展开语法（Spread syntax）

参考文档：https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Operators/Spread_syntax

合并数组：

const first = [1, 2, 3];
const second = [4, 5, 6];

[...first, ...second]; // [1, 2, 3, 4, 5, 6]

合并数组的同时插入元素：

['a', ...first, ...second]; // ["a", 1, 2, 3, 4, 5, 6]

['a', ...first, 'b', ...second, 'c']; // ["a", 1, 2, 3, "b", 4, 5, 6, "c"]

复制数组：

[...first]; // [1, 2, 3]

合并对象：

const first = {name: 'tom'};
const second = {age: 18};

{...first, ...second}; // {name: "tom", age: 18}

合并对象并插入新属性：

{...first, ...second, location: 'china'}; // {name: "tom", age: 18, location: "china"}

复制对象：

{...first}; // {name: "tom"}