feat: medium-join

src/05310-medium-join.ts

@@ -0,0 +1,23 @@
+// ============= Test Cases =============
+import type { Equal, Expect } from './test-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'>>,
+  Expect<Equal<Join<[], 'u'>, ''>>
+]
+
+// ============= Your Code Here =============
+type Join<
+  T extends any[],
+  U extends string | number,
+  Result extends string = ''
+> = T extends [infer A, ...infer B]
+  ? Join<
+      B,
+      U,
+      Result extends '' ? `${A & string}` : `${Result}${U}${A & string}`
+    >
+  : Result