add join

join.ts

@@ -0,0 +1,35 @@
+import type { Equal, Expect } from '@type-challenges/utils'
+
+type cases = [
+  Expect<Equal<Join<['a', 'p', 'p', 'l', 'e'], '-'>, 'a-p-p-l-e'>>,
+  Expect<Equal<Join<['Hello', 'World'], ' '>, 'Hello World'>>,
+  Expect<Equal<Join<['2', '2', '2'], 1>, '21212'>>,
+  Expect<Equal<Join<['o'], 'u'>, 'o'>>,
+];
+
+type Stringable = (string | number | bigint | boolean | null | undefined);
+
+type JoinInternal<
+  T extends Stringable[],
+  U extends Stringable,
+  Acc extends string
+> = T extends [infer First, infer Second, ...infer Tail]
+  ? First extends Stringable
+    ? Second extends Stringable
+      ? Tail extends Stringable[]
+        ? JoinInternal<[Second, ...Tail], U, `${Acc}${First}${U}`>
+        : never
+      : never
+    : never
+  : T extends [infer Head, ...infer Tail]
+    ? Head extends Stringable
+      ? `${Acc}${Head}`
+      : never
+    : Acc
+;
+
+
+type Join<
+  T extends Stringable[],
+  U extends Stringable,
+> = JoinInternal<T, U, ''>