Complete JavaScript-based credit card verification code

Complete JavaScript-based credit card verification code

function isValidCreditCard(type, ccnum) {   if (type == "Visa") {      // Visa: length 16, prefix 4, dashes optional.      var re = /^4\d{3}-?\d{4}-?\d{4}-?\d{4}$/;   } else if (type == "MC") {      // Mastercard: length 16, prefix 51-55, dashes optional.      var re = /^5[1-5]\d{2}-?\d{4}-?\d{4}-?\d{4}$/;   } else if (type == "Disc") {      // Discover: length 16, prefix 6011, dashes optional.      var re = /^6011-?\d{4}-?\d{4}-?\d{4}$/;   } else if (type == "AmEx") {      // American Express: length 15, prefix 34 or 37.      var re = /^3[4,7]\d{13}$/;   } else if (type == "Diners") {      // Diners: length 14, prefix 30, 36, or 38.      var re = /^3[0,6,8]\d{12}$/;   }   if (!re.test(ccnum)) return false;   // Remove all dashes for the checksum checks to eliminate negative numbers   ccnum = ccnum.split("-").join("");   // Checksum ("Mod 10")   // Add even digits in even length strings or odd digits in odd length strings.   var checksum = 0;   for (var i=(2-(ccnum.length % 2)); i<=ccnum.length; i+=2) {      checksum += parseInt(ccnum.charAt(i-1));   }   // Analyze odd digits in even length strings or even digits in odd length strings.   for (var i=(ccnum.length % 2) + 1; i