Add `KeyOf` and `ValueOf` definitions to built-in lib

[SalathielGenese]

Search Terms

None -
Empirical experience -

Suggestion

I want to see added KeyOf and ValueOf

Use Cases

Let them distribute union

let a_or_b_002!: KeyOf<{ a: 'Aa' } | { b: 'Bb' }>;      // "a" | "b"
let a_or_b_003!: KeyOf<({ a: 'Aa' } | { b: 'Bb' })>;    // "a" | "b"

Examples

let a_or_b_002!: KeyOf<{ a: 'Aa' } | { b: 'Bb' }>;      // "a" | "b"
let a_or_b_003!: KeyOf<({ a: 'Aa' } | { b: 'Bb' })>;    // "a" | "b"

instead of

let a_or_b_001!: keyof ({ a: 'Aa' } | { b: 'Bb' });     // "a" & "b"

Checklist

My suggestion meets these guidelines:

• [x ] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [ x] This wouldn't change the runtime behavior of existing JavaScript code
• [ x] This could be implemented without emitting different JS based on the types of the expressions
• [ x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• [x ] This feature would agree with the rest of TypeScript's Design Goals.

[SalathielGenese]

Here's implementation

declare type KeyOf<T> = T extends any ? keyof T : never;

declare type ValueOf<T> = T extends any ? T[ keyof T ] : never;

[carpben]

I use $Values by "utility-types" quite often, and find it useful. Would be nice to see a build-in Typescript version.

[kisaragi-hiu]

A quick search on GitHub shows a mountain of projects defining their own ValueOf or importing type-fest's ValueOf.

• ValueOf should exist because keyof already exists, it is implementable and has been implemented as a one liner.
• KeyOf should exist if ValueOf exists for syntax symmetry. (I'd argue keyof should've never been an operator since it's syntax on top of JS that could've just been a utility type, but that ship has already sailed.)
• A quick search for "typescript valueof" shows significant interest, including a sort of duplicate issue (valueof Type Operator like keyof #53521)
• valueof Type Operator like keyof #53521 is a feature request asking for a valueof operator. It was for some reason marked as a Question then closed as if it's just a resolved question for how to implement ValueOf yourself.

[kisaragi-hiu]

This is something that would likely be quite welcomed by the TypeScript community.

[RyanCavanaugh]

New utility types are declined by policy. See https://github.com/microsoft/TypeScript/wiki/FAQ#new-utility-types