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

title, tags, expertise, cover, excerpt, firstSeen, lastUpdated

title	tags	expertise	cover	excerpt	firstSeen	lastUpdated
Luhn check	math,algorithm	advanced	blog_images/blank-card.jpg	Implements the Luhn Algorithm, used to validate a variety of identification numbers.	2018-01-03T11:02:35+02:00	2022-01-30T13:37:39+02:00

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.shift() 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 => {
  const arr = (num + '')
    .split('')
    .reverse()
    .map(x => parseInt(x));
  const lastDigit = arr.shift();
  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