es6新特性整理

ECMAScript6（ECMAScript 2015 ，ES5，ES2016）技术已经在前端圈子很流行了，他给前端开发人员带来了很多惊喜，提供的语法糖使复杂的操作变得简单。

本文没有详细描述这些新特性，因为网上都已经有很多相关的介绍了。主要针对ES6 新特性收集了相关范例代码，他可以让你快速了解这个新的javascript规范。

箭头函数

function() 函数的简写表示法，但它不绑定 this。

ES6 代码:

var odds = evens.map(v => v + 1);  // no parentes and no brackets
var nums = evens.map((v, i) => v + i);
var pairs = evens.map(v => ({even: v, odd: v + 1}));
 
// Statement bodies
nums.forEach(v => {
  if (v % 5 === 0)
    fives.push(v);
});

this 是如何工作的？

ES6 代码:

var object = {
    name: "Name", 
    arrowGetName: () => this.name,
    regularGetName: function() { return this.name },
    arrowGetThis: () => this,
    regularGetThis: function() { return this }
}
console.log(object);
console.log(object.name)
console.log(object.arrowGetName());
console.log(object.arrowGetThis());
console.log(this);
console.log(object.regularGetName());
console.log(object.regularGetThis());

结果：

console.log(object);  =>
Object {
    name: "Name",
    arrowGetName: function,
    regularGetName: function,
    arrowGetThis: function,
    regularGetThis: function
}

console.log(object.name) => 
Name

console.log(object.arrowGetName());  =>
undefined

console.log(object.arrowGetThis());  =>  [object Window] 
Window {
    stop: function,
    open: function,
    alert: function,
    confirm: function,
    prompt: function,
    ...
}

console.log(this);  => 同上

console.log(object.regularGetName());   =>
Name

console.log(object.regularGetThis());  =>
Object {
    name: "Name",
    arrowGetName: function,
    regularGetName: function,
    arrowGetThis: function,
    regularGetThis: function
}

我们知道“真正”语言中的类(Classes)。在 ES6 中类(Classes)其实是原型继承的语法糖。

ES6 代码:

class SkinnedMesh extends THREE.Mesh {
  constructor(geometry, materials) {
    super(geometry, materials);
 
    this.idMatrix = SkinnedMesh.defaultMatrix();
    this.bones = [];
    this.boneMatrices = [];
    //...
  }
  update(camera) {
    //...
    super.update();
  }
  get boneCount() {
    return this.bones.length;
  }
  set matrixType(matrixType) {
    this.idMatrix = SkinnedMesh[matrixType]();
  }
  static defaultMatrix() {
    return new THREE.Matrix4();
  }
}

增强的对象字面量

ES6 代码:

var theProtoObj = {
  toString: function() {
    return "The ProtoOBject To string"
  }
}
var handler = () => "handler"
var obj = {
    // __proto__
    __proto__: theProtoObj,
    // Shorthand for ‘handler: handler’
    handler,
    // Methods
    toString() {
     // Super calls
     return "d " + super.toString();
    },
    // Computed (dynamic) property names
    [ "prop_" + (() => 42)() ]: 42
};
 
console.log(obj.handler)
console.log(obj.handler())
console.log(obj.toString())
console.log(obj.prop_42)

结果：

obj.handler -> () => "handler"
obj.handler() -> handler
obj.toString() -> d The ProtoOBject To string
obj.prop_42 -> 
(() => 42)()  是一个立即执行函数 (() => 42)() 相当于
(function() {
    return 42;
})()
42

字符串插值

var name = "Bob", time = "today";
 
var multiLine = `This Line Spans Multiple Lines`；
 
console.log(`Hello ${name},how are you ${time}?`)
console.log(multiLine)

结果：

`Hello ${name},how are you ${time}?` -> Hello Bob,how are you today?
multiLine -> This Line Spans Multiple Lines

解构 Destructuring

// list "matching"
var [a, , b] = [1,2,3];
console.log(a);
console.log(b);

