JavaScript中三个点(...)是什么意思?

三个点（...）是扩展运算符，是ES6语法，它可以在函数调用/数组构造时，将数组表达式或者string在语法层面展开；还可以在构造字面量对象时将对象表达式按照key-value的方式展开。说白了就是把数组中大括号（[]）、对象中花括号（{}）去掉！

//数组
var nums = [1, 2, 3, 4, 5]
console.log(...nums) 
//1 2 3 4 5

//对象
var person = { name: 'A', age: 30 }
console.log({ ...person }) 
// {name:'A',age:30}

它有什么用呢？

• 数组或对象的复制

//数组的复制
var arr1 = ['hello']
var arr2 = [...arr1]
console.log(arr2) 
// ['hello']

//对象的复制
var obj1 = { name: 'A' }
var obj2 = { ...obj1 }
console.log(obj2) 
// {name:'A'}

• 数组或对象合并

//数组的合并
var arr1 = ['hello']
var arr2 = ['world']
var mergeArr = [...arr1, ...arr2]
console.log(mergeArr)
// ['hello','world']

// 对象合并
var obj1 = { name: 'A' }
var obj2 = { age: 30 }
var mergeObj = { ...obj1, ...obj2 }
console.log(mergeObj) 
// {name: "A", age: 30}

• 字符转数组用它

var arr1 = [...'hello']
console.log(arr1) 
// ["h", "e", "l", "l", "o"]