Two-phase 'new':
// 1) Call @@create to allocate object
// 2) Invoke constructor on new instance
var arr = new MyArray()
arr[1] = 12
arr.length == 2Many new library additions, including core Math libraries, Array conversion helpers, String helpers, and Object.assign for copying.
Number.EPSILON
Number.isInteger(Infinity) // false
Number.isNaN('NaN') // false
Math.acosh(3) // 1.762747174039086
Math.hypot(3, 4) // 5
Math.imul(Math.pow(2, 32) - 1, Math.pow(2, 32) - 2) // 2
'abcde'.includes('cd') // true
'abc'.repeat(3) // 'abcabcabc'
Array.from(document.querySelectorAll('*')) // Returns a real Array
Array.of(1, 2, 3) // Similar to new Array(...), but without special one-arg behavior
[0, 0, 0].fill(7, 1) // [0,7,7]
[1, 2, 3].find(x => x == 3) // 3
[1, 2, 3].findIndex(x => x == 2) // 1
[1, 2, 3, 4, 5].copyWithin(3, 0) // [1, 2, 3, 1, 2]
['a', 'b', 'c'].entries() // iterator [0, 'a'], [1,'b'], [2,'c']
['a', 'b', 'c'].keys() // iterator 0, 1, 2
['a', 'b', 'c'].values() // iterator 'a', 'b', 'c'
Object.assign(Point, { origin: new Point(0,0) })Two new numeric literal forms are added for binary (b) and octal (o).
0b111110111 === 503 // true
0o767 === 503 // truePromises are a library for asynchronous programming. Promises are a first class representation of a value that may be made available in the future. Promises are used in many existing JavaScript libraries.
function timeout(duration = 0) {
return new Promise((resolve, reject) => {
setTimeout(resolve, duration)
})
}var p = timeout(1000).then(() => {
return timeout(2000)
}).then(() => {
throw new Error('hmm')
}).catch(err => {
return Promise.all([timeout(100), timeout(200)])
})Full reflection API exposing the runtime-level meta-operations on objects. This is effectively the inverse of the Proxy API, and allows making calls corresponding to the same meta-operations as the proxy traps. Especially useful for implementing proxies.
// No sample yetCalls in tail-position are guaranteed to not grow the stack unboundedly. Makes recursive algorithms safe in the face of unbounded inputs.
function factorial(n, acc = 1) {
'use strict'
if (n <= 1) return acc
return factorial(n - 1, n * acc)
}
// Stack overflow in most implementations today, but safe on arbitrary inputs in ES6
factorial(100000)