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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.

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})$/;
      return true;
        alert("Not a valid Amercican Express credit card number!");
        return false;


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