结果：

a -> 1
b -> 3

对象也能很好的解构

nodes = () => { return {op: "a", lhs: "b", rhs: "c"}}
var { op: a, lhs: b , rhs: c } = nodes()
console.log(a)
console.log(b)
console.log(c)

结果：

a -> a
b -> b
c -> c

使用速记表示法。

nodes = () => { return {lhs: "a", op: "b", rhs: "c"}}
 
// binds `op`, `lhs` and `rhs` in scope
var {op, lhs, rhs} = nodes()
 
console.log(op)
console.log(lhs)
console.log(rhs)

结果：

op -> b
lhs -> a
rhs -> c

可在参数位置使用

function g({name: x}) {
  return x
}
 
function m({name}) {
  return name
}
 
console.log(g({name: 5}))
console.log(m({name: 5}))

结果：

g({name: 5}) -> 5
m({name: 5}) -> 5

故障弱化解构

var [a] = []
var [b = 1] = []
var c = [];
console.log(a)
console.log(b);
console.log(c);

结果：

a -> undefined
b -> 1
c -> []

参数默认值（Default）

function f(x, y=12) {
  return x + y;
}
 
console.log(f(3))
console.log(f(3,2))

结果：

f(3) -> 15
f(3,2) -> 5

扩展（Spread）

在函数中：

function f(x, y, z) {
  return x + y + z;
}
// 传递数组的每个元素作为参数
console.log(f(...[1,2,3]))

结果：

f(...[1,2,3]) -> 6

在数组中：

var parts = ["shoulders", "knees"];
var lyrics = ["head", ...parts, "and", "toes"]; 
 
console.log(lyrics)

结果：

lyrics -> ["head","shoulders","knees","and","toes"]

扩展 + 对象字面量

我们可以使用这个创造很酷的对象。

let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }
 
// Spread properties
let n = { x, y, ...z };
console.log(n); // { x: 1, y: 2, a: 3, b: 4 }
console.log(obj)
可惜的是它还不支持：

npm install --save-dev babel-plugin-transform-object-rest-spread

Rest

我们可以使用 rest 操作符来允许无限参数。

function demo(part1, ...part2) {
    return {part1, part2}
}
 
console.log(demo(1,2,3,4,5,6))

结果：

demo(1,2,3,4,5,6) -> {"part1":1,"part2":[2,3,4,5,6]}

Let

let是新的var。 因为它有块级作用域。

{
   var globalVar = "from demo1"
}
 
{
   let globalLet = "from demo2";
}
 
console.log(globalVar)
console.log(globalLet)

结果：

globalVar -> from demo1
globalLet -> ReferenceError: globalLet is not defined

但是，它不会向window分配任何内容：

let me = "go";  // 全局作用域
var i = "able"; // 全局作用域
 
console.log(window.me); 
console.log(window.i);

结果：

window.me -> undefined
window.i -> able

不能使用let重新声明一个变量：

let me = "foo";
let me = "bar"; 
console.log(me);

结果：

SyntaxError: Identifier 'me' has already been declared

var me = "foo";
var me = "bar"; 
console.log(me)

结果：

me -> bar

Const

const 是只读变量。

const a = "b"
a = "a"

结果：

TypeError: Assignment to constant variable.

应该注意，const 对象仍然可以被改变的。

const a = { a: "a" }
a.a = "b"
console.log(a)

结果：

a -> {"a":"b"}

For..of

迭代器的新类型，可以替代for..in。 它返回的是值而不是keys。

let list = [4, 5, 6];
console.log(list)
for (let i in list) {
   console.log(i);
}

结果：

list -> [4,5,6]
i -> 0
i -> 1
i -> 2

let list = [4, 5, 6];
console.log(list)
for (let i of list) {
   console.log(i); 
}

结果：

list -> [4,5,6]
i -> 4
i -> 5
i -> 6

迭代器（Iterators）

迭代器是一个比数组更动态的类型。

let infinite = {
  [Symbol.iterator]() {
    let c = 0;
    return {
      next() {
        c++;
        return { done: false, value: c }
      }
    }
  }
}
 
console.log("start");
 
for (var n of infinite) {
  // truncate the sequence at 1000
  if (n > 10)
    break;
  console.log(n);
}

