第 14 题：情人节福利题，如何实现一个 new

// 实现一个new
var Dog = function (name) {
  this.name = name;
};
Dog.prototype.bark = function () {
  console.log("wangwang");
};
Dog.prototype.sayName = function () {
  console.log("my name is " + this.name);
};
let sanmao = new Dog("三毛");
sanmao.sayName();
sanmao.bark();
// 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sanmao
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 simao = _new(Dog, "simao");
simao.bark();
simao.sayName();
console.log(simao instanceof Dog); // true

简洁写法：

function _new(fn, ...arg) {
  const obj = Object.create(fn.prototype);
  const ret = fn.apply(obj, arg);
  return ret instanceof Object ? ret : obj;
}