本文只介绍是什么，有什么。
ES5
ECMAScript第五个版本，新增如下特性
- strict mode
引入严格模式，通过 'use strict'; 限制- Array新增用法
增加了every、some 、forEach、filter 、indexOf、lastIndexOf、isArray、map、reduce、reduceRight方法- Object方法
Object.getPrototypeOf
Object.create
Object.getOwnPropertyNames
Object.defineProperty
Object.getOwnPropertyDescriptor
Object.defineProperties
Object.keys
Object.preventExtensions / Object.isExtensible
Object.seal / Object.isSealed
Object.freeze / Object.isFrozen- 还有其他方法 Function.prototype.bind、String.prototype.trim、Date.now
ES6
在保证向下兼容的前提下，ES6提供大量新特性，兼容如下：
image.png
ES6 常用特性梳理：
- 块级作用域 let const
见作用域篇 - 对象字面量的属性赋值简写
var obj = {
// __proto__
__proto__: theProtoObj,
// Shorthand for ‘handler: handler’
handler,
// Method definitions
toString() {
// Super calls
return "d " + super.toString();
},
// Computed (dynamic) property names
[ 'prop_' + (() => 42)() ]: 42
};
- 赋值解构
- 函数参数 - 默认值、参数打包、 数组展开（Default 、Rest 、Spread）
//Default
function findArtist(name='lu', age='26') {
...
}
//Rest
function f(x, ...y) {
// y is an Array
return x * y.length;
}
f(3, "hello", true) == 6
//Spread
function f(x, y, z) {
return x + y + z;
}
// Pass each elem of array as argument
f(...[1,2,3]) == 6
- 箭头函数 Arrow functions
- 字符串模板 Template strings
- Iterators（迭代器）+ for..of
for (var n of ['a','b','c']) {
console.log(n);
}
// a, b, c
- Generators
- Class
Class，有constructor、extends、super，但本质上是语法糖
class Artist {
constructor(name) {
this.name = name;
}
perform() {
return this.name + " performs ";
}
}
class Singer extends Artist {
constructor(name, song) {
super.constructor(name);
this.song = song;
}
perform() {
return super.perform() + "[" + this.song + "]";
}
}
let james = new Singer("Etta James", "At last");
james instanceof Artist; // true
james instanceof Singer; // true
james.perform(); // "Etta James performs [At last]"
- Modules
ES6的内置模块功能借鉴了CommonJS和AMD各自的优点:
- 具有CommonJS的精简语法、唯一导出出口(single exports)和循环依赖(cyclic dependencies)的特点。
- 类似AMD，支持异步加载和可配置的模块加载。
// 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));
Module Loaders:
// Dynamic loading – ‘System’ is default loader
System.import('lib/math').then(function(m) {
alert("2π = " + m.sum(m.pi, m.pi));
});
// Directly manipulate module cache
System.get('jquery');
System.set('jquery', Module({$: $})); // WARNING: not yet finalized
- Map + Set + WeakMap + WeakSet
四种集合类型，WeakMap、WeakSet作为属性键的对象如果没有别的变量在引用它们，则会被回收释放
// 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;
//WeakMap
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
- Math + Number + String + Array + Object APIs
一些新的API
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) })
- Proxies
使用代理（Proxy）监听对象的操作,可监听的操作： get、set、has、deleteProperty、apply、construct、getOwnPropertyDescriptor、defineProperty、getPrototypeOf、setPrototypeOf、enumerate、ownKeys、preventExtensions、isExtensible。
var target = {};
var handler = {
get: function (receiver, name) {
return `Hello, ${name}!`;
}
};
var p = new Proxy(target, handler);
p.world === 'Hello, world!';
- Symbols
Symbol是一种基本类型。Symbol 通过调用symbol函数产生，它接收一个可选的名字参数，该函数返回的symbol是唯一的。
var key = Symbol("key");
var key2 = Symbol("key");
key == key2 //false
- Promises
Promises是处理异步操作的对象，使用了 Promise 对象之后可以用一种链式调用的方式来组织代码，让代码更加直观（类似jQuery的deferred 对象）。
Refs:
http://www.cnblogs.com/lovesong/p/4908871.html?utm_source=caibaojian.com
https://www.infoq.cn/article/es6-in-depth-let-and-const
