luhnCheck

Implements the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.

• Use String.prototype.split(''), Array.prototype.reverse() and Array.prototype.map() in combination with parseInt() to obtain an array of digits.
• Use Array.prototype.splice(0, 1) to obtain the last digit.
• Use Array.prototype.reduce() to implement the Luhn Algorithm.
• Return true if sum is divisible by 10, false otherwise.

const luhnCheck = num => {
  let arr = (num + '')
    .split('')
    .reverse()
    .map(x => parseInt(x));
  let lastDigit = arr.splice(0, 1)[0];
  let sum = arr.reduce(
    (acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val *= 2) > 9 ? val - 9 : val)),
    0
  );
  sum += lastDigit;
  return sum % 10 === 0;
};

luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); //  true
luhnCheck(123456789); // false