TypeScript:隐式数字枚举转换 [英] TypeScript: Implicit number to enum cast

问题描述

为什么下面会在 TypeScript 中编译?

Why does the following compile in TypeScript?

enum xEnum {
  X1,X2
}

function test(x: xEnum) {
}

test(6);

它不应该抛出错误吗?恕我直言，这个隐式转换在这里是错误的，不是吗?

Shouldn't it throw an error? IMHO this implicit cast is wrong here, no?

这里是 游乐场链接.>

推荐答案

这是语言规范(3.2.7 Enum Types)的一部分:

This is part of the language specification (3.2.7 Enum Types):

Enum 类型可分配给 Number 原始类型，反之亦然反之，但不同的枚举类型不能相互赋值

Enum types are assignable to the Number primitive type, and vice versa, but different enum types are not assignable to each other

因此，允许在 number 和 Enum 之间进行隐式转换的决定是经过深思熟虑的.

So the decision to allow implicit conversion between number and Enum and vice-versa is deliberate.

这意味着您需要确保该值有效.

This means you will need to ensure the value is valid.

function test(x: xEnum) {
    if (typeof xEnum[x] === 'undefined') {
        alert('Bad enum');
    }
    console.log(x);
}

虽然你可能不同意这个实现，但值得注意的是枚举在这三种情况下很有用:

Although you might not agree with the implementation, it is worth noting that enums are useful in these three situations:

// 1. Enums are useful here:
test(xEnum.X2);

// 2. ...and here
test(yEnum.X2);

和 3. - 当你输入 test( 时，它会告诉你枚举类型，你可以用来保证你选择一个存在的枚举类型.

And 3. - when you type test( it will tell you the enum type you can use to guarantee you pick one that exists.

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