Published at May 10 2020
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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:

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

Install assignment dependencies:

```
$ npm install
```

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

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', () => {
expect(valid('1')).toEqual(false);
});
xtest('a single zero is invalid', () => {
expect(valid('0')).toEqual(false);
});
xtest('a simple valid SIN that remains valid if reversed', () => {
expect(valid('059')).toEqual(true);
});
xtest('a simple valid SIN that becomes invalid if reversed', () => {
expect(valid('59')).toEqual(true);
});
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', () => {
expect(valid('059a')).toEqual(false);
});
xtest('valid strings with punctuation included become invalid', () => {
expect(valid('055-444-285')).toEqual(false);
});
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', () => {
expect(valid('091')).toEqual(true);
});
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', () => {
expect(valid(':9')).toEqual(false);
});
});
```

```
export const valid = (string) => {
if (/[^0-9\s]/g.test(string)) {
return false;
}
const parsed = string.replace(/\D/g, '').split('')
if (parsed.length < 2) {
return false;
}
const result = parsed.reduceRight((sum, currentDigit, position, arr) => {
const parsedDigit = Number(currentDigit)
const nextPosition = arr.length - position;
if (nextPosition > 0 && nextPosition % 2 === 0) {
const doubled = parsedDigit * 2;
if (doubled > 9) {
return sum + doubled - 9;
}
return sum + doubled;
}
return sum + parsedDigit
}, 0)
return result % 10 === 0;
};
```

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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