How can I make a Record with union as its key type in TypeScript, from a function?

Question

I have a function like below

const getQueryParams = (names) => {
  const urlParams = new URLSearchParams(window.location.search)
  return names.reduce((acc, curr) => {
    return {
       ...acc,
       [curr]: urlParams.get(curr),
    }
  }, {})
}

I'd like to set TS types in order to get the following type for the output of the above function.

const params: Record<'a'|'b', string> = getQueryParams(['a', 'b'])

Is such a thing possible? If so, how can I do it?

Answer 1

This seems to work.

type GetQueryParams = <T extends string>(args: T[]) => Record<T, string | null>

export const getQueryParams: GetQueryParams = names => {
  const urlParams = new URLSearchParams(window.location.search)
  return names.reduce((acc, curr) => {
    return {
      ...acc,
      [curr]: urlParams.get(curr),
    }
  }, {} as ReturnType<GetQueryParams>)
}