ES6新特性

ECMAScript出现了许多有趣的新特性，梳理一下：

1.支持constant声明

ES5中，想实现constant变量必须通过修改var的property实现

Object.defineProperty(this, "PI", {
    value: 3.1415926,
    configurable: false,
    writable: false
});

2.支持块作用域声明let，

JS只有函数作用域，没有块作用域，var声明有变量提升的效果，即如果var声明了一个未声明过的变量，会自动将此声明提升到顶端，所以想实现块作用域在ES5中是通过闭包的形式。

(function(param) {})(i);

同时ES6也支持了块级作用域的函数

3.一种function的新写法

function中支持this的使用

nums.forEach(function(v) {
    return foo(v);
});

nums.forEach(v => {
    foo(v)
});

4.undefined参数默认配置

ES6:

function(x, y = 7, z = 10) {
    return x + y + z;
}

等效于ES5：

function(x, y, z) {
    if (y === undefined) {
        y = 7;
    }
    if (z === undefined) {
        z = 10;
    }
    return x + y + z;
}

5.支持剩余参数写法

即...p写法会被解释为

p.prototype.slice

ES6：

function f (x, y, ...a) {
    return (x + y) * a.length
}
f(1, 2, "hello", true, 7) === 9

ES5：

function f (x, y) {
    var a = Array.prototype.slice.call(arguments, 2);
    return (x + y) * a.length;
};
f(1, 2, "hello", true, 7) === 9;

6.可以直接在字符串中加入${表达式}

即console.log("Error: ${e.toString()}");

7.引用

//lib/math.js
export function sum(x, y) {return x + y};
export var pi = 3.1415926;

//app.js
import * as math from "lib/math"
console.log(math.sum(math.pi, math.pi););

8.支持class 继承，setter、getter

class Shape {
    constructor (id, x, y) {
        this.id = id;
        this.move(x, y);
    }
    move (x, y) {
        this.x = x;
        this.y = y;
    }
}
class Rectangle extends Shape {
    constructor (id, x, y, width, height) {
        super(id, x, y);
        this.width  = width;
        this.height = height;
    }
    set width(width) {this.width = width}
    get width()      {return this.width}
}
class Circle extends Shape {
    constructor (id, x, y, radius) {
        super(id, x, y);
        this.radius = radius;
    }
}

9.支持Promise Generators（待学习）

10.bufferArray

class Example {
    constructor (buffer = new ArrayBuffer(24)) {
        this.buffer = buffer
    }
    set buffer (buffer) {
        this._buffer    = buffer
        this._id        = new Uint32Array (this._buffer,  0,  1)
        this._username  = new Uint8Array  (this._buffer,  4, 16)
        this._amountDue = new Float32Array(this._buffer, 20,  1)
    }
    get buffer ()     { return this._buffer       }
    set id (v)        { this._id[0] = v           }
    get id ()         { return this._id[0]        }
    set username (v)  { this._username[0] = v     }
    get username ()   { return this._username[0]  }
    set amountDue (v) { this._amountDue[0] = v    }
    get amountDue ()  { return this._amountDue[0] }
}

let example = new Example()
example.id = 7
example.username = "John Doe"
example.amountDue = 42.0