之前在bind和apply以及call函数使用中详解总结过bind和apply以及call函数的使用，下面手写一下三个函数。
一、首先call函数

Function.prototype.MyCall = function (thisArg, ...args) {
  let fn = this  //this指的是当前函数
  thisArg = (thisArg === undefined || thisArg === null) ? window : Object(thisArg)
  thisArg.fn = fn
  args = args || [] //如果arg不存在，就将其设置为[],方便结构
  let res = thisArg.fn(...args)
  delete thisArg.fn  //执行完之后就删除该对象上的属性
  return res
}

二、手写apply函数

Function.prototype.MyApply = function (obj, arg) {
  let fn = this //this表示函数
  // 如果要是obj为undefined或者null时，设置其为window,
  // 如果是基本数据类型，则将其设置为对象类型
  let thisArg = (obj === undefined || obj === null) ? window : Object(obj)
  thisArg.fn = fn
  arg = arg || [] //如果arg不存在，则直接赋值为[]
  let res = thisArg.fn(...arg)
  delete thisArg.fn
  return res
}

三、手写bind函数

Function.prototype.MyBind = function(obj, ...args1) {
    let fn = this
    let thisArg = (obj === undefined || obj === null) ? window : Object(obj)
    
    return function(...args2) {
      thisArg.fn = fn
      let args = [...args1, ...args2]
      let result = thisArg.fn(...args)
      delete thisArg.fn
      return result
    }
}