手动实现new操作符原理详解

function myNew(Constructor, ...args) {
  // 1. 创建空对象
  const obj = {};

  // 2. 设置原型链：obj.__proto__ = Constructor.prototype
  Object.setPrototypeOf(obj, Constructor.prototype);

  // 3. 执行构造函数，绑定 this 并传参
  const result = Constructor.apply(obj, args);

  // 4. 判断返回值：如果是对象或函数，且不为 null，则返回它；否则返回 obj
  if (result !== null && (typeof result === 'object' || typeof result === 'function')) {
    return result;
  }

  return obj;
}

function myNew(Constructor, ...args) {
  const obj = Object.create(Constructor.prototype);
  const result = Constructor.apply(obj, args);
  if (result != null && (typeof result === 'object' || typeof result === 'function')) {
    return result;
  }
  return obj;
}

function Person(name) {
  this.name = name;
}
Person.prototype.say = function() { return 'Hi'; };

const p = myNew(Person, 'Alice');
console.log(p.name);     // 'Alice'
console.log(p.say());    // 'Hi'
console.log(p instanceof Person); // true