为什么TypeScript中的"instanceof"会给我错误“"Foo"仅指代一种类型，但在此处被用作值".

问题描述

我写了这段代码

interface Foo {
    abcdef: number;
}

let x: Foo | string;

if (x instanceof Foo) {
    // ...
}

但是TypeScript给了我这个错误:

But TypeScript gave me this error:

'Foo' only refers to a type, but is being used as a value here.

为什么会这样?我以为instanceof可以检查我的值是否具有给定类型，但是TypeScript似乎不喜欢这样.

Why is this happening? I thought that instanceof could check whether my value has a given type, but TypeScript seems not to like this.

推荐答案

instanceof适用于类，而不适用于接口.

instanceof works with classes, not interfaces.

问题在于 instanceof 是JavaScript的构造，在JavaScript中，instanceof期望右侧操作数为 value .具体来说，在x instanceof Foo中，JavaScript将执行运行时检查，以查看Foo.prototype原型链中是否存在Foo.prototype.

The issue is that instanceof is a construct from JavaScript, and in JavaScript, instanceof expects a value for the right-side operand.Specifically, in x instanceof Foo JavaScript will perform a runtime check to see whether Foo.prototype exists anywhere in the prototype chain of x.

但是，在TypeScript中，interface没有发出.这意味着Foo和Foo.prototype在运行时都不存在，因此该代码肯定会失败.

However, in TypeScript, interfaces have no emit. That means that neither Foo nor Foo.prototype exist at runtime, so this code will definitely fail.

TypeScript试图告诉您这可能永远不会. Foo只是一种类型，根本不是值！

TypeScript is trying to tell you this could never work. Foo is just a type, it's not a value at all!

您可以查看类型保护器和用户定义的类型保护器.

您可能会很想从interface切换到class，但是您应该意识到，在TypeScript的结构类型系统(主要基于 shape 的结构)中，您可以生成任何与给定类具有相同形状的对象:

You might be tempted to switch from an interface to a class, but you should realize that in TypeScript's structural type system (where things are primarily shape based), you can produce any an object that has the same shape as a given class:

class C {
    a: number = 10;
    b: boolean = true;
    c: string = "hello";
}

let x = new C()
let y = {
    a: 10, b: true, c: "hello",
}

// Works!
x = y;
y = x;

在这种情况下，您的x和y具有相同的类型，但是如果您尝试在任何一个上使用instanceof，则在另一个上会得到相反的结果.因此，如果您要利用TypeScript中的结构类型，instanceof不会真正告诉您有关类型的更多信息.

In this case, you have x and y that have the same type, but if you try using instanceof on either one, you'll get the opposite result on the other. So instanceof won't really tell you much about the type if you're taking advantage of structural types in TypeScript.

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