30-seconds-of-code/snippets/luhnCheck.md

luhnCheck

Implementation of 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.split(''), Array.reverse() and Array.map() in combination with parseInt() to obtain an array of digits. Use Array.splice(0,1) to obtain the last digit. Use Array.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 || 9), 0);
  sum += lastDigit;
  return sum % 10 === 0;
};

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