TypeScript "this" 范围问题

Question

我的代码片段：

class MyClass {
    name = "MyClass";

    // traditional method definition
    getName1(){
        return this.name;
    }

    // method defined as an arrow function
    getName2 = () => {
        return this.name;
    }

    // traditional method definition using a this parameter
    getName3(this: MyClass){
        return this.name;
    }
}

const c = new MyClass();
const obj = {
    name: "obj",
    getName1: c.getName1,
    getName2: c.getName2,
    getName3: c.getName3
};

const g = c.getName3;

console.log(c.getName1());
// "MyClass" because this is called in the context of c
// using a traditional method definition getName1

console.log(c.getName2());
// "MyClass" because this is called in the context of c
// using an arrow function getName2

console.log(c.getName3());
// "MyClass" because this is called in the context of c
// using a traditional method definition getName3, having
// a this parameter that statically enforce the context of c

console.log(obj.getName1()); 
// "obj" because this is called in the context of obj
// using the MyClass traditional method definition getName1()
// and assigning it to obj's getName1 property

console.log(obj.getName2()); 
// "MyClass" because this is called in the context of obj
// using an arrow function getName2 and assigning it 
// to obj's getName2 property, but which will preserve c's context
// nonetheless

console.log(obj.getName3()); 
// "obj" but why? Isn't the context of this enforced as that of MyClass? Shouldn't I get
// « The 'this' context of type 'obj' is not assignable to method's 'this' of type 'MyClass' »

console.log(g()); 
// Error, the 'this' context of type 'void' is not assignable to method's 'this' of type 'MyClass'.
// but isn't g declared in the global this context?

我的问题在评论中突出显示，但我还是将它们列如下：

为什么console.log(obj.getName3());显示"obj"？ this 的上下文是否与 MyClass 的上下文一样强制执行？我不应该得到类似的东西吗：

The 'this' context of type 'obj' is not assignable to method's 'this' of type 'MyClass'

我知道 g 不是在 object/class 的上下文中声明的，而是在全局对象上下文中声明的，因此 this 应该指globalThis？如果是这样，那为什么我会得到以下信息：

The 'this' context of type 'void' is not assignable to method's 'this' of type 'MyClass'.

Answer 1

我认为打字稿将“外部”对象推断为 this，因此您可以从 obj.getName3() 获得“obj”，这没问题，因为它也与 class 匹配。它有名称 attr 和所有方法，所以没问题。

如果您将 obj.name 更改为 obj.noname（或 fn 名称之一），您将无法调用 obj.getName3()，因为 obj 不再与 class 匹配.

g() 也是如此。 g 的外部对象是 void 并且没有名称属性（和 fns），因此您会收到其他错误消息。

阅读更多：https://www.typescriptlang.org/docs/handbook/2/functions.html#declaring-this-in-a-function

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