方法一，用JSON

const b = JSON.parse(JSON.stringify(a))

• 不支持Date、正则、undefined、函数等数据
• 不支持引用（即环状结构）

方法二，用递归

const deepClone = (a, cache) => {
  if(!cache){
    cache = new Map() // 缓存不能全局，最好临时创建并递归传递
  }
  if(a instanceof Object) {
    if(cache.get(a)){return cache.get(a)}
    let result
    if(a instanceof Function){
      if(a.prototype){// 有prototype就是普通函数
        result = function(){return a.apply(this, arguments)}
      }else { // 箭头函数
        result = (...args) => {return a.call(undefined, ...args)}
      }
    }else if(a instanceof Array){// 数组
      result []
    }else if(a instanceof Date){// 日期
      result = new Date(a - 0)
    }else if(a instanceof RegExp){// 正则
      result = new RegExp(a.source, a.flags)
    }else {// 普通对象
      result = {}
    }
    cache.set(a, result)
    for(let key in a){
      if(a.hasOwnProperty(key)){
        result[key] = deepClone(a[key], cache)
      }
    }
    return result
  }else {
    return a
  }
}

const a ={
  nnumber: 1, bool: false, str:'hi', empty1: undefined, enpty2: null, 
  array: [
    {name:'jack', age: 18}
    {name:'tom', age: 19}
  ],
  date: new Data(2000, 0, 1, 20, 30, 0),
  regex: /\.(j|t)sx/i,
  obj: {name:'jack', age: 18}
  f1: (a,b) => a + b,
  f2: function(a, b){return a + b}
}
a.self = a

const b =deepClone(a)

b.self === b // true
b.self = 'hi'
a.self !== 'hi' // true