# Credit card number check

For people building an e-commerce website, a payment portal from scratch, here's some thing which you'll find useful! No plugins required!

This is the **luhn algorithm** or **luhn formula** that checks if the credit card number is valid or not. You'll find this built-in validator in almost all payment processing platforms, like *Stripe*, *Paypal* to name a few.

### The Algorithm

- Starting from the second last digit, moving left double the value of every second digit.
- If the doubled result is greater than 9, subtract 9 from the doubled result
- Find sum all the digits
- Take the modulo 10 of the sum and if it equates to 0, then the number is valid according to the algorithm.

### An Example

Take 79927398713 as an example, walkthrough of the algorithm is as follows: |7|9|9|2|7|3|9|8|7|1|3|

- |7|
**18**|9|**4**|7|**6**|9|**16**|7|**2**|3| - |7|
**9**|9|4|7|6|9|**7**|7|2|3| - Sum of all digits = 70
- 70%10 = 0 Therefore, valid number.

Here's a javascript implementation depicting use of map and reduce. You can try in your favourite language as well!

```
function luhn(no){
no = no.toString();
arr = no.split('').map(x=>parseInt(x));
arr.reverse();
rArr = rArr.map(function(x,index){
if(index%2!=0){
if(x*2 >= 10) x = x*2 - 9;
else x = x*2;
}
return x;
});
sum = rArr.reduce((accu,curr)=>accu+curr);
if(sum%10==0) return true;
return false;
}
```

If you wish to know more about this algorithm, you can read more about this here.

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