what's the ES2015


ES2015是在2015年6月正式批准的最新 `javascript`标准,自从 `es5`在2009年被批准之后, 这是一次较大的更新.刚开始还叫` es6`来着后来改名字了.


> https://github.com/lukehoban/es6features	
> 来自 https://babeljs.io/docs/learn-es2015/

#features:

## Arrow和 Lexical this

通过`=>`语法是一种函数的简写. 类似 C#, Java8还有 CoffeeScript. 他们都支持表达式或者声明. 不同于函数的就是, 剪头共享同一个语义的` this`

```js
// Expression bodies
var odds = evens.map(v => v + 1);
var nums = evens.map((v, i) => v + i);

// 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));
  }
};

```


## 类

ES2015里的类就是一个基于` prototype`的面向对象模型简单的语法糖, 有一个简单的约束声明, 可以让定义类更加方便. 它支持原型类型的继承,调用父类, 实例, 静态方法还有构造函数.

```
class SkinnedMesh extends THREE.Mesh {
  constructor(geometry, materials) {
    super(geometry, materials);

    this.idMatrix = SkinnedMesh.defaultMatrix();
    this.bones = [];
    this.boneMatrices = [];
    //...
  }
  update(camera) {
    //...
    super.update();
  }
  static defaultMatrix() {
    return new THREE.Matrix4();
  }
}
```


## 增强对象表述

对象表述是用来在构建函数的时候设置原型用的. 简写成` foo:foo`, 这样的赋值, 定义方法或者调用父类,另外这样也更加拉进来了对象表述和类定义间的距离. 让大家感觉到这样的便利性,在定义对象的时候.

```js
var obj = {
    // Sets the prototype. "__proto__" or '__proto__' would also work.
    __proto__: theProtoObj,
    // Computed property name does not set prototype or trigger early error for
    // duplicate __proto__ properties.
    ['__proto__']: somethingElse,
    // Shorthand for ‘handler: handler’
    handler,
    // Methods
    toString() {
     // Super calls
     return "d " + super.toString();
    },
    // Computed (dynamic) property names
    [ "prop_" + (() => 42)() ]: 42
};
```


> 这个__ proto__ 属性需要原生的支持.有些浏览器并不支持.

## 模板字符串

模板字符串提供给构造字符串时一种语法糖. 这个类似于 perl或者 python 里对于 string 的解析功能. 另外一个 tag 添加到里面可以自定义它. 避免被注入.


```
// Basic literal string creation
`This is a pretty little template string.`

// Multiline strings
`In ES5 this is
 not legal.`

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

// Unescaped template strings
String.raw`In ES5 "\n" is a line-feed.`

// Construct an HTTP request prefix is used to interpret the replacements and construction
GET`http://foo.org/bar?a=${a}&b=${b}
    Content-Type: application/json
    X-Credentials: ${credentials}
    { "foo": ${foo},
      "bar": ${bar}}`(myOnReadyStateChangeHandler);
```


## 解析赋值

析构是和模式匹配绑定使用. 支持数组和对象的匹配. 匹配不到返回` undefined`

```
// list matching
var [a, ,b] = [1,2,3];
a === 1;
b === 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;

// Destructuring + defaults arguments
function r({x, y, w = 10, h = 10}) {
  return x + y + w + h;
}
r({x:1, y:2}) === 23
```


## 默认值

函数默认值,数组作为连续参数, 转化 trailing 的参数为数组.
###spread运算符

> https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator
> http://jsbin.com/riyokow/edit?js,console


```
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

```

## let 和 const 赋值

块构造, 用`let`来定义 new var, `const `是一个单独的的声明赋值.禁止没有声明就赋值.

```
function f() {
  {
    let x;
    {
      // okay, block scoped name
      const x = "sneaky";
      // error, const
      x = "foo";
    }
    // okay, declared with `let`
    x = "bar";
    // error, already declared in block
    let x = "inner";
  }
}
```

> 这个例子写的乱七八糟

## 遍历器 + 新的循环语法 for..of

遍历对象可以允许自定义的遍历如 CLR 或者 Java 的 iterable, 通过` for..of`来遍历.
遍历对象不一定式一个声明的数组. 可以是一个可遍历对象.

```

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);
}
```

## 生成器

通过` function*` 或者` yield` 来实行生成器, 它不会返回一个值代替的是返回一个生成器.

```
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);
}
```


## unicode

支持全 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);
}

```

## 模块的支持

语言级的 module 支持. 从 AMD CommonJS 那里流行的 javascript module 的写法给归纳到语法中

```
// lib/math.js
export function sum(x, y) {
  return x + y;
}
export var pi = 3.141593;
// app.js
import * as math from "lib/math";
console.log("2π = " + math.sum(math.pi, math.pi));
// otherApp.js
import {sum, pi} from "lib/math";
console.log("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.exp(x);
}
// app.js
import exp, {pi, e} from "lib/mathplusplus";
console.log("e^π = " + exp(pi));

```

#### 模块加载


## 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
```

## 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";
```

## Symbols

Symbol 可以控制对象状态的访问, Symbol 可以让对象属性的 key 是字符串还是 symbol, `Symbol` 是一个新的预定于类型.

```
(function() {

  // module scoped symbol
  var key = Symbol("key");

  function MyClass(privateData) {
    this[key] = privateData;
  }

  MyClass.prototype = {
    doStuff: function() {
      ... this[key] ...
    }
  };

  // Limited support from Babel, full support requires native implementation.
  typeof key === "symbol"
})();

var c = new MyClass("hello")
c["key"] === undefined
```


## 一些内置类可以被继承

如 Array Date DOM 的 Element 等

```
// User code of Array subclass
class MyArray extends Array {
    constructor(...args) { super(...args); }
}

var arr = new MyArray();
arr[1] = 12;
arr.length == 2
```

## Math + Number + String + Object APIs


内置了 core 的 Math 库, 还有很多 API, Array 的相关操作帮助类, Object.assign 的相关复制功能.

```
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].findIndex(x => x == 2) // 1
["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) })
```

## Binary and Octal Literals

新增两种数据类型


## Promise

它是一个异步编程的库,

```
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)]);
})
```


## Reflect API

反射 API, 可以提供运行时object的元数据


```
var O = {a: 1};
Object.defineProperty(O, 'b', {value: 2});
O[Symbol('c')] = 3;

Reflect.ownKeys(O); // ['a', 'b', Symbol(c)]

function C(a, b){
  this.c = a + b;
}
var instance = Reflect.construct(C, [20, 22]);
instance.c; // 42

```

##  Tail Calls

保证递归运行更加安全?

```
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 ES2015
factorial(100000)
```