00847-hard-string-join.ts

// ============= Test Cases =============
import type { Equal, Expect } from './test-utils'

// Edge cases
const noCharsOutput = join('-')()
const oneCharOutput = join('-')('a')
const noDelimiterOutput = join('')('a', 'b', 'c')

// Regular cases
const hyphenOutput = join('-')('a', 'b', 'c')
const hashOutput = join('#')('a', 'b', 'c')
const twoCharOutput = join('-')('a', 'b')
const longOutput = join('-')('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h')

type cases = [
  Expect<Equal<typeof noCharsOutput, ''>>,
  Expect<Equal<typeof oneCharOutput, 'a'>>,
  Expect<Equal<typeof noDelimiterOutput, 'abc'>>,
  Expect<Equal<typeof twoCharOutput, 'a-b'>>,
  Expect<Equal<typeof hyphenOutput, 'a-b-c'>>,
  Expect<Equal<typeof hashOutput, 'a#b#c'>>,
  Expect<Equal<typeof longOutput, 'a-b-c-d-e-f-g-h'>>
]

// ============= Your Code Here =============
type Join<
  Delimiter extends string = '',
  Parts extends readonly string[] = []
> = Parts extends [infer Head extends string, ...infer Tail extends string[]]
  ? Tail extends []
    ? Head
    : `${Head}${Delimiter}${Join<Delimiter, Tail>}`
  : ''

declare function join<Delimiter extends string>(
  delimiter: Delimiter
): <Parts extends readonly string[]>(...parts: Parts) => Join<Delimiter, Parts>