# new 实现原理

#### 原理

1. 创建一个空对象，构造函数中的 `this` 指向这个空对象
2. 这个新对象被执行 `[[原型]]` 连接
3. 执行构造函数方法，属性和方法被添加到 `this` 引用的对象中
4. 如果构造函数中没有返回其它对象，那么返回 `this`，即创建的这个的新对象，否则，返回构造函数中返回的对象

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

`Object.create()` 方法创建一个新对象，使用现有的对象来提供新创建的对象的 `__proto__`


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