“keyof typeof"是什么意思?在打字稿中是什么意思?

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问题描述

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示例:

向我解释 keyof typeof 在 TypeScript 中的含义

Explain to me what keyof typeof means in TypeScript

enum ColorsEnum {
    white = '#ffffff',
    black = '#000000',
}

type Colors = keyof typeof ColorsEnum;

最后一行相当于:

type Colors = "white" | "black"

但它是如何工作的?

我希望 typeof ColorsEnum 返回类似 Object" 的东西，然后 keyof Object" 不会做任何有趣的事情.但我显然错了.

I would expect typeof ColorsEnum to return something like "Object" and then keyof "Object" to not do anything interesting. But I am obviously wrong.

推荐答案

keyof 接受一个对象类型并返回一个接受任何对象键的类型.

keyof takes an object type and returns a type that accepts any of the object's keys.

type Point = { x: number; y: number };
type P = keyof Point; // type '"x" || "y"'

const coordinate: P = 'z' // Type '"z"' is not assignable to type '"x" | "y"'.

typeof 与 TypeScript 类型

typeof 在 javascript 对象上调用时的行为与在 typescript 类型上调用时的行为不同.

typeof with TypeScript types

typeof behaves differently when called on javascript objects, to when it is called on typescript types.

TypeScript 在调用时使用 javascript 的 typeof在运行时处理 javascript 值并返回 undefined"、object"、boolean"、number"、bigint"、string"、symbol"、function"之一;TypeScript 的 typeof 在类型值上被调用，但可以也可以在类型表达式中调用 javascript 值.它还可以推断 javascript 对象的类型，返回更详细的对象类型. TypeScript uses javascript's typeof when called on javascript values at runtime and returns one of "undefined", "object", "boolean", "number", "bigint", "string", "symbol", "function" TypeScript's typeof is called on type values, but can also be called on javascript values when in a type expression. It can also infer the type of javascript objects, returning a more detailed object type.

type Language = 'EN' | 'ES'; 
const userLanguage: Language = 'EN';
const preferences = { language: userLanguage, theme: 'light' };

console.log(typeof preferences); // "object"
type Preferences = typeof preferences; // type '{language: 'EN''; theme: string; }'

因为第二个 typeofreferences 在类型表达式中，所以它实际上是 TypeScript 自己的 typeof 被调用，而不是 javascript 的.

Because the second typeof preferences is in a type expression it is actually TypeScript's own typeof that get called, and not javascript's.

因为 keyof 是我们将调用 TypeScript 的 typeof 版本的 TypeScript 概念.

Because keyof is a TypeScript concept we will be calling TypeScript's verion of typeof.

keyof typeof 将推断 javascript 对象的类型并返回其键的联合类型.因为它可以推断出键的确切值，所以它可以返回它们的 文字类型的联合 而不是仅仅返回字符串".

keyof typeof will infer the type of a javascript object and return a type that is the union of its keys. Because it can infer the exact value of the keys it can return a union of their literal types instead of just returning "string".

type PreferenceKeys = keyof typeof preferences; // type '"language" | "theme"'

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