new 理解

//实现一个new
var Dog=function(name){
this.name=name
}

Dog.prototype.wang=function(){
console.log("汪汪汪")
}
Dog.prototype.sayName=function(){
console.log("My name is"+this.name)
}

//实例化对象
let dongge=new Dog("冬哥");
dongge.sayName();//My name is 冬哥
dongge.wang(); //汪汪汪

new 的作用

创建一个新对象obj

把obj的 _ proto _ 指向Dog.prototype 实现继承

执行构造函数，传递参数，改变this指向 Dog.call(obj, ...args)

最后把obj赋值给dongge

var _new = function() {
  let constructor = Array.prototype.shift.call(arguments)
  let args = arguments
  const obj = new Object()
  obj.__proto__ = constructor.prototype
  constructor.call(obj, ...args)
  return obj
}
var dongge = _new(Dog, 'dongge')
dongge.bark()
dongge.sayName()
console.log(dongge instanceof Dog) // true