ES6特性

ES6特性如下：

1.块级作用域 关键字let, 常量const

2.对象字面量的属性赋值简写（property value shorthand）

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
};

3.赋值解构

let singer = { first: "Bob", last: "Dylan" };
let { first: f, last: l } = singer; // 相当于 f = "Bob", l = "Dylan"
let [all, year, month, day] =  /^(\d\d\d\d)-(\d\d)-(\d\d)$/.exec("2015-10-25");
let [x, y] = [1, 2, 3]; // x = 1, y = 2

4.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

5.箭头函数 Arrow functions

(1).简化了代码形式，默认return表达式结果。

(2).自动绑定语义this，即定义函数时的this。如上面例子中，forEach的匿名函数参数中用到的this。

6.字符串模板 Template strings

var name = "Bob", time = "today";
`Hello ${name}, how are you ${time}?`
// return "Hello Bob, how are you today?"

7. Iterators（迭代器）+ for..of

迭代器有个next方法，调用会返回：

(1).返回迭代对象的一个元素：{ done: false, value: elem }

(2).如果已到迭代对象的末端：{ done: true, value: retVal }

for (var n of ['a','b','c']) {
  console.log(n);
}
// 打印a、b、c

8.生成器 （Generators）

9.Class

Class，有constructor、extends、super，但本质上是语法糖（对语言的功能并没有影响，但是更方便程序员使用）。

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]"

10.Modules

ES6的内置模块功能借鉴了CommonJS和AMD各自的优点：

(1).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。

(2).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

11.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，则会被回收释放掉。

// 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

12.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) })

13. Proxies

使用代理（Proxy）监听对象的操作，然后可以做一些相应事情。

var target = {};
var handler = {
  get: function (receiver, name) {
    return `Hello, ${name}!`;
  }
};

var p = new Proxy(target, handler);
p.world === 'Hello, world!';

可监听的操作： get、set、has、deleteProperty、apply、construct、getOwnPropertyDescriptor、defineProperty、getPrototypeOf、setPrototypeOf、enumerate、ownKeys、preventExtensions、isExtensible。

14.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

15.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）。

function fakeAjax(url) {
  return new Promise(function (resolve, reject) {
    // setTimeouts are for effect, typically we would handle XHR
    if (!url) {
      return setTimeout(reject, 1000);
    }
    return setTimeout(resolve, 1000);
  });
}

// no url, promise rejected
fakeAjax().then(function () {
  console.log('success');
},function () {
  console.log('fail');
});