extract keys of type (key type, not its field type)

[regevbr]

🍩 Feature Request

Describe the solution you'd like

Since keys of an object can be of type string|number|symbol and sometimes types require only string values, it will be handy to have a type that extracts only the string key (or of any type for that matter)

type Index = string | number | symbol; // I think we should make that type public!

type KeysOfType<T extends object, M extends Index> = Extract<keyof T, M>;

type t = KeysOfType<{
  a: number;
  2: number;
}, string>; // "a"

@pirix-gh thoughts?

[millsp]

@regevbr , I currently achieve this by doing [keys] & [type]. For example:

('1' | 1) & string // '1'

I rather raise awareness of such techniques instead of writing such types.

So I am marking this as a question :)

[regevbr]

That makes sense, although less intuitive in my view. Thought this type will make it more verbose and easier to discover in your library

[millsp]

You can still be verbose (but more generic) with https://pirix-gh.github.io/ts-toolbelt/modules/_union_filter_.html#filter

[millsp]

Sorry @regevbr, I meant https://pirix-gh.github.io/ts-toolbelt/modules/_union_select_.html#select

[regevbr]

@pirix-gh do you think we can at least expose

type Index = string | number | symbol;

in the public interface?

[millsp]

@regevbr Yes, I also found myself needing it a few times