什么是es5和es6

   let a = 10;
   var b = 1;
}
a // ReferenceError: a is not defined.
b // 1

var a = [];
for (var i = 0; i < 10; i++) {
  a[i] = function () {
    console.log(i);
  };
}
a[2](); // 10
a[6](); // 10

var a = [];
for (let i = 0; i < 10; i++) {
    a[i] = function () {
        console.log(i);
    }
}
a[2](); // 2
a[6](); // 6

for (let i = 0; i < 3; i++) {
    let i = &#39;abc&#39;;  // 和上一行for循环计数器的i不在同一作用域，所以可以声明成功。类比下面要讲的function(arg){let arg;}会报错，就是作用域问题
    console.log(i);
}
// abc
// abc
// abc

function func() {
  var a = 10;
  let a = 1;
}
func(); //Identifier 'a' has already been declared

function func() {
  let a = 10;
  let a = 1;
}
func(); //Identifier 'a' has already been declared
 
function func(arg) {
  let arg; 
}
func(); //Identifier 'arg' has already been declared
 
function func(arg) {
  {
    let arg; 
  }
}
func(); //不报错

console.log(foo); //undefined
var foo = 2;
console.log(bar); // ReferenceError: bar is not defined
let bar =  2;

var tmp = 123;
if(true){
    tmp = 'abc'; // ReferenceError: tmp is not defined
    let tmp;
}

var a = 1;
window.a; // 1
let b = 2;
window.b; // undefined

// 之前，为变量赋值，可以直接指定值
let a = 1;
let b = 2;
let c = 3;

// ES6中允许这样
let [a, b, c] = [1, 2, 3];  // 可以从数组中提取值，按照对应位置，对变量赋值。

let [foo, [[bar], baz]] = [1, [[2], 3]];
foo // 1
bar // 2
baz // 3
 
let [ , , third] = ["foo", "bar", "baz"];
third // "baz"
 
let [head, ...tail] = [1, 2, 3, 4];
head // 1
tail // [2, 3, 4]
 
let [x, y, ...z] = ['a'];
x // "a"
y // undefined  - 如果解构不成功，变量的值就等于 undefined。
z // []

let [a, [b], d] = [1, [2, 3], 4];
a // 1
b // 2
d // 4

let [foo] = 1;
let [foo] = false;
let [foo] = NaN;
let [foo] = undefined;
let [foo] = null;
let [foo] = {};

let [foo = true] = [];
foo;// true
 
let [x, y = 'b'] = ['a'];
x;//'a'
y;//'b'

let [x = 1] = [undefined];
x // 1

let [x = 1] = [null];
x // null - 默认值就不会生效，因为null不严格等于undefined

let [x = 1, y = x] = [1, 2];
x;//1
y;//2
 
let [x = y, y = 1] = [];// ReferenceError: y is not defined -- let的暂时性死区
let [x = y, y = 1] = [1, 2];

let { bar, foo } = { foo: "aaa", bar: "bbb" };
foo // "aaa"
bar // "bbb"

let { baz } = { foo: "aaa", bar: "bbb" };
baz // undefined

let { foo: baz } = { foo: 'aaa', bar: 'bbb' };
baz // "aaa"

let obj = { first: 'hello', last: 'world' };
let { first: f, last: l } = obj;
f // 'hello'
l // 'world'

let { foo: foo, bar: bar } = { foo: "aaa", bar: "bbb" };

let { foo: baz } = { foo: "aaa", bar: "bbb" };
baz // "aaa"
foo // error: foo is not defined

let {x, y = 5} = {x: 1};
x;// 1
y;// 5

let { message: msg = 'Something went wrong' } = {};
msg;//"Something went wrong"

let x;
{x} = {x:1};
//上面的代码会报错，因为JavaScript引擎会将 {x} 理解为一个代码块，从而发生语法错误。只有不将大括号写在行首，避免JavaScript将其解释为代码块，才能解决这个问题。
 
