翻译自 https://github.com/lukehoban/es6features
ECMAScript 6,也被称为 ECMAScript 2015,是 ECMAScript 标准的最新版本。ES6 是 JavaScript 语言的重大革新,同时也是 ES5 在 2009 年被标准化以来的首次更新。主要的 JavaScript 引擎对这些特性的实现正在进行中。
点击 ES6 标准 以查看 ECMAScript 6 的完整说明。
ES6 包含了如下新特性:
- 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
箭头函数是使用 =>
语法简写的函数,这在语法上和 C#,Java 8 以及 CoffeeScript 的相关特性类似。箭头函数既支持语句块,也支持表达式。与普通函数不同的是,箭头函数中的 this
始终指向函数定义时所在的对象,而非函数执行时所在的对象。
// 表达式
const odds = evens.map(v => v + 1);
const nums = evens.map((v, i) => v + i);
const pairs = evens.map(v => ({even: v, odd: v + 1}));
// 语句块
nums.forEach(v => {
if (v % 5 === 0) {
fives.push(v);
}
});
// 箭头函数中的this
var bob = {
_name: 'Bob',
_friends: ['Tom', 'Kathy'],
printFriends() {
this._friends.forEach(f => {
// 此处的this指向bob,而非window
console.log(`${this._name} knows ${f}`);
});
}
}
更多信息:MDN 箭头函数
ES6 的类是现有的基于原型的面向对象模式的一个简单的语法糖。简单且方便的声明形式使得类模式更易于使用,并鼓励互操作性。类支持基于原型的继承、父类调用、实例方法、静态方法、构造函数等。
// 定义一个继承自THREE.Mesh的类
class SkinnedMesh extends THREE.Mesh {
// 类的构造函数
constructor(geometry, materials) {
// 调用父类构造函数
super(geometry, materials);
this.idMatrix = SkinnedMesh.defaultMatrix();
this.bones = [];
this.boneMatrices = [];
// ...
}
// 普通(实例)方法,通过 new SkinnedMesh().update 调用
update(camera) {
// ...
super.update();
}
// get访问器
get boneCount() {
return this.bones.length;
}
// set访问器
set matrixType(matrixType) {
this.idMatrix = SkinnedMesh[matrixType]();
}
// 静态方法,直接通过 SkinnedMesh.defaultMatrix 调用
static defaultMatrix() {
return new THREE.Matrix4();
}
}
更多信息:MDN 类
在 ES2015,对象字面量被扩展到支持在创建对象时设置原型,简写 foo: foo
赋值,简写方法的定义,调用父函数,动态计算的属性名。这些增强使得对象字面量和类声明紧密联系起来,让基于对象的设计也在这种便利中收益。
var obj = {
// 设置原型
__proto__: theProtoObj,
// 'handler: handler'的简写
handler,
// 简写的方法定义,省略了function
toString() {
// 调用父函数
return "d " + super.toString();
},
// 动态计算的属性名
[ 'prop_' + (() => 42)() ]: 42
};
更多信息:MDN 语法和数据类型:对象字面量
模板字符串提供了构建字符串的语法糖。这类似于 Perl,Python 等语言中的字符串插值特性。作为可选项,你还可以加入标签来自定义字符串的构建,这可以避免注入攻击,或者从字符串内容构建高阶数据结构。
// 基本的模板字符串
`In JavaScript '\n' is a line-feed.`
// 多行字符串
`In JavaScript this is
not legal.`
// 字符串插值
var name = "Bob", time = "today";
`Hello ${name}, how are you ${time}?`
// 构建一个HTTP请求,其中标签POST是一个自定义的函数,用来进行替换和构建
POST`http://foo.org/bar?a=${a}&b=${b}
Content-Type: application/json
X-Credentials: ${credentials}
{ "foo": ${foo},
"bar": ${bar}}`(myOnReadyStateChangeHandler);
更多信息:MDN 模板字符串
解构使用模式匹配进行绑定,支持匹配数组和对象。解构具有良好的容错性,类似于标准对象的查询 foo['bar']
,如果找不到则返回 undefined
。
// 数组匹配
var [a, , b] = [1, 2, 3];
// 对象匹配
var {op: a, lhs: {op: b}, rhs: c} = getASTNode();
// 对象匹配的简写
// 绑定 `op`, `lhs` and `rhs` 到作用域
var {op, lhs, rhs} = getASTNode();
// 可以被用在参数位置
function g({name: x}) {
console.log(x);
}
g({name: 5});
// 解构的容错性
var [a] = [];
a === undefined;
// 带默认值时的容错性
var [a = 1] = [];
a === 1;
更多信息:MDN 解构赋值
函数可以设置参数的默认值。调用函数时可以传入一个扩展参数,这将被视为连续的参数形式。函数尾部的剩余参数会被绑定到一个数组,它取代了 arguments
的功能,可以更直接地处理常见情况。
function f(x, y = 12) {
// y 将被赋值为 12 如果没有传值(或者传值为 undefined)
return x + y;
}
f(3) == 15
function f(x, ...y) {
// y 是一个数组
return x * y.length;
}
f(3, "hello", true) == 6
function f(x, y, z) {
return x + y + z;
}
// 将数组的每一个元素作为连续参数传入方法f
f(...[1,2,3]) == 6
两者都用来声明块级作用域的变量。let
是新的 var
,const
是单次赋值,且必须在赋值之后才能使用。
function f() {
{
let x;
{
// 块级作用域
const x = "sneaky";
// 错误,不允许再次赋值
x = "foo";
}
// 错误,在当前作用域下已有定义
let x = "inner";
}
}
更多信息:MDN let 语句,MDN const 语句
迭代器对象让 JavaScript 能够像 CLR 的 IEnumerable 接口或 Java 的 Iterable 接口一样进行自定义迭代。通常我们将 for..in
转换成自定义的基于迭代器的 for..of
迭代。不需要实现一个像 LINQ 那样的惰性设计模式的数组。
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) {
// 在1000处停止
if (n > 1000)
break;
console.log(n);
}
迭代器是基于这些鸭子类型的接口(这里使用 TypeScript 类型的语法只是用以阐述问题):
interface IteratorResult {
done: boolean;
value: any;
}
interface Iterator {
next(): IteratorResult;
}
interface Iterable {
[Symbol.iterator](): Iterator
}
更多信息:MDN for...of
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).
Note: Can also be used to enable ‘await’-like async programming, see also ES7 await
proposal.
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);
}
The generator interface is (using TypeScript type syntax for exposition only):
interface Generator extends Iterator {
next(value?: any): IteratorResult;
throw(exception: any);
}
More info: MDN Iteration protocols
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);
}
More info: MDN RegExp.prototype.unicode
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);
More MDN info: import statement, export statement
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 finalized
Efficient 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 set
More MDN info: Map, Set, WeakMap, WeakSet
Proxies 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:...
}
More info: MDN Proxy
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"] === undefined
More info: MDN Symbol
In ES6, built-ins like Array
, Date
and DOM Element
s 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 == 2
Many 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) })
More MDN info: Number, Math, Array.from, Array.of, Array.prototype.copyWithin, Object.assign
Two new numeric literal forms are added for binary (b
) and octal (o
).
0b111110111 === 503 // true
0o767 === 503 // true
Promises 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)]);
})
More info: MDN Promise
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 yet
More info: MDN Reflect
Calls 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)