# How to Validate a Credit Card Number in Javascript?

Validating credit card in javascript with Luhn’s algorithm.

**. Start from the last digit of the number.**

. Get a double of every alternate digit, if the double value is greater than 9 then take its remainder and add it to 1.

. Create a sum of this doubled values, remainders and each digit then divide it by 10.

. If it is divisible then it is valid.

. Get a double of every alternate digit, if the double value is greater than 9 then take its remainder and add it to 1.

. Create a sum of this doubled values, remainders and each digit then divide it by 10.

. If it is divisible then it is valid.

Following is the implementation of luhn’s algorithm for validation of credit card in javascript.

```
const validateCardNumber = number => {
//Check if the number contains only numeric value
//and is of between 13 to 19 digits
const regex = new RegExp("^[0-9]{13,19}$");
if (!regex.test(number)){
return false;
}
return luhnCheck(number);
}
const luhnCheck = val => {
let checksum = 0; // running checksum total
let j = 1; // takes value of 1 or 2
// Process each digit one by one starting from the last
for (let i = val.length - 1; i >= 0; i--) {
let calc = 0;
// Extract the next digit and multiply by 1 or 2 on alternative digits.
calc = Number(val.charAt(i)) * j;
// If the result is in two digits add 1 to the checksum total
if (calc > 9) {
checksum = checksum + 1;
calc = calc - 10;
}
// Add the units element to the checksum total
checksum = checksum + calc;
// Switch the value of j
if (j == 1) {
j = 2;
} else {
j = 1;
}
}
//Check if it is divisible by 10 or not.
return (checksum % 10) == 0;
}
```

JavaScript: HTML Form - Credit Card Number validation

```
function cardnumber(inputtxt)
{
var cardno = /^(?:3[47][0-9]{13})$/;
if(inputtxt.value.match(cardno))
{
return true;
}
else
{
alert("Not a valid Amercican Express credit card number!");
return false;
}
}
```