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to Luhn in the JavaScript Track

Published at May 20 2020 · 0 comments
Test suite

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:



Install assignment dependencies:

$ npm install

Making the test suite pass

Execute the tests with:

$ npm test

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by changing xtest to test.


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.


import { valid } from './luhn';

describe('Luhn', () => {
  test('single digit strings can not be valid', () => {

  xtest('a single zero is invalid', () => {

  xtest('a simple valid SIN that remains valid if reversed', () => {

  xtest('a simple valid SIN that becomes invalid if reversed', () => {

  xtest('a valid Canadian SIN', () => {
    expect(valid('055 444 285')).toEqual(true);

  xtest('invalid Canadian SIN', () => {
    expect(valid('055 444 286')).toEqual(false);

  xtest('invalid credit card', () => {
    expect(valid('8273 1232 7352 0569')).toEqual(false);

  xtest('valid number with an even number of digits', () => {
    expect(valid('095 245 88')).toEqual(true);

  xtest('valid number with an odd number of spaces', () => {
    expect(valid('234 567 891 234')).toEqual(true);

  xtest('valid strings with a non-digit added at the end invalid', () => {

  xtest('valid strings with punctuation included become invalid', () => {

  xtest('valid strings with symbols included become invalid', () => {
    expect(valid('055# 444$ 285')).toEqual(false);

  xtest('single zero with space is invalid', () => {
    expect(valid(' 0')).toEqual(false);

  xtest('more than a single zero is valid', () => {
    expect(valid('0000 0')).toEqual(true);

  xtest('input digit 9 is correctly converted to output digit 9', () => {

  xtest('using ascii value for non-doubled non-digit isn\'t allowed', () => {
    expect(valid('055b 444 285')).toEqual(false);

  xtest('using ascii value for doubled non-digit isn\'t allowed', () => {
// This is only a SKELETON file for the 'Luhn' exercise. It's been provided as a
// convenience to get you started writing code faster.

export const valid = (number) => {
  var luhn = number.split(" ").join("").split("").reverse();
  var sum = 0;
  if (luhn.length<2) return false;
  for (var i=0;i<luhn.length; i++){
      if (i%2!==0){
          sum+=luhn[i]>=5 ? parseInt(luhn[i])*2-9 : parseInt(luhn[i])*2;  
      else sum += parseInt(luhn[i]);
  if (sum%10===0) return true;
  else return false;

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