结果：

"start" -> start
n -> 1
n -> 2
n -> 3
n -> 4
n -> 5
n -> 6
n -> 7
n -> 8
n -> 9
n -> 10

使用Typescript，我们可以看到它接口的样子：

Typescript 代码:
interface IteratorResult {
  done: boolean;
  value: any;
}
interface Iterator {
  next(): IteratorResult;
}
interface Iterable {
  [Symbol.iterator](): Iterator
}

生成器（Generators）

生成器创建迭代器，并且比迭代器更具动态性。他们不必以相同的方式跟踪状态 并不支持 done 的概念。

var infinity = {
  [Symbol.iterator]: function*() {
    var c = 1;
    for (;;) {   
      yield c++;
    }
  }
}
 
console.log("start")
for (var n of infinity) {
  // truncate the sequence at 1000
  if (n > 10)
    break;
  console.log(n);
}

结果：

"start" -> start
n -> 1
n -> 2
n -> 3
n -> 4
n -> 5
n -> 6
n -> 7
n -> 8
n -> 9
n -> 10

使用Typescript 再次显示接口：

Typescript 代码:

interface Generator extends Iterator {
    next(value?: any): IteratorResult;
    throw(exception: any);
}

function* Iterators and generator

一个产量的例子*

function* anotherGenerator(i) {
  yield i + 1;
  yield i + 2;
  yield i + 3;
}
 
function* generator(i) {
  yield i;
  yield* anotherGenerator(i);
  yield i + 10;
}
 
var gen = generator(10);
 
console.log(gen.next().value); 
console.log(gen.next().value); 
console.log(gen.next().value); 
console.log(gen.next().value); 
console.log(gen.next().value);

结果：

gen.next().value -> 10
gen.next().value -> 11
gen.next().value -> 12
gen.next().value -> 13
gen.next().value -> 20

Unicode

ES6 为Unicode 提供了更好的支持。

var regex = new RegExp('\u{61}', 'u');
 
console.log(regex.unicode)
console.log("\uD842\uDFD7")
console.log("\uD842\uDFD7".codePointAt())

结果：

regex.unicode -> true
"" -> 
"".codePointAt() -> 134103

模块和模块加载器

原生支持模块。

import defaultMember from "module-name";
import * as name from "module-name";
import { member } from "module-name";
import { member as alias } from "module-name";
import { member1 , member2 } from "module-name";
import { member1 , member2 as alias2 , [...] } from "module-name";
import defaultMember, { member [ , [...] ] } from "module-name";
import defaultMember, * as name from "module-name";
import "module-name";

export { name1, name2, …, nameN };
export { variable1 as name1, variable2 as name2, …, nameN };
export let name1, name2, …, nameN; // also var
export let name1 = …, name2 = …, …, nameN; // also var, const
 
export expression;
export default expression;
export default function (…) { … } // also class, function*
export default function name1(…) { … } // also class, function*
export { name1 as default, … };
 
export * from …;
export { name1, name2, …, nameN } from …;
export { import1 as name1, import2 as name2, …, nameN } from …;
Import Export

Set

Set 为数学对应，其中所有项目都是唯一的。对于知道SQL的人来说，这相当于distinct。

var set = new Set();
set.add("Potato").add("Tomato").add("Tomato");
console.log(set.size)
console.log(set.has("Tomato"))
 
for(var item of set) {
   console.log(item)
}

结果：

set.size -> 2
set.has("Tomato") -> true
item -> Potato
item -> Tomato

WeakSet

WeakSet对象允许您在集合中存储弱持有的对象。没有引用的对象将被垃圾回收。

var item = { a:"Potato"}
var set = new WeakSet();
set.add({ a:"Potato"}).add(item).add({ a:"Tomato"}).add({ a:"Tomato"});
console.log(set.size)
console.log(set.has({a:"Tomato"}))
console.log(set.has(item))
 
for(let item of set) {
   console.log(item)
}

结果：

set.size -> undefined
set.has({a:"Tomato"}) -> false
set.has(item) -> true
TypeError: set[Symbol.iterator] is not a function

