const luhn: (candidate: string) => boolean = (candidate: string) =>
  parseInt(candidate[candidate.length - 1]) === candidate.
    slice(0, candidate.length - 1).
    split('').
    map((c: string) => parseInt(c)).
    reverse().
    map((n: number, index: number) => index % 2 === 0 ? 2 * n : n).
    map((n: number) => n > 9 ? n - 9 : n).
    reduce((a: number, b: number) => a + b)
    * 9 % 10
;