TypeScript Template literals Array Join

Question

I found a definition for a type level Split function:

type Split<S extends string, D extends string> =
    string extends S ? string[] :
    S extends '' ? [] :
    S extends `${infer T}${D}${infer U}` ? [T, ...Split<U, D>] : [S];

Is there also a way to create a type level Join<string[], string> function, so I can use them for example for changing underscores to hyphens?

For example:

type ChangeHyphensToUnderscore<T> = { [P in keyof T & string as `${Join(Split<P, '-'>, '_')}`]: T[P] };

kelsny · Accepted Answer · 2023-05-22T15:03:29.657

3

Sure there is:

type Stringable = string | number | bigint | boolean | null | undefined;

type Join<A, Sep extends string = ""> = A extends [infer First, ...infer Rest] ? Rest extends [] ? `${First & Stringable}` : `${First & Stringable}${Sep}${Join<Rest, Sep>}` : "";

You can also improve performance slightly by utilizing tail call optimization:

type Join<A, Sep extends string = "", R extends string = ""> = A extends [infer First, ...infer Rest] ? Join<Rest, Sep, R extends "" ? `${First & Stringable}` : `${R}${Sep}${First & Stringable}`> : R;

Here's a playground for you to play.

edited May 22 '23 at 15:03

answered May 19 '22 at 15:07

kelsny

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This works when the elements of the array are statically known, but when providing like `Join<('a'|'b')[],','>` it assumes it returns always an empty string `""`. Any idea to make it work dynamically ? :D – Emmanuel Meric de Bellefon Jul 28 '22 at 20:14
@EmmanuelMericdeBellefon What would the expected result even be? – kelsny Jul 28 '22 at 22:38
yeah that would be an infinite set then `'' | 'a' | 'b' | 'a,a' | 'a,b' | 'b,a' | 'b,b' | 'a,a,a' | 'a,a,b' ...`, not sure if these can be represented in Typescript. – Emmanuel Meric de Bellefon Jul 29 '22 at 05:13

TypeScript Template literals Array Join

1 Answers1