Published at Jul 13 2018
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Instructions

Test suite

Solution

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.

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.

```
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!

```
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:

http://exercism.io/languages/javascript/installation

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

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');
expect(luhn.valid).toEqual(false);
});
xit('A single zero is invalid', function () {
const luhn = new Luhn('0');
expect(luhn.valid).toEqual(false);
});
xit('valid Canadian SIN', function () {
const luhn = new Luhn('046 454 286');
expect(luhn.valid).toEqual(true);
});
xit('invalid Canadian SIN', function () {
const luhn = new Luhn('046 454 287');
expect(luhn.valid).toEqual(false);
});
xit('invalid credit card', function () {
const luhn = new Luhn('8273 1232 7352 0569');
expect(luhn.valid).toEqual(false);
});
xit('valid strings with a non-digit added become invalid', function () {
const luhn = new Luhn('046a 454 286');
expect(luhn.valid).toEqual(false);
});
});
```

```
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();
}
module.exports=Luhn;
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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