What is record in TypeScript

Question

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}

Answer 1

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.

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 }.

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