用于對象賦值

可以傳入多個對象用于合并，同一個屬性key，后傳入的對象的value會覆蓋前面的。

example

let obj1 = {key1: 1, dupkey: 999};
let obj2 = {key2: 2, dupkey: 888};

let obj = Object.assign(obj1, obj2);
console.log(obj);
//{key1: 1, dupkey: 888, key2: 2}
console.log(obj1);
//{key1: 1, dupkey: 888, key2: 2}

也就是說，assign不僅會返回合并后的多個對象，還會對傳入的第一個對象進行覆蓋。

所以如果需要不覆蓋原有參數(shù)，兩種寫法：

let obj1 = {key1: 1};
let obj2 = {key2: 2};
let obj = Object.assign({}, obj1, obj2);
console.log(obj);
//{key1: 1, key2: 2}
console.log(obj1);
//{key1: 1}

let obj1 = {key1: 1};
let obj2 = {key2: 2};
let obj = {};
Object.assign(obj, obj1, obj2);
console.log(obj);
//{key1: 1, key2: 2}
console.log(obj1);
//{key1: 1}