JavaScript implementation of credit card verification method_javascript skills

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<ccnum.length; i+=2) {
   var digit = parseInt(ccnum.charAt(i-1)) * 2;
   if (digit < 10) { checksum += digit; }
   else { checksum += (digit-9); }
  }
  if ((checksum % 10) == 0) return true; else return false;
}