es6学习笔记(一)

1 var arr=[]
2 for(var i=0;i<5;i++){
3     arr[i]=function(){
4         console.log(i);
5     }  
6 }
7 arr[3](); //6

1 const arr = Object.freeze([‘apple‘,‘banana‘])
2 
3 arr.push(‘x‘); //报错
4 
5 const arr1 = [1,2]
6 
7 arr1.push(3); //正确操作

let a=12,b=5,c=6;
let arr=[12,5,6]
//解构赋值将以上两者结合
let [a,b,c] = [12,5,6]
console.log(a); //12
console.log(b); //5
console.log(c); //6
//解构赋值左右两边格式要保持一致
let json={
    name:‘x‘,
    age:18,
    job:‘y‘
}
let {name,age,job} = json;
console.log(name) //x
console.log(age) //18
console.log(job) //y

//别名
let {name,age,job:a}=json //a为job的别名
console.log(a) //y

//默认值
let [a,b,c=0] = [1,2]
console.log(a,b,c); //1,2,0

1 let x=1,y=2;
2 console.log(`abcd${x}efdh${y}`); //‘abcd1efdh2‘

 1 //字符串查找
 2 let str="apple pear pitch";
 3 
 4 str.indexOf(‘apple‘); //0
 5 
 6 str.includes(‘apple‘); //true
 7 
 8 //判断浏览器是否为Chrome
 9 if(navigator.userAgent.includes(‘chrome‘)){
10   //……  
11 }
12 
13 str.startsWith(‘apple‘); //true
14 
15 sre.endWidth(‘apple‘); //false
16 
17 let a=‘x‘;
18 a.repeat(3); //‘xxx‘
19 
20 a=‘09‘;
21 a.padStart(5,‘0‘); //00009
22 a.padEnd(5,‘0‘); //09000

 1 //默认参数
 2 function show(a,b){//es5
 3   a = a || 1
 4   b = b || 2
 5   //....  
 6 }
 7 function show(a=1,b=2){//es6
 8 //...
 9 }
10 function test({x=0,y=0}){
11     console.log(x,y)
12 }
13 test({}) //0 0
14 function test1({x=0,y=0}={}){
15 
16 }

1 //默认参数已定义的情况
2 function show(a=5){
3     let a=10  //报错
4     console.log(a);
5 }

//扩展运算符 ... 作用：[展开数组，剩余参数]
let arr = [‘x‘,‘y‘,‘z‘]
console.log(...arr)  //展开x,y,z

//优化arguments转数组
function show(...a){
     console.log(a); //[1,2,3,4,5]
}
show(1,2,3,4,5)

//es5伪数组转数组
Array.prototype.slice.all(null,arguments)

//箭头函数
()=>{ //...  }
let json = {
    id:1,
    show:function(){
        setTimeout(function(){
            console.log(this.id);//this指向window
        },2000);      
    }
}

let json1 = {
    id:1,
    show:()=>{
        setTimeout(function(){
            console.log(this.id);//this指向定义时对象，window
        })
    }
}

//复制数组
let arr = [1,2,3]
let arr1 = [...arr]

//伪数组转数组
let arr2 = Array.from(fake)
let arr3 = [...fake]

//es5
[].slcie.call(fake)

let obj = {
  0:8,
  1:9,
  2:10,
  length:3
}
let a1 = Array.from(obj);
console.log(a1) //[8,9,10]

let a2 = Array.of(1,2,3,4,5); //[1,2,3,4,5]

a1.find((val,index,arr)=>{
    return val>8;
});//9

 1 //对象的简洁语法
 2 let name = ‘name‘;
 3 let age=18
 4 let obj = {
 5     name,
 6     age,
 7     showA(){//注意这里千万不要用箭头函数
 8         return this.name
 9     }
10 }
11 
12 //Object.is()  用于比较两个值是否相等
13 NaN == NaN //false
14 Object.is(NaN,NaN) //true
15 +0 == -0 //true
16 Object.is(+0,-0) //false
17 
18 //Object.assign()用于合并对象
19 Object.assign(target,source1,source2,...)