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# Luhn
Welcome to Luhn on Exercism's JavaScript Track.
If you need help running the tests or submitting your code, check out `HELP.md`.
## Instructions
Given a number determine whether or not it is valid per the Luhn formula.
The [Luhn algorithm](https://en.wikipedia.org/wiki/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
```text
4539 3195 0343 6467
```
The first step of the Luhn algorithm is to double every second digit,
starting from the right. We will be doubling
```text
4_3_ 3_9_ 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:
```text
8569 6195 0383 3437
```
Then sum all of the digits:
```text
8+5+6+9+6+1+9+5+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
```text
8273 1232 7352 0569
```
Double the second digits, starting from the right
```text
7253 2262 5312 0539
```
Sum the digits
```text
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.
## Source
### Created by
- @matthewmorgan
### Contributed to by
- @ankorGH
- @gabriel376
- @ovidiu141
- @rchavarria
- @ryanplusplus
- @SleeplessByte
- @tejasbubane
- @xarxziux
### Based on
The Luhn Algorithm on Wikipedia - http://en.wikipedia.org/wiki/Luhn_algorithm
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