在 TypeScript 中，“extends keyof"是做什么的?和“in keyof"意思?

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问题描述

在 TypeScript 中，某些类型是使用 extends keyof 或 in keyof 定义的.我试图理解它们的意思，但到目前为止我没有成功.

In TypeScript, some types are defined using extends keyof or in keyof. I have tried to understand what they mean, but so far I didn't succeed.

我得到的是 keyof 单独返回一个联合类型，该类型将所有名称作为可能的值存在于您在 keyof 之后指定的类型上的属性名称中.

What I got is that keyof alone returns a union type which has all the names as possible values that are existent as property names on the type that you specify after keyof.

type T = keyof string;

T 因此等价于 startsWith |结束于 |修剪 |子串 |....

这是正确的吗?

现在，如果我尝试思考 extends keyof 和 in keyof 是什么意思，我的直觉是这样的:

Now, if I try to think about what extends keyof and in keyof mean, my gut feeling says the following:

extends keyof 是从 T 派生的任何类型，即它具有所有这些可能的值，但可能更多.
in keyof 是从 T 中获取值的任何类型，但不一定是全部(可能，但可能更少).

extends keyof is any type that derives from T, i.e. it has all these possible values, but maybe more.
in keyof is any type that takes values from T, but not necessarily all of them (it's possible, but maybe less).

因此，从这个 POV extends keyof 将描述一个 >= 关系，in keyof 将描述一个 <= 关系.这样对吗?如果不是，什么是正确的?

So, from this POV extends keyof would describe a >= relation, in keyof would describe a <= relation. Is this correct? If not, what would be correct?

推荐答案

对于任何类型 T，keyof T 是 的已知公共属性名称的并集>T.

For any type T, keyof T is the union of known, public property names of T.

示例:

interface Person {
  age: number;
  name: string;
}

type PersonKeys = keyof Person; // "age" | "name"

您假设 keyof string 产生 startsWith |结束于 |修剪 |... 因此是正确的.您可以在查找类型发行说明.

Your assumption that keyof string yields startsWith | endsWith | trim | ... is therefore correct. You can learn more about it in the lookup type release notes.

extends 用于限制泛型参数的类型.示例:

K 因此只能是T 的公共属性名称.它与扩展类型或继承无关，与扩展相反接口.

K can therefor only be a public property name of T. It has nothing to do with extending a type or inheritance, contrary to extending interfaces.

extends keyof 的用法如下:

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

const person: Person = {
  age: 22,
  name: "Tobias",
};

// name is a property of person
// --> no error
const name = getProperty(person, "name");

// gender is not a property of person
// --> error
const gender = getProperty(person, "gender");

除了关于索引类型的文档，我找到了这篇有用的文章.

in 用于定义索引签名，我们想用字符串、数字或符号文字的联合输入.结合keyof，我们可以使用它来创建所谓的映射类型，它重新映射原始类型的所有属性.

in is used when we're defining an index signature that we want to type with a union of string, number or symbol literals. In combination with keyof we can use it to create a so called mapped type, which re-maps all properties of the original type.

in keyof 的用法如下:

type Optional<T> = {
  [K in keyof T]?: T[K]
};

const person: Optional<Person> = {
  name: "Tobias"
  // notice how I do not have to specify an age,
  // since age's type is now mapped from 'number' to 'number?'
  // and therefore becomes optional
};

除了关于映射类型的文档，我再次找到了这篇有用的文章.

有趣的事实:我们刚刚构建的 Optional 类型具有与官方 部分实用程序类型！

这篇关于在 TypeScript 中，“extends keyof"是做什么的?和“in keyof"意思?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！

StartsWith