Can I get the strict keys for an array somehow?

[nandorojo]

🍩 Feature Request

I want to get the keys of an array of fixed length, without adding as const to the end of the array. I'm able to get the strict values of it, but not the keys.

Describe the solution you'd like

I'd like to use a type factory to get the readonly keys of an array:

import { List } from "ts-toolbelt";

function makeTheme<T>(t: T): T {
  return t;
}
 
const theme = makeTheme({
  space: [1, 2]
});

type Keys = List.StrictKeys<typeof theme['space']> // "0" | "1"

Describe alternatives you've considered

import { List } from "ts-toolbelt";

function makeTheme<T>(t: T): T {
  return t;
}

// comment in the 'as const' to solve it 😕
const theme = makeTheme({
  space: [1, 2] // as const
});

type Space = List.CompulsoryKeys<typeof theme["space"]>;

// this should work
const space: Space = "0";

// this should only allow "0" and "1", but it allows any number
const key: keyof typeof theme["space"] = 10;

Code: https://codesandbox.io/s/ts-toolbelt-test-forked-89wql?file=/src/index.ts:0-396

Screenshots

Thank you!!

[tonivj5]

Hey! @millsp did it some time ago! You can use Function.Narrow

Some context: microsoft/TypeScript#30680 (comment)

You can check it working here:
https://codesandbox.io/s/ts-toolbelt-test-forked-hnvml?file=/src/index.ts

[nandorojo]

You just made my day.

@millsp what would we do without you?