05310-medium-join.ts

/*
  5310 - Join
  -------
  by Pineapple (@Pineapple0919) #medium #array
  
  ### Question
  
  Implement the type version of Array.join, Join<T, U> takes an Array T, string or number U and returns the Array T with U stitching up.
  
  ```ts
  type Res = Join<["a", "p", "p", "l", "e"], "-">; // expected to be 'a-p-p-l-e'
  type Res1 = Join<["Hello", "World"], " ">; // expected to be 'Hello World'
  type Res2 = Join<["2", "2", "2"], 1>; // expected to be '21212'
  type Res3 = Join<["o"], "u">; // expected to be 'o'
  ```
  
  > View on GitHub: https://tsch.js.org/5310
*/


/* _____________ Your Code Here _____________ */

type Join<T extends readonly string[], U extends string | number, Output extends string = ''> = 
  T extends [infer First, ...infer Rest]
    ? Rest extends readonly string[]
      ? First extends string
        ? Join<Rest, U, Output extends '' ? `${First}` : `${Output}${U}${First}`>
        : never
      : never
    : Output

/* _____________ Test Cases _____________ */
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'>>,
]



/* _____________ Further Steps _____________ */
/*
  > Share your solutions: https://tsch.js.org/5310/answer
  > View solutions: https://tsch.js.org/5310/solutions
  > More Challenges: https://tsch.js.org
*/