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] };
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;