Typescript: list of keys into the object with the same keys and values based on key content

Question

We need to type the function, which requires a list of keys (strings) and produces object with the same keys where values are string | undefined when key starts with ? or otherwise just string.

export function useParams<T extends string>(
  expected: T[]
): { [key in T as NonOptional<T>]: string } {
  // ... our custom implementation
  return null as any;
}

type NonOptional<T extends string> = T extends `?${infer Inner}` ? Inner : T;

// Should produce type { a: string; b: string | undefined }
const result = useParams(['a', '?b']);

However, I'm not sure how to type the output.

aleksxor · Accepted Answer · 2021-06-16 08:25:43Z

1

You may write it iterating over tuple's keys:

export function useParams<T extends readonly string[]>(
  expected: T
): { [K in T[number] as NonOptional<K>]: string | OrUndefined<K> } {
  return null as any;
}

type NonOptional<T extends string> = T extends `?${infer Inner}` ? Inner : T;
type OrUndefined<T extends string> = T extends `?${string}` ? undefined : never

const result = useParams(['a', '?b'] as const);

playground link

Added section to address the problem described in comments:

I'm pretty sure developers will forgot to add as const and then it will stop working correctly.

To allow passing non-readonly tuples you may use variadic tuple types:

export function useParams<T extends string[]>(
  expected: [...T]
): { [K in T[number] as NonOptional<K>]: string | OrUndefined<K> } {
  return null as any;
}

type NonOptional<T extends string> = T extends `?${infer Inner}` ? Inner : T;
type OrUndefined<T extends string> = T extends `?${string}` ? undefined : never

const result = useParams(['a', '?b']);

playground link

edited Jun 16, 2021 at 8:25

answered Jun 15, 2021 at 16:28

aleksxor

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1 Comment

Deftomat Over a year ago

Thanks! Looks good but unfortunately, I'm pretty sure developers will forgot to add as const and then it will stop working correctly. Any idea how to make it works without as const?

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