Map

Map 也称为词典。

var map = new Map();
map.set("Potato", 12);
map.set("Tomato", 34);
 
console.log(map.get("Potato"))
 
 
for(let item of map) {
   console.log(item)
}
 
 
for(let item in map) {
   console.log(item)
}

结果：

map.get("Potato") -> 12
item -> ["Potato",12]
item -> ["Tomato",34]

可以使用除字符串之外的其他类型。

var map = new Map();
var key = {a: "a"}
map.set(key, 12);

console.log(map.get(key))
console.log(map.get({a: "a"}))

结果：

map.get(key) -> 12
map.get({a: "a"}) -> undefined

WeakMap

使用键的对象，并且只保留对键的弱引用。

var wm = new WeakMap();

var o1  = {}
var o2  = {}
var o3  = {}
wm.set(o1, 1);
wm.set(o2, 2);
wm.set(o3, {a: "a"});
wm.set({}, 4);
 
console.log(wm.get(o2));
console.log(wm.has({}))
 
delete o2;
 
console.log(wm.get(o3));
 
for(let item in wm) {
   console.log(item)
}
 
 
for(let item of wm) {
   console.log(item)
}

结果：

wm.get(o2) -> 2
wm.has({}) -> false
wm.get(o3) -> {"a":"a"}
TypeError: wm[Symbol.iterator] is not a function

代理（Proxy）

代理可以用来改变对象的行为。 它们允许我们定义 trap 。

var obj = function ProfanityGenerator() {
    return {
       words: "Horrible words"    
    }
}()
 
var handler = function CensoringHandler() {
        return {
        get: function (target, key) {
            return target[key].replace("Horrible", "Nice");
        },
    }
 
}()
 
var proxy = new Proxy(obj, handler);
 
console.log(proxy.words);

结果：

proxy.words -> Nice words

提供以下 trap ：

var handler =
{
  get:...,
  set:...,
  has:...,
  deleteProperty:...,
  apply:...,
  construct:...,
  getOwnPropertyDescriptor:...,
  defineProperty:...,
  getPrototypeOf:...,
  setPrototypeOf:...,
  enumerate:...,
  ownKeys:...,
  preventExtensions:...,
  isExtensible:...
}

Symbols 是一个新类型。 可用于创建匿名属性。

var typeSymbol = Symbol("type");
 
class Pet {
 
  constructor(type) {
 
    this[typeSymbol] = type;
 
  }
  getType() {
     return this[typeSymbol];
  }
 
}
 
 
var a = new Pet("dog");
console.log(a.getType());
console.log(Object.getOwnPropertyNames(a))
 
 
console.log(Symbol("a") === Symbol("a"))

结果：

a.getType() -> dog
Object.getOwnPropertyNames(a) -> []
Symbol("a") === Symbol("a") -> false

可继承内置函数

我们现在可以继承原生类。

class CustomArray extends Array {
 
}
 
var a = new CustomArray();
 
a[0] = 2
console.log(a[0])

结果：

a[0] -> 2

不能使用数组的代理(Proxy)来覆盖getter函数。

新类库

各种新的方法和常量

console.log(Number.EPSILON)
console.log(Number.isInteger(Infinity))
console.log(Number.isNaN("NaN"))
 
console.log(Math.acosh(3))
console.log(Math.hypot(3, 4))
console.log(Math.imul(Math.pow(2, 32) - 1, Math.pow(2, 32) - 2))
 
console.log("abcde".includes("cd") )
console.log("abc".repeat(3) )
 
 
console.log(Array.of(1, 2, 3) )
console.log([0, 0, 0].fill(7, 1) )
console.log([1, 2, 3].find(x => x == 3) )
console.log([1, 2, 3].findIndex(x => x == 2)) 
console.log([1, 2, 3, 4, 5].copyWithin(3, 0)) 
console.log(["a", "b", "c"].entries() )
console.log(["a", "b", "c"].keys() )
console.log(["a", "b", "c"].values() )
 
console.log(Object.assign({}, { origin: new Point(0,0) }))

