js手写源码

new

function defineNew(fn, ...args) {
    const obj = {}
    obj.__proto__ = fn.prototype
    fn.apply(obj, args)
    return obj
}

call

Function.prototype.gromyCall = function(target,...args){
    //此时this是调用者，Function
  let context = Symbol('this')  //避免key冲突
  target[context] = this   //把调用者的作用域地址送给目标的一个属性
  return target[context](...args)  //执行并返回
}

apply

Function.prototype.gromyApply = function(target,args){
  let context = Symbol('this');
  target[context] = this
  return target[context](...args)
}

bind

Function.prototype.gromyBind = function(target,...args){
  let context = Symbol('this')
  target[context] = this
  let result = function(){
      return target[context](...args)
  }
  if(this.prototype){  //保证
      result.prototype = this.prototype
  }
  return result
}