 # PatrickMcSweeny's solution

## to Luhn in the JavaScript Track

Published at Aug 20 2019 · 0 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution 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.

## Setup

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

https://exercism.io/tracks/javascript/installation

## Requirements

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`.

## Source

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.

### luhn.spec.js

``````import { Luhn } from './luhn';

describe('Luhn', () => {
test('single digit strings can not be valid', () => {
const luhn = new Luhn('1');
expect(luhn.valid).toEqual(false);
});

xtest('a single zero is invalid', () => {
const luhn = new Luhn('0');
expect(luhn.valid).toEqual(false);
});

xtest('a simple valid SIN that remains valid if reversed', () => {
const luhn = new Luhn('059');
expect(luhn.valid).toEqual(true);
});

xtest('a simple valid SIN that becomes invalid if reversed', () => {
const luhn = new Luhn('59');
expect(luhn.valid).toEqual(true);
});

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

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

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

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

xtest('valid strings with a non-digit included become invalid', () => {
const luhn = new Luhn('055a 444 285');
expect(luhn.valid).toEqual(false);
});

xtest('valid strings with a non-digit added at the end invalid', () => {
const luhn = new Luhn('059a');
expect(luhn.valid).toEqual(false);
});

xtest('valid strings with punctuation included become invalid', () => {
const luhn = new Luhn('055-444-285');
expect(luhn.valid).toEqual(false);
});

xtest('valid strings with symbols included become invalid', () => {
const luhn = new Luhn('055£ 444\$ 285');
expect(luhn.valid).toEqual(false);
});

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

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

xtest('input digit 9 is correctly converted to output digit 9', () => {
const luhn = new Luhn('091');
expect(luhn.valid).toEqual(true);
});

xtest('strings with non-digits is invalid', () => {
const luhn = new Luhn(':9');
expect(luhn.valid).toEqual(false);
});
});``````
``````const add = (accumulator, currentValue) => accumulator + currentValue;

export class Luhn {
constructor(input) {
this.numbers = input
.replace(/\s/g, "")
.split("")
.reverse()
.map(char => parseInt(char));
this.multiplyEvens();
this.subtractNine();
}

get valid() {
if (this.numbers.length < 2)
return false;

return this.numbers.reduce(add) % 10 == 0;
}

multiplyEvens() {
this.numbers = this.numbers.map(function(number, i) {
return i % 2 !== 0 ? number * 2 : number;
});
}

subtractNine() {
this.numbers = this.numbers.map(function(number) {
return number > 9 ? number - 9 : number;
});
}
}``````