Request: keyword or operator for iterating unions

Suggestion

I'm sorry, I feel like this has definitely been suggested but I couldn't find it in issues.

I think there should be a more explicit way to force iterating through types of a union in generics than what we currently have. The easiest example I can think of is the way keyof interacts with unions.

interface A {
  foo: string;
}

interface B {
  bar: number;
}

type C = A | B;

type Keys = keyof C; // never

I understand why it works this way. I'm not arguing that TypeScript is doing anything wrong. However, sometimes I want to get all the keys of all the members of a union, and the workaround is to abuse conditional types to force iteration.

type KeyofUnion<T> = T extends T ? keyof T : never;

type Keys = KeyofUnion<A | B>; // 'foo' | 'bar';

The expression T extends T ? /* something */ : never is silly enough that I feel it justifies creating a keyword. More importantly, it's not clear that types would behave this way if you didn't look it up. Why would conditional types force union iteration? I assume there is a good reason, but for someone not super familiar with TS, I would never expect them to see that and understand why anyone wrote it.

I believe this suggestion would be an improvement under #7 of the "non-goals" of the contributing guidelines.

🔍 Search Terms

Iterating unions, distributing unions, T extends T ? T : never

✅ Viability Checklist

My suggestion meets these guidelines:

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

⭐ Suggestion

Add a keyword that explicitly iterates types in a union instead of relying on conditional types.

type A {
  foo: string;
}
type B {
  bar: number;
}
type C = A | B;

type Keys = keyof in C;
type Keys = keyof union C;
type Keys = keyof memberof C;

I'm not happy with any of the ideas I have for the syntax but hopefully you can see what I'm going for.

📃 Motivating Example

I already put one example in the first section, but I'll also give a more concrete example of something I was trying to do that made me think of this.

I wanted to create a generic type that takes in two args.

• T, an object
• and F, a type that exists as a member on T.

The resulting type is a union of all keys in T whose members are type F.

Example

interface Foo {
  a: string;
  b: number;
  c: Date;
  d: string;
  e: string;
  f: number;
}

type NumberKeys = KeysOfType<Foo, number>; // 'b' | 'f';
type StringKeys = KeysOfType<Foo, string>; // 'a' | 'd' | 'e';
type BoolKeys = KeysOfType<Foo, boolean>; // Error: Type 'boolean' does not satisfy the constraint 'string | number | Date'.

This was my first version of this type

type KeysOfType<T, F extends T[keyof T]> = {
  [K in keyof T]: T[K] extends F ? K : never;
}[keyof T];

But, when I tried to pass a union in as the first argument, it didn't work the way I wanted it to. This led me to change it to this:

type KeysOfType<T, F extends T extends T ? T[keyof T] : never> = T extends T
  ? {
      [K in keyof T]: T[K] extends F ? K : never;
    }[keyof T]
  : never;

This is much more cumbersome.

I'd love to see a way to force this without abusing conditional types.

type KeysOfType<T, F extends T[keyof in T]> = {
 [K in keyof in T]: T[K] extends F ? K : never;
}[keyof T];

type KeysOfType<T, F extends T[keyof union T]> = {
 [K in keyof union T]: T[K] extends F ? K : never;
}[keyof T];

Again, I really don't have any good ideas for the syntax, but hopefully the spirit of this request is clear at this point.

💻 Use Cases

I'm having a hard time thinking of another use case than the one I provided here, so maybe it's edge enough to be too low priority. I still think it'd be nice to have an explicit way to do this.

Thanks for reading this and have a good day.

Start by reviewing the issue's conditional-type workaround, keyof behavior on unions, and the linked TypeScript design goals. Compare the proposed in, union, and memberof syntax ideas against the motivating examples. Done requires an agreed language design and implementation scope, which this issue does not yet specify.