Proposal: KeysOfUnion, `keyof` Union type

[ajitzero]

Context: https://stackoverflow.com/q/49401866

interface Foo {
  foo: string;
}

interface Bar {
   bar: string;
}
type Batz = Foo | Bar;
type AvailableKeysFoo  = keyof Foo;  // Works as expected
type AvailableKeysBatz = keyof Batz; // Gives 'never', expected (keyof Foo | keyof Bar)

Solution: https://stackoverflow.com/a/49402091

type Batz = Foo | Bar;
type KeysOfUnion<T> = T extends T ? keyof T : never;

type AvailableKeys = KeysOfUnion<Batz>; // keyof Foo | keyof Bar -> "foo" | "bar"

[ajitzero]

I am willing to open a PR with this change if this is fine "as-is". This solution belongs to the OP in the linked StackOverflow answer.

[sindresorhus]

This type is being added internally in https://github.com/sindresorhus/type-fest/pull/259/files#diff-49eaac4718b21c2c3d9ed8024c97600050979229a82a0255cc3c538e36350131R45-R53. After that PR is merged, we can expose it. If you want to see this happen sooner than later, help with reviewing the referenced PR is welcome.

[sindresorhus]

This can now be done: https://github.com/sindresorhus/type-fest/blob/f171b707fdbc9b7e895a34374ee865272a9b1d5a/source/internal.d.ts#L46-L53

[ajitzero]

@sindresorhus Thank you! I think with this the pending changes would be documentation?

• [ ] Mention/link in Repository README
• [ ] Update on paka.dev? Or is that auto-generated from the TSDoc?

I can take this up this weekend if you can point to a relevant docs PR.

[sindresorhus]

Read: https://github.com/sindresorhus/type-fest/blob/main/.github/contributing.md#submitting-a-new-type

It needs tests and expanded doc comments.

[sindresorhus]

Update on paka.dev? Or is that auto-generated from the TSDoc?

Auto-generated