//正确的写法，将需要解构的部分用括号（）包起来
let x;
({x} = {x:1});

const [a, b, c, d, e] = 'hello';
a // "h"
b // "e"
c // "l"
d // "l"
e // "o"

let {length: len} = 'hello';
len;// 5

let {toString: s} = 123;
s === Number.prototype.toString // true

let {toString: s} = true;
s === Boolean.prototype.toString // true

function add([x, y]){
   return x + y;
}
 
add([1, 2]); // 3
 
[[1, 2], [3, 4]].map(([a, b]) => a + b);
// [ 3, 7 ]

function move({x=0, y=0}={}) {
  return [x, y];
}
move({x:3,y:8});// [3, 8]
move({x:3});// [3, 0]
move({});// [0, 0]
move();// [0, 0]

function move({x, y} = { x: 0, y: 0 }) {
  return [x, y];
}
 
move({x: 3, y: 8}); // [3, 8]
move({x: 3}); // [3, undefined]
move({}); // [undefined, undefined]
move(); // [0, 0]

let x = 1;
let y = 2;
[x, y] = [y, x];
x;// 2
y;// 1

//返回一个数组
function example() {
    return [1, 2, 3];
}
let [a, b, c] = example();
a;//1
b;//2
c;//3
 
//返回一个对象
function example(){
    return {
        foo: 1,
        bar: 2
    }
}
let {foo, bar} = example();
foo;//1
bar;//2

//参数是一组有序值
function f([x, y, z]) {}
f([1, 2, 3]);
 
//参数是一组无序值
function f({x, y, z}) {}
f({x: 1, y: 2, z: 3});

let data = {
    id: 1,
    status: 'ok',
    data: [867, 5612]
};
let {id, status, data:number} = data;
console.log(id, status, number);// 1 "ok"  [867, 5612]

const map = new Map();
map.set('first','hello');
map.set('second','world');
for (let [key, value] of map) {
    console.log(key + ' is ' + value );
}
//first is hello
//second is world

// 获取键名
for (let [key] of map) {}
 
// 获取键值
for (let [,value] of map) {}

function log(x, y) {
  y = y || 'World';
  console.log(x, y);
}

log('Hello') // Hello World
log('Hello', 'China') // Hello China
log('Hello', '') // Hello World

if (typeof y === 'undefined') {
  y = 'World';
}

function log(x, y = 'World') {
  console.log(x, y);
}

log('Hello') // Hello World
log('Hello', 'China') // Hello China
log('Hello', '') // Hello -- 上面说过，ES6 内部使用严格相等运算符(===)，判断一个位置是否有值。所以，只有当一个数组成员严格等于 undefined ，默认值才会生效。

function foo({x, y = 5}) {
  console.log(x, y);
}

foo({}) // undefined 5
foo({x: 1}) // 1 5
foo({x: 1, y: 2}) // 1 2
foo() // TypeError: Cannot read property 'x' of undefined

function foo({x, y = 5} = {}) {
  console.log(x, y);
}

foo() // undefined 5

function fetch(url, { body = '', method = 'GET', headers = {} }) {
  console.log(method);
}

fetch('http://example.com', {})
// "GET"

fetch('http://example.com')
// 报错

function fetch(url, { body = '', method = 'GET', headers = {} } = {}) {
  console.log(method);
}

fetch('http://example.com')
// "GET"

let greeting = `\`Yo\` World!`;  // `Yo` World -- 使用反斜杠转义反引号

// 多行字符串
$('#result').append(`
  There are <b>${basket.count}</b> items
   in your basket, <em>${basket.onSale}</em>
  are on sale!
`);

//普通字符串
`In JavaScript '\n' is a line-feed.`;
 `Hello ${'World'}`; // "Hello World"

//字符串中嵌入变量
let name = "Bob", time = "today";
console.log(`Hello ${name}, how are you ${time}?`); //  Hello Bob, how are you today?

//字符串中嵌入 JavaScript 表达式
`${x} + ${y} = ${x + y}`;// "1 + 2 = 3"
`${x} + ${y * 2} = ${x + y * 2}`;// "1 + 4 = 5"

let obj = {x: 1, y: 2};
`${obj.x + obj.y}`;// "3"

// 字符串中嵌入函数
function fn(){
    return "hello world"
}
`foo ${fn()} bar`;// "foo hello world bar"

// 字符串中嵌入对象
`${obj}`;// "[object Object]"

