How can I create a credit card validator using Luhn's algorithm?

Question

I am trying to create a simple credit card validator using Luhn's algorithm. If the check digit matches the last inputted number, then it should alert the user that it is valid. Else, say that it isn't valid. Currently, I am getting an error with my sum (total) coming up as NaN. I assume that is the only problem with the code.

<input type="number" id="creditCard" placeholder="0000 0000 0000 0000">
<input type="submit" id="checkButton" value="CHECK VALIDITY" onclick="checkNumber()">

function checkNumber() {
    let number = document.getElementById("creditCard").value;
    let multiplier = "212121212121212";
    let multipliedNumber;
    let multipliedString;

    if (number.length != 16) {
        alert("Please enter a Credit Card number that is 16 digits in length.");
    } else {
        for (count = 0; count < number.length - 1; count++) {
            multipliedNumber = number[count] * multiplier[count];
            console.log(multipliedNumber);
            if (multipliedNumber > 9) {
                multipliedNumber = multipliedNumber[0] + multipliedNumber[1];
                multipliedString = multipliedString + multipliedNumber;
            } else {
                multipliedString = multipliedString + multipliedNumber;

            }
        }
        console.log(multipliedString);
        let checkDigit = 10 - (multipliedString % 10);

        if (checkDigit == number[15]) {
            alert(`${number} is a valid Credit Card number.`);
        } else {
            alert(`${number} is not a valid Credit Card number.`);
        }
    }
}

`multiplier` is a string. `multiplier[count]` is a string. `number` is a string. `number[count]` is a string. What do you expect to happen when you multiply a string with a string? — Andreas, Aug 31 '21 at 07:24
[How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) — Thomas Sablik, Aug 31 '21 at 07:24
Trincot has explained the issues with your code. But you might also look at the [many other Luhn Algorithm JS questions](https://stackoverflow.com/search?q=%5Bjavascript%5D+luhn) already available. Many of these do not share the 16-digit limitation. I was pretty happy with [an answer](https://stackoverflow.com/a/66175134/1243641) I wrote some time ago. — Scott Sauyet, Aug 31 '21 at 12:19

trincot · Accepted Answer · 2021-08-31T11:15:47.687

There are several issues:

multipliedNumber is a product, so it is a number type. Therefore accessing properties like [0] or [1] on it, will just evaluate to undefined. Either turn that value to string first, or (better) use arithmetic to extract the two digits:
```
multipliedNumber = multipliedNumber % 10 + Math.floor(multipliedNumber/10);
```
multipliedString is not initialised, so adding things to it will not give the desired outcome. Secondly, you define it as a string, but it should be a number, as with Luhn's algorithm you are supposed to sum up the resulting digits, not concatenate them. So initialise a variable like this:
```
sum = 0;
```
... and use it like you did -- although you could benefit from the += operator, and since the operation is the same for both cases, you can do it outside of the if..else blocks.
The calculation of the check digit is wrong when the modulo operation evaluates to 0: 10 - (multipliedString % 10) then returns 10, but in that case the check digit is supposed to be 0. It is much easier to just treat that last digit also in the loop and then check that you have reached a multiple of 10. This is also how the algorithm is explained on Wikipedia

Corrected version:

function checkNumber() {
    let number = document.getElementById("creditCard").value;
    let multiplier = "2121212121212121";  // One more character added...
    let multipliedNumber;
    let sum = 0 // Initialise it as a number.

    if (number.length != 16) {
        console.log("Please enter a Credit Card number that is 16 digits in length.");
    } else {
        for (count = 0; count < number.length; count++) { // Include last digit in loop
            multipliedNumber = number[count] * multiplier[count];
            if (multipliedNumber > 9) {
                // Use arithmetic to add the two digits
                multipliedNumber = multipliedNumber % 10 + Math.floor(multipliedNumber/10);
            }
            sum += multipliedNumber;
        }
        let check = sum % 10; // Simpler now because all digits were processed
        if (check == 0) { // Sum is multiple of 10
            console.log(`${number} is a valid Credit Card number.`);
        } else {
            console.log(`${number} is not a valid Credit Card number.`);
        }
    }
}

<input type="number" id="creditCard" placeholder="0000 0000 0000 0000">
<input type="submit" id="checkButton" value="CHECK VALIDITY" onclick="checkNumber()">

How can I create a credit card validator using Luhn's algorithm?

1 Answers1