Typesafe string joining

joinArray.ts

type _JoinableElement = string | number | null | undefined | boolean;
type _JoinableArray = _JoinableElement[];

export type JoinArray<
  TArray extends _JoinableArray,
  TDelimiter extends string = " "
> = _JoinArray<TArray, TDelimiter>;

type _JoinArray<
  TArray extends _JoinableArray,
  TDelimiter extends string,
  _TValue = _CircularJoin<TArray, TDelimiter>
> = _TValue extends null ? string : _TValue;

type _CircularJoin<
  TArray extends _JoinableArray,
  TDelimiter extends string,
  R extends string | null = null
> = TArray extends [
  infer TNext extends _JoinableElement,
  ...infer TAfter extends _JoinableArray
]
  ? R extends null
    ? _CircularJoin<TAfter, TDelimiter, `${TNext}`>
    : _CircularJoin<TAfter, TDelimiter, `${R}${TDelimiter}${TNext}`>
  : TArray extends [infer TNext extends _JoinableElement]
  ? R extends null
    ? TNext
    : `${R}${TDelimiter}${TNext}`
  : R;

export function join<
  TArray extends _JoinableArray,
  TDelimiter extends string = " "
>(
  array: TArray,
  separator: TDelimiter = " " as TDelimiter
): JoinArray<TArray, TDelimiter> {
  return array.join(separator) as JoinArray<TArray, TDelimiter>;
}