this 指向什么 · 八股题库 / Drills

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一句话：this 是调用时决定的，不是定义时。看「谁调用的」。

四条规则，按优先级从高到低—— 这个顺序是标准答案：

new 绑定——new Foo()，this 是新创建的对象。
显式绑定——call / apply /bind 指定的那个。
隐式绑定——obj.fn()，this 是 obj（看点号左边）。
默认绑定—— 都不满足时，严格模式下是 undefined，否则是 window / global。

箭头函数是例外，它不参与这四条—— 它没有自己的 this，用的是定义时外层作用域的 this，而且 call / bind改不了它。

最经典的陷阱：隐式丢失。const fn = obj.method 之后单独调fn()，点号没了，this 就丢了。把方法当回调传出去（setTimeout(obj.method)、onClick={this.handle}）都是这个问题 ——这就是 React 类组件必须在构造器里bind 的原因。

会追问：「DOM 事件回调里 this 是什么？」—— 普通函数是绑定事件的那个元素（等于 e.currentTarget），箭头函数则是外层的 this。

In one line: this is decided when the function is called, not where it was written. Look at who called it.

Four rules, highest priority first — this order is the model answer:

new binding — with new Foo(), this is the object that was just created.
Explicit binding — whatever you handed to call / apply / bind.
Implicit binding — obj.fn() makes this the obj (look left of the dot).
Default binding — when none of the above applies: undefined in strict mode, otherwise window / global.

Arrow functions are the exception; they do not play by those four rules — an arrow has no this of its own, it uses the this of the scope it was defined in, and call / bind cannot change that.

The classic trap: the implicit binding gets lost. Do const fn = obj.method and then call fn() on its own — the dot is gone, so this is gone. Handing a method off as a callback (setTimeout(obj.method), onClick={this.handle}) is the same bug — and it is exactly why React class components had to bind in the constructor.

Follow-up: “What is this inside a DOM event handler?” — in a normal function it is the element the listener is attached to (the same as e.currentTarget); in an arrow function it is the outer this.

JavaScript四条规则与隐式丢失The four rules, and losing the implicit binding示意Illustrative

1const obj = {

2 label: "obj",

3 show() { console.log(this.label); },

4};

6obj.show(); // "obj" 隐式绑定，看点号左边

7const f = obj.show;

8f(); // 不是 "obj"：严格模式（模块、class）下 this 是 undefined，直接抛 TypeError；非严格时是全局对象，打印 undefined

9f.call({ label: "call" }); // "call" 显式绑定

10setTimeout(obj.show, 0); // undefined 传出去就丢了

11setTimeout(() => obj.show(), 0); // "obj" 包一层就保住了

13// 箭头函数不参与规则，call 也改不了

14const arrow = () => console.log(this);

15arrow.call({ a: 1 }); // 还是外层的 this

17// React 类组件为什么要 bind

18class Btn extends React.Component {

19 constructor(p) {

20 super(p);

21 this.handle = this.handle.bind(this); // 不 bind，onClick 里 this 就是 undefined

22 }

23 handle() { console.log(this.props); }

24}

1const obj = {

2 label: "obj",

3 show() { console.log(this.label); },

4};

6obj.show(); // "obj" implicit binding: look left of the dot

7const f = obj.show;

8f(); // not "obj": in strict mode (modules, classes) this is undefined and it throws a TypeError; in sloppy mode this is the global object and it prints undefined

9f.call({ label: "call" }); // "call" explicit binding

10setTimeout(obj.show, 0); // undefined pass it out and the binding is gone

11setTimeout(() => obj.show(), 0); // "obj" one wrapper keeps it

13// An arrow function ignores these rules, and call cannot change it either

14const arrow = () => console.log(this);

15arrow.call({ a: 1 }); // still the outer this

17// Why a React class component needs bind

18class Btn extends React.Component {

19 constructor(p) {

20 super(p);

21 this.handle = this.handle.bind(this); // without bind, this is undefined in onClick

22 }

23 handle() { console.log(this.props); }

24}