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to Luhn in the JavaScript Track

Published at Jul 13 2018 · 0 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Given a number determine whether or not it is valid per the Luhn formula.

The Luhn algorithm is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.

The task is to check if a given string is valid.

Validating a Number

Strings of length 1 or less are not valid. Spaces are allowed in the input, but they should be stripped before checking. All other non-digit characters are disallowed.

Example 1: valid credit card number

4539 1488 0343 6467

The first step of the Luhn algorithm is to double every second digit, starting from the right. We will be doubling

4_3_ 1_8_ 0_4_ 6_6_

If doubling the number results in a number greater than 9 then subtract 9 from the product. The results of our doubling:

8569 2478 0383 3437

Then sum all of the digits:

8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

Example 2: invalid credit card number

8273 1232 7352 0569

Double the second digits, starting from the right

7253 2262 5312 0539

Sum the digits

7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57

57 is not evenly divisible by 10, so this number is not valid.


Go through the setup instructions for JavaScript to install the necessary dependencies:


Running the test suite

The provided test suite uses Jasmine. You can install it by opening a terminal window and running the following command:

npm install -g jasmine

Run the test suite from the exercise directory with:

jasmine luhn.spec.js

In many test suites all but the first test have been marked "pending". Once you get a test passing, activate the next one by changing xit to it.


The Luhn Algorithm on Wikipedia http://en.wikipedia.org/wiki/Luhn_algorithm

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


var Luhn = require('./luhn');

describe('Luhn', function () {
  it('single digit strings can not be valid', function () {
    const luhn = new Luhn('1');

  xit('A single zero is invalid', function () {
    const luhn = new Luhn('0');

  xit('valid Canadian SIN', function () {
    const luhn = new Luhn('046 454 286');

  xit('invalid Canadian SIN', function () {
    const luhn = new Luhn('046 454 287');

  xit('invalid credit card', function () {
    const luhn = new Luhn('8273 1232 7352 0569');

  xit('valid strings with a non-digit added become invalid', function () {
    const luhn = new Luhn('046a 454 286');
function Luhn(luhn){
    this.checkDigit = luhn % 10;
    this.addends = addends(luhn);
    this.checksum = 
	this.addends.reduce(function(sum,digit){return sum+digit},0);
    this.valid = !(this.checksum % 10);

Luhn.create = function(num) {
    var check = addends(num*10).join('') / 10;

    var checksum = String(check).split('').reduce(function(sum,digit){
	    return sum + Number(digit) },0) % 10;

    if (checksum) checksum = 10-checksum;

    return Number(num + String(checksum));

var addends = function(num) {
    return String(num).split('').reverse().map(function(x,index){
	    return ( (index % 2) ? ((x*=2)>9 ? x-9 : x) : x|0 ) }).reverse();


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