I find this code very helpful, specially if you have s shopping card website:

TODO: just copy and paste this code into notepad and save it as validator.html and open it with your browser

want to see it in action? ok here we go, i will show you a demo:

DEMO

Please Enter Credit Card Number:




Here is the code i used:


<html>
<!-- TWO STEPS TO INSTALL CREDIT CARD NUMBER VALIDATION:

  1.  Copy the coding into the HEAD of your HTML document
  2.  Add the last code into the BODY of your HTML document  -->

<!-- STEP ONE: Paste this code into the HEAD of your HTML document  -->

<HEAD>

<script type="text/javascript">
<!--
/* Created by: David Leppek

Basically, the alorithum takes each digit, from right to left and muliplies each second
digit by two. If the multiple is two-digits long (i.e.: 6 * 2 = 12) the two digits of
the multiple are then added together for a new number (1 + 2 = 3). You then add up the 
string of numbers, both unaltered and new values and get a total sum. This sum is then
divided by 10 and the remainder should be zero if it is a valid credit card. Hense the
name Mod 10 or Modulus 10. */
function Mod10(ccNumb) {  // v2.0
var valid = "0123456789"  // Valid digits in a credit card number
var len = ccNumb.length;  // The length of the submitted cc number
var iCCN = parseInt(ccNumb);  // integer of ccNumb
var sCCN = ccNumb.toString();  // string of ccNumb
sCCN = sCCN.replace (/^\s+|\s+$/g,'');  // strip spaces
var iTotal = 0;  // integer total set at zero
var bNum = true;  // by default assume it is a number
var bResult = false;  // by default assume it is NOT a valid cc
var temp;  // temp variable for parsing string
var calc;  // used for calculation of each digit

// Determine if the ccNumb is in fact all numbers
for (var j=0; j<len; j++) {
  temp = "" + sCCN.substring(j, j+1);
  if (valid.indexOf(temp) == "-1"){bNum = false;}
}

// if it is NOT a number, you can either alert to the fact, or just pass a failure
if(!bNum){
  /*alert("Not a Number");*/bResult = false;
}

// Determine if it is the proper length 
if((len == 0)&&(bResult)){  // nothing, field is blank AND passed above # check
  bResult = false;
} else{  // ccNumb is a number and the proper length - let's see if it is a valid card number
  if(len >= 15){  // 15 or 16 for Amex or V/MC
    for(var i=len;i>0;i--){  // LOOP throught the digits of the card
      calc = parseInt(iCCN) % 10;  // right most digit
      calc = parseInt(calc);  // assure it is an integer
      iTotal += calc;  // running total of the card number as we loop - Do Nothing to first digit
      i--;  // decrement the count - move to the next digit in the card
      iCCN = iCCN / 10;                               // subtracts right most digit from ccNumb
      calc = parseInt(iCCN) % 10 ;    // NEXT right most digit
      calc = calc *2;                                 // multiply the digit by two
      // Instead of some screwy method of converting 16 to a string and then parsing 1 and 6 and then adding them to make 7,
      // I use a simple switch statement to change the value of calc2 to 7 if 16 is the multiple.
      switch(calc){
        case 10: calc = 1; break;       //5*2=10 & 1+0 = 1
        case 12: calc = 3; break;       //6*2=12 & 1+2 = 3
        case 14: calc = 5; break;       //7*2=14 & 1+4 = 5
        case 16: calc = 7; break;       //8*2=16 & 1+6 = 7
        case 18: calc = 9; break;       //9*2=18 & 1+8 = 9
        default: calc = calc;           //4*2= 8 &   8 = 8  -same for all lower numbers
      }                                               
    iCCN = iCCN / 10;  // subtracts right most digit from ccNum
    iTotal += calc;  // running total of the card number as we loop
  }  // END OF LOOP
  if ((iTotal%10)==0){  // check to see if the sum Mod 10 is zero
    bResult = true;  // This IS (or could be) a valid credit card number.
  } else {
    bResult = false;  // This could NOT be a valid credit card number
    }
  }
}
// change alert to on-page display or other indication as needed.
if(bResult) {
  alert("This IS a valid Credit Card Number!");
}
if(!bResult){
  alert("This is NOT a valid Credit Card Number!");
}
  return bResult; // Return the results
}
// -->
</script>
</HEAD>

<!-- STEP TWO: Copy this code into the BODY of your HTML document  -->

<BODY>

<div align="center">
<form name="Form1">
<table width="50%" border="0" cellspacing="0" cellpadding="5">
  <tr>
    <td width="50%" align="right">Credit Card Number:   </td>
    <td width="50%">
      <input name="CreditCard" type="text" value="4012888888881881" size="18" maxlength="16" style="border: 1px solid #000098; padding: 3px;">
    </td>
  </tr>
  <tr>
    <td colspan="2" align="center">
      <input type="button" name="Button" style="color: #000; background: #EEE; font-weight:bold; border: solid 1px #000;" value="TEST CARD NUMBER" onClick="return Mod10(document.Form1.CreditCard.value);">
    </td>
  </tr>
</table>
</form>
</div>


</body>
</html>