---
title: luhnCheck
tags: math,algorithm,advanced
firstSeen: 2018-01-03T11:02:35+02:00
lastUpdated: 2020-12-28T13:49:24+02:00
---

Implementation of the [Luhn Algorithm](https://en.wikipedia.org/wiki/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.

```js
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;
};
```

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