var ccErrorNo = 0;
var ccErrors = new Array ()

ccErrors [0] = "Unknown card type";
ccErrors [1] = "No card number provided";
ccErrors [2] = "Credit card number is in invalid format";
ccErrors [3] = "Credit card number is invalid";
ccErrors [4] = "Credit card number has an inappropriate number of digits";
ccErrors [5] = "Warning! This credit card number is associated with a scam attempt";

function checkCreditCard (cardnumber, cardname) {
     
  // Array to hold the permitted card characteristics
  var cards = new Array();

  // Define the cards we support. You may add addtional card types as follows.
  
  //  Name:         As in the selection box of the form - must be same as user's
  //  Length:       List of possible valid lengths of the card number for the card
  //  prefixes:     List of possible prefixes for the card
  //  checkdigit:   Boolean to say whether there is a check digit
  
  cards [0] = {name: "Visa", 
               length: "13,16", 
               prefixes: "4",
               checkdigit: true};
  cards [1] = {name: "MasterCard", 
               length: "16", 
               prefixes: "51,52,53,54,55",
               checkdigit: true};
  cards [2] = {name: "DinersClub", 
               length: "14,16", 
               prefixes: "36,38,54,55",
               checkdigit: true};
  cards [3] = {name: "CarteBlanche", 
               length: "14", 
               prefixes: "300,301,302,303,304,305",
               checkdigit: true};
  cards [4] = {name: "AmEx", 
               length: "15", 
               prefixes: "34,37",
               checkdigit: true};
  cards [5] = {name: "Discover", 
               length: "16", 
               prefixes: "6011,622,64,65",
               checkdigit: true};
  cards [6] = {name: "JCB", 
               length: "16", 
               prefixes: "35",
               checkdigit: true};
  cards [7] = {name: "enRoute", 
               length: "15", 
               prefixes: "2014,2149",
               checkdigit: true};
  cards [8] = {name: "Solo", 
               length: "16,18,19", 
               prefixes: "6334,6767",
               checkdigit: true};
  cards [9] = {name: "Switch", 
               length: "16,18,19", 
               prefixes: "4903,4905,4911,4936,564182,633110,6333,6759",
               checkdigit: true};
  cards [10] = {name: "Maestro", 
               length: "12,13,14,15,16,18,19", 
               prefixes: "5018,5020,5038,6304,6759,6761,6762,6763",
               checkdigit: true};
  cards [11] = {name: "VisaElectron", 
               length: "16", 
               prefixes: "4026,417500,4508,4844,4913,4917",
               checkdigit: true};
  cards [12] = {name: "LaserCard", 
               length: "16,17,18,19", 
               prefixes: "6304,6706,6771,6709",
               checkdigit: true};
               
  // Establish card type
  var cardType = -1;
  for (var i=0; i<cards.length; i++) {

    // See if it is this card (ignoring the case of the string)
    if (cardname.toLowerCase () == cards[i].name.toLowerCase()) {
      cardType = i;
      break;
    }
  }
  
  // If card type not found, report an error
  if (cardType == -1) {
     ccErrorNo = 0;
     return false; 
  }
   
  // Ensure that the user has provided a credit card number
  if (cardnumber.length == 0)  {
     ccErrorNo = 1;
     return false; 
  }
    
  // Now remove any spaces from the credit card number
  cardnumber = cardnumber.replace (/\s/g, "");
  
  // Check that the number is numeric
  var cardNo = cardnumber
  var cardexp = /^[0-9]{13,19}$/;
  if (!cardexp.exec(cardNo))  {
     ccErrorNo = 2;
     return false; 
  }
       
  // Now check the modulus 10 check digit - if required
  if (cards[cardType].checkdigit) {
    var checksum = 0;                                  // running checksum total
    var mychar = "";                                   // next char to process
    var j = 1;                                         // takes value of 1 or 2
  
    // Process each digit one by one starting at the right
    var calc;
    for (i = cardNo.length - 1; i >= 0; i--) {
    
      // Extract the next digit and multiply by 1 or 2 on alternative digits.
      calc = Number(cardNo.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};
    } 
  
    // All done - if checksum is divisible by 10, it is a valid modulus 10.
    // If not, report an error.
    if (checksum % 10 != 0)  {
     ccErrorNo = 3;
     return false; 
    }
  }  
  
