typescript - 什么是记录类型？

Question

什么是Record<K, T>在 typescript 中是什么意思？

Typescript 2.1 引入了 Record类型，在示例中对其进行描述:

// 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>

参见 Typescript 2.1

还有 Advanced Types页面提及 Record在 Readonly 旁边的映射类型标题下, Partial , 和 Pick ，在它的定义中:

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>

就是这样。除了上面的引述，没有其他提及 Record在 typescriptlang.org .

问题

1. 谁能给出一个简单的定义Record是吗？

2. 是Record<K,T>仅仅是一种说法“此对象上的所有属性都将具有类型 T”？可能不是所有 属性，因为K有一些目的...

3. K通用禁止对象上不是 K 的附加键, 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', string>
   

   是不是和这个一模一样？:

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

Answer 1

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

A Record<K, T>是一个对象类型，其属性键是 K其属性值为 T .即 keyof Record<K, T>相当于K , 和 Record<K, T>[K]是(基本上)等同于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...

如您所见，K有一个目的......将属性键限制为特定值。如果你想接受所有可能的字符串值键，你可以做类似 Record<string, T> 的事情。 , 但惯用的方法是使用 index signature喜欢{ [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?

它并不完全“禁止”附加键:毕竟，通常允许一个值具有其类型中未明确提及的属性……但它不会识别此类属性的存在:

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

它会将它们视为 excess properties有时会被拒绝:

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

有时接受:

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}
   

是的!