ES6扩展运算符使用。

1.基本使用

　　数组

let arr = [1,2,3,4,5];
let arr2 = [...arr];

console.log(arr2);
//[ 1, 2, 3, 4, 5 ]
console.log(arr2 == arr);
//false

　　对象

let obj = {a:1,b:2};
let obj2 = {...obj};
console.log(obj2)；
//{ a: 1, b: 2 }
console.log(obj == obj2)；
//false

　　字符串转数组

let str = "hello";
let str2 = [...str];
console.log(str2);
//[ 'h', 'e', 'l', 'l', 'o' ]

2.函数传值；

function fn(a,b,c){
    console.log(a)
    console.log(b)
    console.log(c)
}
let arr = [1,2,3];
//ES5
// fn(arr[0],arr[1],arr[2]);
// fn.apply(null,arr);
//ES6
fn(...arr);
//
1
2
3

3.数组拷贝

let arr = [1,2,3,4];
let arr2 = [...arr];
arr2.push(5);

console.log(arr);
//[ 1, 2, 3, 4 ]
console.log(arr2);
//[ 1, 2, 3, 4, 5 ]
console.log(arr == arr2);
//false

4.对象拷贝

let obj = {a:1,b:2};
let obj2 = {...obj};
obj2.c = 3;
console.log(obj);
//{ a: 1, b: 2 }
console.log(obj2);
//{ a: 1, b: 2, c: 3 }
console.log(obj == obj2);
//false

5.代替数组的unshift方法

let arr = [1,2,3];
let arr2 = [4,5,...arr];
console.log(arr);
//[ 1, 2, 3 ]
console.log(arr2);
[ 4, 5, 1, 2, 3 ]
console.log(arr == arr2);
false

6.代替数组的push方法

let arr = [1,2,3];
let arr2 = [...arr,4,5,"a",null,undefined,function(){},Symbol];
console.log(arr);
//[ 1, 2, 3 ]
console.log(arr2);
[ 1, 2, 3, 4, 5, 'a', null, undefined, [Function], [Function: Symbol] ]
console.log(arr == arr2);
//false

7.配合解构使用

　　数组

let [a,...arr] = [1,2,3,4,5];
console.log(a);
//1
console.log(arr)；
//[ 2, 3, 4, 5 ]

　　对象

let {a,...obj} = {a:1,b:"1",c:function(){},d:null,e:undefined,e:Symbol};
console.log(a);
//1
console.log(obj);
//{ b: '1', c: [Function: c], d: null, e: [Function: Symbol] }

8.总结

　　放在被赋值一方为rest运算符，放在赋值一方为扩展运算符