ECMAScript 6, also known as ECMAScript 2015, is the latest version of the ECMAScript standard. ES6 is a significant update to the language, and the first update to the language since ES5 was standardized in 2009. Implementation of these features in major JavaScript engines is underway now.
Content is copied shamelessly from lukehoban/es6features
ES6 includes the following new features:
- arrows
- classes
- enhanced object literals
- template strings
- destructuring
- default + rest + spread
- let + const
- iterators + for..of
- generators
- unicode
- modules
- module loaders
- map + set + weakmap + weakset
- proxies
- symbols
- subclassable built-ins
- promises
- math + number + string + array + object APIs
- binary and octal literals
- reflect api
- tail calls
ECMAScript 6 Features
Arrows
// Expression bodies
var odds = evens.map(v => v + 1);
var nums = evens.map((v, i) => v + i);
var pairs = evens.map(v => ({even: v, odd: v + 1}));
// Statement bodies
nums.forEach(v => {
if (v % 5 === 0)
fives.push(v);
});
// Lexical this
var bob = {
_name: "Bob",
_friends: [],
printFriends() {
this._friends.forEach(f =>
console.log(this._name + " knows " + f));
}
}
More info: MDN Arrow Functions
Classes
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();
}
}
More info: MDN Classes
Enhanced Object Literals
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
};
More info: MDN Grammer and types: Object literals
Template Strings
// 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";
`Hello ${name}, how are you ${time}?`
// Construct an HTTP request prefix is used to interpret the replacements and construction
POST`http://foo.org/bar?a=${a}&b=${b}
Content-Type: application/json
X-Credentials: ${credentials}
{ "foo": ${foo},
"bar": ${bar}}`(myOnReadyStateChangeHandler);
More info: MDN Template Strings
Destructuring
// 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 === 1;
More info: MDN Destructuring assignment
Default + Rest + Spread
function f(x, y=12) {
// y is 12 if not passed (or passed as undefined)
return x + y;
}
f(3) == 15
function f(x, ...y) {
// y is an Array
return x * y.length;
}
f(3, "hello", true) == 6
function f(x, y, z) {
return x + y + z;
}
// Pass each elem of array as argument
f(...[1,2,3]) == 6
More MDN info: Default parameters, Rest parameters, Spread Operator
Let + Const
function f() {
{
let x;
{
// okay, block scoped name
const x = "sneaky";
// error, const
x = "foo";
}
// error, already declared in block
let x = "inner";
}
}
More MDN info: let statement, const statement
Iterators + For..Of
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);
}
interface IteratorResult {
done: boolean;
value: any;
}
interface Iterator {
next(): IteratorResult;
}
interface Iterable {
[Symbol.iterator](): Iterator
}
More info: MDN for…of
Generators
Generators simplify iterator-authoring using function* and yield. A function declared as function* returns a Generator instance. Generators are subtypes of iterators which include additional next and throw. These enable values to flow back into the generator, so yield is an expression form which returns a value (or throws).
var fibonacci = {
[Symbol.iterator]: function*() {
var pre = 0, cur = 1;
for (;;) {
var temp = pre;
pre = cur;
cur += temp;
yield cur;
}
}
}
for (var n of fibonacci) {
// truncate the sequence at 1000
if (n > 1000)
break;
console.log(n);
}
interface Generator extends Iterator {
next(value?: any): IteratorResult;
throw(exception: any);
}
More info: MDN Iteration protocols
Unicode
// 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);
}
More info: MDN RegExp.prototype.unicode
Modules
// 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));
// 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);
More MDN info: import statement, export statement
Module Loaders
Module loaders support:
- Dynamic loading
- State isolation
- Global namespace isolation
- Compilation hooks
- Nested virtualization
// 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 finalized
Map + Set + WeakMap + WeakSet
// 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 set
More MDN info: Map, Set, WeakMap, WeakSet
Proxies
// 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';
var handler =
{
get:...,
set:...,
has:...,
deleteProperty:...,
apply:...,
construct:...,
getOwnPropertyDescriptor:...,
defineProperty:...,
getPrototypeOf:...,
setPrototypeOf:...,
enumerate:...,
ownKeys:...,
preventExtensions:...,
isExtensible:...
}
More info: MDN Proxy
Symbols
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"] === undefined
More info: MDN Symbol
Subclassable Built-ins
In 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
// 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 == 2
Math + Number + String + Array + Object APIs
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) })
More MDN info: Number, Math, Array.from, Array.of, Array.prototype.copyWithin, Object.assign
Binary and Octal Literals
0b111110111 === 503 // true
0o767 === 503 // true
Promises
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)]);
})
More info: MDN Promise
Reflect API
// No sample yet
More info: MDN Reflect
Tail Calls
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)