Implementing the Luhn Algorithm in JavaScript

The Luhn Algorithm is a simple checksum formula, used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.

In order to implement it in JavaScript, you can start by splitting the number into an array of digits, using String.prototype.split(). Use Array.prototype.reverse() to reverse it and Array.prototype.map() to convert each digit to a number with the help of Number.parseInt().

You can then use Array.prototype.shift() to obtain the last digit of the number, and Array.prototype.reduce() to implement the algorithm. This simply means iterating over the array of digits, and for each one, you adding it to the accumulator if the index is even or doubling it and adding the result to the accumulator if the index is odd.

Finally, return true if the sum is divisible by 10, and false otherwise.

const luhnCheck = num => {
  const arr = `${num}`
    .split('')
    .reverse()
    .map(x => Number.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