Implementation of The Luhn Algorithm

validate.js

/*
  Implementation of The Luhn Algorithm (https://en.wikipedia.org/wiki/Luhn_algorithm)
 */

function validate(n) {

  /**
   * Check if the given number is of valid type
   */
  if (!n || typeof n !== 'number') {
    // purposely not checking the length of the string
    return false
  }

  /**
   * Placeholders
   *     @type {Array} doubled
   *     @type {Number} finalSum
   */
  let doubled = [],
    finalSum = 0;

  const
    /**
     * Takes number and returns the number doubled
     * @param  {[Number]} num
     * @return {[Number]}
     */
    double = num => num + num,
    /**
     * Checks if given number is even
     * @param  {[Num]} num
     * @return {[Boolean]}
     */
    isEven = num => num % 2 === 0,
    /**
     * Convert the given number to array for easier looping and
     * individual element's manipulation
     * @type {[Array]}
     */
    stringify = n.toString().split(''),
    /**
     * Takes array and boolean and increments its elements
     * @param  {[Array]} array
     * @param  {[Boolean]} lengthIsOdd
     * @return {[Boolean]}
     */
    incrementor = (array, lengthIsOdd) => {
      /*
        Adding an dummy element to array to start index from 1.
        If lengthIsOdd === true, double every other digit starting with the second
        else double every other digit starting with the first.
       */
      array.unshift(0)
      array.map((el, idx) => {
        if (lengthIsOdd ? isEven(idx) : !isEven(idx)) {
          doubled.push(double(Number(array[idx++])))
        } else {
          doubled.push(Number(array[idx]))
        }
      })
      /* Restore the array */
      doubled.shift()
    },

    /**
     * If element is a double digit number, substract it by 9
     * and return it.
     * @param  {[Number]} el
     * @return {[Number]}
     */
    normalizeElements = el => el > 9 ? el - 9 : el,
    /**
     * Add all the elements
     * @param  {[Number]} sum [Initial value]
     * @param  {[Number]} el  [Supplied elements]
     * @return {[Number]}
     */
    addAllElements = (sum, el) => sum = sum + el

    if (isEven(stringify.length)) {
      incrementor(stringify, false)
    } else {
      incrementor(stringify, true)
  }

  finalSum = doubled
              .map(el => normalizeElements(el))
              .reduce(addAllElements, 0)

  /* Finally, take that sum and divide it by 10. If the remainder equals zero, the original credit card number is valid. */
  return finalSum % 10 === 0
}

// console.log('Final', validate(5200828282828210))