手写new实现原理

function modelNew(Parent, ...test) {
  if(typeof Parent !== 'function'){
      throw 'newOperator function the first param must be a function';
    }
  // 1.创建一个空对象
  let target = {};

  // 2.把Parent的this指向target
  target.__proto__ = Parent.prototype;

  // 3.执行parent的代码为target添加属性,如果构造函数有返回结果，则接收
  let result = Parent.apply(target, test)
  
  if (typeof result === "object" || typeof result === "function" && result !== null) {
    return result
  }
  
  // 4.返回这个对象
  return target
}

function Person(name, age) {
  this.name = name;
  this.age = age;
}

Person.prototype.sayName = function () {
  console.log(this.name);
}

let child = modelNew(Person, 'andy', 21);
console.log(child); // Person {name: "andy", age: 21}
child.sayName() // 'andy'