对象深拷贝

比较完善的版本：

const deepClone = (a, cache) => {
  if(!cache)
    cache = new Map()   
  if(a instanceof Object) {   // 不考虑跨 iframe
    if(cache.has(a))
      return cache.get(a);

    let result;
    if(a instanceof Function) {
      if(a.prototype)
        result = function(){ return a.apply(this, arguments) }
      else
        result = (...args) => { return a.call(undefined, ...args) }
    }
    else if(a instanceof Array)
      result = []
    else if(a instanceof Date)
      result = new Date(a)
    else if(a instanceof RegExp)
      result = new RegExp(a.source, a.flags)
    else
      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 = {
  number:1,
  bool:false,
  str: 'hi',
  empty1: undefined,
  empty2: null,
  array: [
    {name: 'frank', age: 18},
    {name: 'jacky', age: 19},
  ],
  date: new Date(2000, 0, 1, 20, 30, 0),
  regex: /\.(j|t)sx/i,
  obj: { name:'frank', 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

不完善的版本：

const copy = (obj)=>{
    const newObj = {}
    for(let key in obj){
        if(['string', 'number', 'boolean'].indexOf(typeof obj[key]) > -1 || [null, undefined].indexOf(obj[key]) > -1)
            newObj[key] = obj[key]
        else {
            newObj[key] = copy(obj[key])
        }    
    }
    return newObj
}

利用 JSON，不支持函数，undefined，正则等

const copy = (obj)=>{
    return JSON.parse(JSON.stringify(obj))
}