What is the Record type?

What does Record 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(obj: Record, f: (x: T) => U): Record

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 = {
    [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

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

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Can someone give a simple definition of what Record is?

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

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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?

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With the given example:

type ThreeStringProps = Record

Is it exactly the same as this?:

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

• Can someone give a simple definition of what Record is?

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

• Is Record 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, but the idiomatic way of doing that is to use an index signature like { [k: string]: T }.

• 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;
x.b; // error, Property 'b' does not exist on type 'Record'

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

declare function acceptR(x: Record): 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

-
With the given example:

type ThreeStringProps = Record

Is it exactly the same as this?:

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

Yes!

Stack Overflow Q#51936369, score: 717