为什么枚举上的 switch 语句会抛出“无法与类型相比"的错误?

问题描述

在做关于 Typescript 的 Lynda 教程时 (https://www.lynda.com/Visual-Studio-tutorials/TypeScript-types-part-2/543000/565613-4.html#tab )，我遇到了障碍.示例代码说明了 switch 语句在 TypeScript 中是如何工作的，但似乎对教师来说工作正常的代码抛出了一个 Type 'x' 与 type 'y' 不具有可比性的错误.代码如下:

While doing a Lynda tutorial on Typescript ( https://www.lynda.com/Visual-Studio-tutorials/TypeScript-types-part-2/543000/565613-4.html#tab ), I hit a snag. The sample code illustrates how switch statements work in TypeScript but the code that seems to work fine for the instructor throws a Type 'x' is not comparable to type 'y' error. Here's the code:

enum temperature{
    cold,
    hot
}

let temp = temperature.cold;

switch (temp) {
    case temperature.cold:
        console.log("Brrr....");
        break;
    case temperature.hot:
        console.log("Yikes...")
        break;
}

我收到一个错误并在 case temperature.hot: 下显示:

I get an error and squiggles under case temperature.hot: saying:

Type 'temperature.hot' is not comparable to type 'temperature.cold'

是什么?

推荐答案

那是因为编译器已经知道情况 temperature.hot 永远不会发生:变量 temp被赋予枚举文字类型 temperature.cold，它只能被分配给该值本身(如果没有严格的 null 检查，则为 null).由于 temperature.hot 在这里不是一个兼容的值，编译器会抛出一个错误.

That's because the compiler already knows that the case temperature.hot will never happen: the variable temp is given the enum literal type temperature.cold, which can only be assigned that value itself (or null if there are no strict null checks). As temperature.hot is not a compatible value here, the compiler throws an error.

如果我们丢弃有关文字的信息(通过强制转换或从函数中检索值):

If we discard the information about the literal (by casting or retrieving the value from a function):

function how_cold(celsius: number): temperature {
    return celsius > 40 ? temperature.hot : temperature.cold;
}

代码将编译:

let temp = how_cold(35); // type is "temperature"

switch (temp) {
    case temperature.cold:
        console.log("Brrr....");
        break;
    case temperature.hot:
        console.log("Yikes...")
        break;
}

或者，在值前面加上 + 是有效的，因为它将值转换为数字，这也会扩大类型的范围并使其与所有枚举变体以及其他数字兼容.

Alternatively, prepending + to the value works because it converts the value to a number, which will also widen the type's scope and make it compatible with all enum variants, as well as other numbers.

let temp = temperature.cold;

switch (+temp) {
    case temperature.cold:
        console.log("Brrr....");
        break;
    case temperature.hot:
        console.log("Yikes...")
        break;
    case 5:
        console.log("What??");
        break;
}

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