为什么 TypeScript 中的 'instanceof' 给我错误“'Foo' 仅指一种类型，但在此处用作值."? [英] Why does 'instanceof' in TypeScript give me the error "'Foo' only refers to a type, but is being used as a value here."?

问题描述

我写了这段代码

interface Foo {abcdef:数字；}让 x: Foo |细绳;如果(x 实例的 Foo){//...}

但是 TypeScript 给了我这个错误:

'Foo' 仅指一种类型，但在此处用作值.

为什么会这样?我认为 instanceof 可以检查我的值是否具有给定的类型，但 TypeScript 似乎不喜欢这样.

解决方案

instanceof 使用类，而不是接口.

发生了什么

问题在于 instanceof 是来自 JavaScript 的构造，在 JavaScript 中，instanceof 期望右侧操作数有一个 value.具体来说，在 x instanceof Foo 中，JavaScript 将执行运行时检查以查看 Foo.prototype 是否存在于 x 的原型链中.>

然而，在 TypeScript 中，interfaces 没有发射.这意味着 Foo 和 Foo.prototype 在运行时都不存在，所以这段代码肯定会失败.

TypeScript 试图告诉您这永远行不通.Foo 只是一个类型，它根本不是一个值！

"我可以做什么来代替 instanceof?"

您可以查看类型保护和用户定义的类型保护.

"但是如果我只是从 interface 切换到 class 呢?"

您可能很想从 interface 切换到 class，但您应该意识到，在 TypeScript 的结构类型系统中(其中的事情主要是基于形状的)，您可以生成与给定类具有相同形状的任何对象:

class C {a:数字=10；b: 布尔值 = 真；c:字符串=你好"；}让 x = 新 C()让 y = {a: 10, b: true, c: 你好",}//有效！x = y;y = x;

在这种情况下，您的 x 和 y 具有相同的类型，但是如果您尝试在其中任何一个上使用 instanceof，您会得到相反的结果.因此，如果您利用 TypeScript 中的结构类型，instanceof 不会真正告诉您关于类型的太多信息.

I wrote this code

interface Foo {
    abcdef: number;
}

let x: Foo | string;

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

But TypeScript gave me this error:

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

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

解决方案

instanceof works with classes, not interfaces.

What's going on

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.

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

"What can I do instead of instanceof?"

You can look into type guards and user-defined type guards.

"But what if I just switched from an interface to a class?"

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;

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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