// 字符串中嵌入未声明的变量
 `Hello, ${place}`;// ReferenceError: place is not defined

var f = v => v;
//等同于
var f = function (v) {
    return v;
}

var f = () => 5;
//等同于
var f = function (v) {
    return 5
};

var sum = (num1, num2) =>  num1 + num2;
//等同于
var sum = function(num1, num2){
    return num1 + num2;
}

var sum = (num1,num2) => { return num1 + num2; }

// 报错
let getTempItem = id => { id: id, name: "Temp" };

// 不报错
let getTempItem = id => ({ id: id, name: "Temp" });
getTempItem(1);//{id: 1, name: "Temp"}

let getTempItem = id => {
    return { id: id, name: "Temp" };
}
getTempItem(1);// {id: 1, name: "Temp"}

function insert (value) {
    return {into: function (array){
            return {after: function (afterValue) {
                        array.splice(array.indexOf(ahterValue)+1, 0, value);
                        return array;
                    }
               }
        }    
    }
}
insert(2).into([1, 3]).after(1); //[1, 2, 3]

//使用箭头函数改写
let insert = (value) => ({into: (array) => ({after: (afterValue) => {
            array.splice(array.indexOf(afterValue)+1, 0, value);
            return array;
        }
    })
});

const full = ({ first, last }) => first + ' ' + last;
full({first: 1, last: 2});// "1 2"
// 等同于
function full(person) {
  return person.first + ' ' + person.last;
}
full({first: 1, last: 2});// "1 2"

const numbers = (...nums) => nums;
numbers(1,2,3,4);//  [1, 2, 3, 4]

const headAndTail = (head, ...tail) => [head, tail];
headAndTail(1,2,3,4);// [1,[2,3,4]]

const isEven = n => n%2 == 0;
isEven(3);// false
isEven(4);// true

[1, 2, 3].map( x => x*x ); //[1, 4, 9]
[3,2,5].sort((a, b) => a - b); // [2, 3, 5]

var  handler = {
    id: '123',
    init: function () {
        document.addEventListener('click', 
            event => this.doSomethisng(event.type), false);
    },
    doSomethisng: function (type) {
        console.log('Handling ' + type + ' for ' + this.id);
    }
};
handler.init();  //Handling click for 123  -- this指向handle  -- this是属于函数的一个对象，谁调用指向谁（es5中）

// ES6
function foo() {
  setTimeout(() => {
    console.log('id:', this.id);
  }, 100);
}

// ES5
function foo() {
  var _this = this;

  setTimeout(function () {
    console.log('id:', _this.id);
  }, 100);
}

console.log(...[1, 2, 3])  // 1 2 3
console.log(1, ...[2, 3, 4], 5)  // 1 2 3 4 5
[...document.querySelectorAll(&#39;div&#39;)]  // [<div>, <div>, <div>]

  array.push(...items);
}

function add(x, y) {
  return x + y;
}

const numbers = [4, 38];
add(...numbers) // 42

function f(v, w, x, y, z) { }
const args = [0, 1];
f(-1, ...args, 2, ...[3]);

// Uncaught SyntaxError: Unexpected number

console.log((...[1,2]))
// Uncaught SyntaxError: Unexpected number

console.log(...[1,2])  // 不会报错

// ES5 的写法
function f(x, y, z) {
  // ...
}
var args = [0, 1, 2];
f.apply(null, args);

// ES6的写法
function f(x, y, z) {
  // ...
}
let args = [0, 1, 2];
f(...args);

// ES5
const a1 = [1, 2];
const a2 = a1.concat();

// ES6
const a1 = [1, 2];
// 写法一
const a2 = [...a1];
// 写法二
const [...a2] = a1;

a2[0] = 2;
a1 // [1, 2]

const arr1 = ['a', 'b'];
const arr2 = ['c'];
const arr3 = ['d', 'e'];

// ES5 的合并数组
arr1.concat(arr2, arr3);
// [ 'a', 'b', 'c', 'd', 'e' ]

