手写 bind,apply,call 和 new

实现 bind

Function.prototype.MyBind = function() {
  var _this = this
  var context = arguments.shift()
  var args = Array.prototype.slice.call(arguments)
  return function Fn() {
    var _args = [].concat(args, Array.prototype.slice.call(arguments))
    return _this.apply(context, _args)
  }
}

实现 apply

Function.prototype.MyApply = function() {
  var fn = arguments.shift()
  var args = Array.prototype.slice.call(arguments[0])
  fn.__this = this

  var result = fn.__this(...args)
  delete fn.__this
  return result
}

实现 call

Function.prototype.MyCall = function() {
  var fn = arguments.shift()
  var args = Array.prototype.slice.call(arguments)
  fn.__this = this

  var result = fn.__this(...args)
  delete fn.__this
  return result
}

实现 new

function MyNew (Fn, ...args) {
  var obj = Object.create(Fn.prototype)
  let result = Fn.apply(obj, args)
  return typeof result === 'object' ? result : obj
}