TypeScript Template Literal Type - how to infer numeric type?

Question

// from a library
type T = null | "auto" | "text0" | "text1" | "text2" | "text3" | "text4";

//in my code
type N = Extract<T, `text${number}`> extends `text${infer R}` ? R : never

(TS playground)

For the above piece of code N will be equivalent to "0" | "1" | "2" | "3" | "4". How can I convert that to a numeric type, i.e. 0 | 1 | 2 | 3 | 4? Have already tried putting & number in some places, like infer R & number, but none of that works.

Answer 1

Thanks for the answer, @captain-yossarian. I found you can also keep the tuple of Mapped<MAXIMUM_ALLOWED_BOUNDARY> and index using string index. Of course, Range must be a tuple.

type MAXIMUM_ALLOWED_BOUNDARY = 999

type Mapped<
    N extends number,
    Result extends Array<unknown> = [],
    > =
    (Result['length'] extends N
        ? Result
        : Mapped<N, [...Result, Result['length']]>
    )


type NumberRange = Mapped<MAXIMUM_ALLOWED_BOUNDARY>; // <- tuple [0, 1, 2, 3, ...]


type ConvertToNumber<T extends string, Range extends number[]> = 
  T extends keyof Range ? Range[T] : never;

type _ = ConvertToNumber<'5', NumberRange> // 5
type __ = ConvertToNumber<'125', NumberRange> // 125

Playground