结果：

Number.EPSILON -> 2.220446049250313e-16
Number.isInteger(Infinity) -> false
Number.isNaN("NaN") -> false
Math.acosh(3) -> 1.7627471740390859
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.of(1, 2, 3) -> [1,2,3]
[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() -> {}
["a", "b", "c"].keys() -> {}
["a", "b", "c"].values() -> TypeError: ["a","b","c"].values is not a function
Object.assign({}, { origin: new Point(0,0) }) -> ReferenceError: Point is not defined
文档: Number, Math, Array.from, Array.of, Array.prototype.copyWithin, Object.assign

二进制和八进制

二进制和八进制数字的字面量。

console.log(0b11111)
console.log(0o2342)
 
console.log(0xff); // also in es5

结果：

ES6 代码:
0b11111 -> 31
0o2342 -> 1250
0xff -> 255

Promises

异步编程。

var p1 = new Promise((resolve, reject) => {
  setTimeout(() => resolve("1"), 101)
})
var p2 = new Promise((resolve, reject) => {
  setTimeout(() => resolve("2"), 100)
})
 
Promise.race([p1, p2]).then((res) => {
   console.log(res)
})
 
Promise.all([p1, p2]).then((res) => {
   console.log(res)
})

结果：

res -> 2
res -> ["1","2"]

快速的 Promise

var p1 = Promise.resolve("1")
var p2 = Promise.reject("2")
 
Promise.race([p1, p2]).then((res) => {
   console.log(res)
})

结果：

res -> 1

快速失败

如果一个 promise 失败，all和race也将 reject(拒绝)。

var p1 = new Promise((resolve, reject) => {
  setTimeout(() => resolve("1"), 1001)
})
var p2 = new Promise((resolve, reject) => {
  setTimeout(() => reject("2"), 1)
})
 
Promise.race([p1, p2]).then((res) => {
   console.log("success" + res)
}, res => {
   console.log("error " + res)
})
 
Promise.all([p1, p2]).then((res) => {
   console.log("success" + res)
}, res => {
   console.log("error " + res)
})

结果：

"error " + res -> error 2
"error " + res -> error 2

反射（Reflect）

新类型的元编程与新的API现有的还有一些新的方法。

var z = {w: "Super Hello"}
var y = {x: "hello", __proto__: z};
 
console.log(Reflect.getOwnPropertyDescriptor(y, "x"));
console.log(Reflect.has(y, "w"));
console.log(Reflect.ownKeys(y, "w"));
 
console.log(Reflect.has(y, "x"));
console.log(Reflect.deleteProperty(y,"x"))
console.log(Reflect.has(y, "x"));

结果：

Reflect.getOwnPropertyDescriptor(y, "x") -> {"value":"hello","writable":true,"enumerable":true,"configurable":true}
Reflect.has(y, "w") -> true
Reflect.ownKeys(y, "w") -> ["x"]
Reflect.has(y, "x") -> true
Reflect.deleteProperty(y,"x") -> true
Reflect.has(y, "x") -> false

尾调用（Tail Call）优化

尾调用的概念非常简单，一句话就能说清楚，就是指某个函数的最后一步是调用另一个函数。

ES6可以确保尾调用不会造成堆栈溢出。 （不是所有的实现工作）。

function factorial(n, acc = 1) {
    if (n <= 1) return acc; 
    return factorial(n - 1, n * acc);
}
console.log(factorial(10))
console.log(factorial(100))
console.log(factorial(1000))
console.log(factorial(10000))
console.log(factorial(100000))
console.log(factorial(1000000))

结果：

factorial(10) -> 3628800
factorial(100) -> 9.332621544394418e+157
factorial(1000) -> Infinity
factorial(10000) -> Infinity
factorial(100000) -> RangeError: Maximum call stack size exceeded
factorial(1000000) -> RangeError: Maximum call stack size exceeded

原文：ES6 Features