// ES6 的合并数组
[...arr1, ...arr2, ...arr3]
// [ 'a', 'b', 'c', 'd', 'e' ]

const a1 = [{ foo: 1 }];
const a2 = [{ bar: 2 }];

const a3 = a1.concat(a2);
const a4 = [...a1, ...a2];

a3[0] === a1[0] // true
a4[0] === a1[0] // true

const [first, ...rest] = [1, 2, 3, 4, 5];
first // 1
rest  // [2, 3, 4, 5]

const [first, ...rest] = [];
first // undefined
rest  // []

const [first, ...rest] = ["foo"];
first  // "foo"
rest   // []

const [...butLast, last] = [1, 2, 3, 4, 5]; // 报错

const [first, ...middle, last] = [1, 2, 3, 4, 5]; // 报错

[...'hello']  // [ "h", "e", "l", "l", "o" ]

function (){
   let arr = [...arguments];
}

const foo = 'bar';
const baz = {foo};
baz // {foo: "bar"}

// 等同于
const baz = {foo: foo};

function f(x, y) {
  return {x, y};
}

// 等同于
function f(x, y) {
  return {x: x, y: y};
}

f(1, 2) // Object {x: 1, y: 2}

const o = {
  method() {
    return "Hello!";
  }
};

// 等同于
const o = {
  method: function() {
    return "Hello!";
  }
};

let ms = {};

function getItem (key) {
  return key in ms ? ms[key] : null;
}

function setItem (key, value) {
  ms[key] = value;
}

function clear () {
  ms = {};
}

module.exports = { getItem, setItem, clear };
// 等同于
module.exports = {
  getItem: getItem,
  setItem: setItem,
  clear: clear
};

// 方法一
obj.foo = true;

// 方法二
obj['a' + 'bc'] = 123;

var obj = {
  foo: true,
  abc: 123
};

let propKey = 'foo';

let obj = {
  [propKey]: true,
  ['a' + 'bc']: 123
};

// mutation-types.js
export const SOME_MUTATION = 'SOME_MUTATION'

// store.js
import Vuex from 'vuex'
import { SOME_MUTATION } from './mutation-types'

const store = new Vuex.Store({
  state: { ... },
  mutations: {
    // 我们可以使用 ES2015 风格的计算属性命名功能来使用一个常量作为函数名
    [SOME_MUTATION] (state) {
      // mutate state
    }
  }
})

let ary = [1, 2, 3];
...ary // 1, 2, 3console.log(...ary); // 1 2 3,相当于下面的代码console.log(1,2,3);

// 方法一 let ary1 = [1, 2, 3];
let ary2 = [3, 4, 5];
let ary3 = [...ary1, ...ary2];// 方法二 ary1.push(...ary2);

//定义一个集合let arrayLike = {'0': 'a','1': 'b','2': 'c',
length: 3}; 
//转成数组let arr2 = Array.from(arrayLike); // ['a', 'b', 'c']

let arrayLike = { 
"0": 1,"1": 2,"length": 2}
let newAry = Array.from(arrayLike, item => item *2)//[2,4]注意：如果是对象，那么属性需要写对应的索引

let ary = [{
id: 1,
name: '张三'}, { 
id: 2,
name: '李四'}]; 
let target = ary.find((item, index) => item.id == 2);//找数组里面符合条件的值，当数组中元素id等于2的查找出来，注意，只会匹配第一个

let ary = [1, 5, 10, 15];
let index = ary.findIndex((value, index) => value > 9); 
console.log(index); // 2

[1, 2, 3].includes(2) // true [1, 2, 3].includes(4) // false

let ary = [10, 20, 50]; // 把20给删除
 // ary.splice(1, 1);
 // console.log(ary);
 // 把20删除，替换成30
 // ary.splice(1, 1, 30);
 // console.log(ary);
 // 在20前面加上15
 // ary.splice(1, 0, 15);
 // console.log(ary);
 // 在50前面加上30,40ary.splice(2, 0, 30, 40);
console.log(ary);

// 返回数组，包含了符合条件的所有元素。如果没有符合条件的元素则返回空数组
 // let result = ary.filter(item => item > 150);
 // console.log(result)

'x'.repeat(3) // "xxx" 'hello'.repeat(2) // "hellohello"

const s = new Set();

const set = new Set([1, 2, 3, 4, 4]);//{1, 2, 3, 4}

const s = new Set();
s.add(1).add(2).add(3); // 向 set 结构中添加值 s.delete(2) // 删除 set 结构中的2值 s.has(1) // 表示 set 结构中是否有1这个值 返回布尔值 s.clear() // 清除 set 结构中的所有值//注意：删除的是元素的值，不是代表的索引

s.forEach(value => console.log(value))