  // Check it's not a spam number
  if (cardNo == '5490997771092064') { 
    ccErrorNo = 5;
    return false; 
  }

  // The following are the card-specific checks we undertake.
  var LengthValid = false;
  var PrefixValid = false; 
  var undefined; 

  // We use these for holding the valid lengths and prefixes of a card type
  var prefix = new Array ();
  var lengths = new Array ();
    
  // Load an array with the valid prefixes for this card
  prefix = cards[cardType].prefixes.split(",");
      
  // Now see if any of them match what we have in the card number
  for (i=0; i<prefix.length; i++) {
    var exp = new RegExp ("^" + prefix[i]);
    if (exp.test (cardNo)) PrefixValid = true;
  }
      
  // If it isn't a valid prefix there's no point at looking at the length
  if (!PrefixValid) {
     ccErrorNo = 3;
     return false; 
  }
    
  // See if the length is valid for this card
  lengths = cards[cardType].length.split(",");
  for (j=0; j<lengths.length; j++) {
    if (cardNo.length == lengths[j]) LengthValid = true;
  }
  
  // See if all is OK by seeing if the length was valid. We only check the length if all else was 
  // hunky dory.
  if (!LengthValid) {
     ccErrorNo = 4;
     return false; 
  };   
  
  // The credit card is in the required format.
  return true;
}


function testCreditCard() {
  myCardNo = document.getElementById('CardNumber').value;
  myCardType = document.getElementById('CardType').value;
  if (checkCreditCard(myCardNo, myCardType)) {
    alert("Credit card has a valid format")
  } else {
    alert(ccErrors[ccErrorNo])
  };
}

<!-- 
<script src="https://www.braemoor.co.uk/software/_private/creditcard.js"></script>
-->

<!-- COPIED THE DEMO CODE FROM THE SOURCE WEBSITE (https://www.braemoor.co.uk/software/creditcard.shtml) -->

<table>
  <tbody>
    <tr>
      <td style="padding-right: 30px;">American Express</td>
      <td>3400 0000 0000 009</td>
    </tr>
    <tr>
      <td>Carte Blanche</td>
      <td>3000 0000 0000 04</td>
    </tr>
    <tr>
      <td>Discover</td>
      <td>6011 0000 0000 0004</td>
    </tr>
    <tr>
      <td>Diners Club</td>
      <td>3852 0000 0232 37</td>
    </tr>
    <tr>
      <td>enRoute</td>
      <td>2014 0000 0000 009</td>
    </tr>
    <tr>
      <td>JCB</td>
      <td>3530 111333300000</td>
    </tr>
    <tr>
      <td>MasterCard</td>
      <td>5500 0000 0000 0004</td>
    </tr>
    <tr>
      <td>Solo</td>
      <td>6334 0000 0000 0004</td>
    </tr>
    <tr>
      <td>Switch</td>
      <td>4903 0100 0000 0009</td>
    </tr>
    <tr>
      <td>Visa</td>
      <td>4111 1111 1111 1111</td>
    </tr>
    <tr>
      <td>Laser</td>
      <td>6304 1000 0000 0008</td>
    </tr>
  </tbody>
</table>
<hr /> Card Number:
<select tabindex="11" id="CardType" style="margin-left: 10px;">
  <option value="AmEx">American Express</option>
  <option value="CarteBlanche">Carte Blanche</option>
  <option value="DinersClub">Diners Club</option>
  <option value="Discover">Discover</option>
  <option value="EnRoute">enRoute</option>
  <option value="JCB">JCB</option>
  <option value="Maestro">Maestro</option>
  <option value="MasterCard">MasterCard</option>
  <option value="Solo">Solo</option>
  <option value="Switch">Switch</option>
  <option value="Visa">Visa</option>
  <option value="VisaElectron">Visa Electron</option>
  <option value="LaserCard">Laser</option>
</select> <input type="text" id="CardNumber" maxlength="24" size="24" style="margin-left: 10px;"> <button id="mybutton" type="button" onclick="testCreditCard();" style="margin-left: 10px; color: #f00;">Check</button>

<p style="color: red; font-size: 10px;"> COPIED THE DEMO CODE FROM TEH SOURCE WEBSITE (https://www.braemoor.co.uk/software/creditcard.shtml) </p>