打字稿中的记录类型是什么? [英] What is the Record type in typescript?

问题描述

Record 在 Typescript 中是什么意思?

Typescript 2.1 引入了 Record 类型，举例说明:

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//对于T类型的每个属性K，将其转换为Ufunction mapObject(obj: Record, f: (x: T) => U): Record

见 打字稿 2.1

Advanced Types 页面提到了 Recordcode> 位于 Readonly、Partial 和 Pick 旁边的 Mapped Types 标题下，其定义如下:

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type Record= {[K中的P]:T；}

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Readonly、Partial 和 Pick 是同态的，而 Record 不是.Record 不是同态的一个线索是它不需要输入类型来复制属性:

type ThreeStringProps = Record<'prop1' |'prop2' |'prop3', 字符串>

就是这样.除了以上引用之外，在 typescriptlang.org.

问题

1. 谁能给Record一个简单的定义?

2. Record<K,T> 仅仅是一种表达此对象上的所有属性都具有 T 类型"的方式吗?可能不是所有属性，因为 K 有一些目的......

3. K 泛型是否禁止在对象上添加不是 K 的键，或者它是否允许它们并且只是表明它们的属性没有转换为T?

4. 以给定的例子:

   type ThreeStringProps = Record<'prop1' |'prop2' |'prop3', 字符串>

   和这个完全一样吗?:

   type ThreeStringProps = {prop1: string, prop2: string, prop3: string}

解决方案

1. 谁能给Record一个简单的定义?

A Record 是一种对象类型，其属性键为 K，属性值为 T.也就是说，keyof Record 等价于 K，而 Record[K] 是(基本上)相当于T.

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2. Record<K,T> 是否只是一种表示此对象上的所有属性都具有 T 类型"的方式?可能不是所有的对象，因为 K 有一些目的......

正如您所注意到的，K 有一个目的……将属性键限制为特定值.如果您想接受所有可能的字符串值键，您可以执行类似 Record 的操作，但惯用的方法是使用 索引签名 像 { [k: string]: T }.

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3. K 泛型是否禁止在对象上添加不是 K 的键，或者它是否允许它们并且只是表明它们的属性没有转换为 ?

它并没有完全禁止"额外的键:毕竟，通常允许一个值具有在其类型中没有明确提及的属性……但它不会识别出这样的属性存在:

declare const x: Record<"a", string>;x.b;//错误，属性 'b' 在类型 'Record<"a", string>' 上不存在

它会将它们视为多余的属性有时会被拒绝:

declare function acceptR(x: Record<"a", string>): void;acceptR({a: "嘿", b: "你"});//错误，对象字面量只能指定已知属性

有时被接受:

const y = {a: "嘿", b: "你"};接受R(y);//好的

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4. 以给定的例子:

   type ThreeStringProps = Record<'prop1' |'prop2' |'prop3', 字符串>

   和这个完全一样吗?:

   type ThreeStringProps = {prop1: string, prop2: string, prop3: string}

是的！

希望有所帮助.祝你好运！

What does Record<K, T> mean in Typescript?

Typescript 2.1 introduced the Record type, describing it in an example:

// For every properties K of type T, transform it to U
function mapObject<K extends string, T, U>(obj: Record<K, T>, f: (x: T) => U): Record<K, U>

see Typescript 2.1

And the Advanced Types page mentions Record under the Mapped Types heading alongside Readonly, Partial, and Pick, in what appears to be its definition:

type Record<K extends string, T> = {
    [P in K]: T;
}

Readonly, Partial and Pick are homomorphic whereas Record is not. One clue that Record is not homomorphic is that it doesn’t take an input type to copy properties from:

type ThreeStringProps = Record<'prop1' | 'prop2' | 'prop3', string>

And that's it. Besides the above quotes, there is no other mention of Record on typescriptlang.org.

Questions

1. Can someone give a simple definition of what Record is?

2. Is Record<K,T> merely a way of saying "all properties on this object will have type T"? Probably not all properties, since K has some purpose...

3. Does the K generic forbid additional keys on the object that are not K, or does it allow them and just indicate that their properties are not transformed to T?

4. With the given example:

   type ThreeStringProps = Record<'prop1' | 'prop2' | 'prop3', string>
   

   Is it exactly the same as this?:

   type ThreeStringProps = {prop1: string, prop2: string, prop3: string}
   

解决方案

1. Can someone give a simple definition of what Record is?

A Record<K, T> is an object type whose property keys are K and whose property values are T. That is, keyof Record<K, T> is equivalent to K, and Record<K, T>[K] is (basically) equivalent to T.

2. Is Record<K,T> merely a way of saying "all properties on this object will have type T"? Probably not all objects, since K has some purpose...

As you note, K has a purpose... to limit the property keys to particular values. If you want to accept all possible string-valued keys, you could do something like Record<string, T>, but the idiomatic way of doing that is to use an index signature like { [k: string]: T }.

3. Does the K generic forbid additional keys on the object that are not K, or does it allow them and just indicate that their properties are not transformed to T?

It doesn't exactly "forbid" additional keys: after all, a value is generally allowed to have properties not explicitly mentioned in its type... but it wouldn't recognize that such properties exist:

declare const x: Record<"a", string>;
x.b; // error, Property 'b' does not exist on type 'Record<"a", string>'

and it would treat them as excess properties which are sometimes rejected:

declare function acceptR(x: Record<"a", string>): void;
acceptR({a: "hey", b: "you"}); // error, Object literal may only specify known properties

and sometimes accepted:

const y = {a: "hey", b: "you"};
acceptR(y); // okay

4. With the given example:

   type ThreeStringProps = Record<'prop1' | 'prop2' | 'prop3', string>
   

   Is it exactly the same as this?:

   type ThreeStringProps = {prop1: string, prop2: string, prop3: string}
   

Yes!

Hope that helps. Good luck!

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