"If the property is not undefined, use a conditional type to retrieve the numerical index from it, otherwise display a message indicating that there is

Here is a snippet of code I am working with:

type Numbers = [3,65,2,7,3,99,23,555];

interface Options {
  options?: Numbers;
}

type FilterOption = Options['options'] extends undefined ? undefined : Options['options'][number];

I am trying to define a type that involves referencing the indexes of the "options" property in a union. However, since the `options?` property is optional (indicated by the question mark), I cannot directly assign the value: `Options['options'][number]` as it may be undefined.

I attempted to use a conditional type to check for undefined, but encountered an error when accessing the [number] index:

Type 'Numbers | undefined' has no matching index signature for type 'number'

I considered that this issue might be related to "distributivity", so I tried disabling distribution by adding brackets:

type FilterOption = [Options['options']] extends [undefined] ? undefined : Options['options'][number];

However, that did not resolve the problem. Could this limitation stem from "narrowing"? Is there a viable solution?

playground

typescript

Answer №1

Instead of attempting to create your own conditional type, it would be advisable to utilize the NonNullable<T> utility type to eliminate undefined from Options['options'] before accessing it with number:

type FilterOption = NonNullable<Options['options']>[number];
// type FilterOption = 3 | 65 | 2 | 7 | 99 | 23 | 555

Your previous version did not work because the statement 

Options['options'] extends undefined
is untrue. Trying to disable distributivity was also unhelpful as your type was not distributive to begin with. The solution involved debugging through these steps:

// original version
type FO0 = Options['options'] extends undefined ? undefined :
  Options['options'][number]; // error

// turning on distributivity by introducing a type parameter
type _FO1<T> =
  T extends undefined ? undefined : T[number]; // still error,
// just because T isn't undefined doesn't guarantee it has a numeric key

// checking for a numeric key rather than undefined, switching true/false branches
type _FO2<T> =
  T extends { [k: number]: any } ? T[number] : undefined; // no error
type FO2 = _FO2<Options['options']>
// type FO2 = 3 | 65 | 2 | 7 | 99 | 23 | 555 | undefined 
// wait, undefined?

// excluding undefined if the input is undefined.
// adding nothing in that case (i.e., never)
type _FO3<T> =
  T extends { [k: number]: any } ? T[number] : never; // no error
type FO3 = _FO3<Options['options']>
// type FO3 = 3 | 65 | 2 | 7 | 99 | 23 | 555
// success!

Playground link to code

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