This JavaScript function checks a text field for a valid credit card number through several validation methods.

1. // This code validates a credit card number
2. function ValidateCC(fieldToValidate)
3. {
4.  // Validate for numbers only
5.  var ccRE=/\W/gi;
6.  var CCnumber=fieldToValidate.value.replace(ccRE, "");
7.  if(isNaN(CCnumber))
8.  {
9.  alert("Credit card number is not numeric.");
10.                 fieldToValidate.focus();
11.  return false;
12.  }
13.  // Validate for the number of digits in the credit card number
14.  if((CCnumber.length!=16) && (CCnumber.length!=18))
15.  {
16.  alert("Incorrect number of digits in credit card number.");
17.                 fieldToValidate.focus();
18.  return false;
19.  }
20.  // More advanced validation using a mathematical formula that applies to all credit card numbers
21.  var cardMath=0;
22.  for (i=CCnumber.length; i>0; i--)
23.  {
24.  if (i % 2 == 1)
25.  {
26.  var doubled = "" + (parseInt(CCnumber.substring(i - 1, i)) * 2);
27.  if(doubled.length==2)
28.  {
29.                                 doubled = parseInt(doubled.substring(0,1)) + parseInt(doubled.substring(1,2))
30.  }
31.                         cardMath += parseInt(doubled);
32.  }
33.  else
34.  {
35.                         cardMath += parseInt(CCnumber.substring(i - 1, i))
36.  }
37.  }
38.  if(cardMath % 10 != 0)
39.  {
40.  alert("Credit card number is invalid.");
41.                 fieldToValidate.focus();
42.  return false;
43.  }
44. }
46. /*
47. <form onSubmit="return ValidateCC(this.txtCC);">
48. <input type="text" name="txtCC">
49. <input type="submit" value="Validate">
50. </form>
51. */