Entry

How to check credit card digit validity

May 19th, 2000 21:17
Hiroto Sekine,

function ValidCard(CreditCard){
// Valid Input Syntax:
// Type1: **************** (16 columns)
// Type2: **** **** **** **** (19)
// Type3: ****-****-****-**** (19)
// Input is a credit card number in the form of a string.
// Validates numbers which use a "double-add-double MOD 10"
// check digit Output is False if not valid, True if valid.
var Validity = false;       // Assume invalid card
var LN = CreditCard.length;	 // Get input value length
  if ((16 <= LN) && (LN <= 19)){
    LN --;
    CheckSum = 0;	// start with 0 checksum
    Dbl = false;	// Start with a non-doubling
    //------------------------------------------------------------
    //	Beginning backward loop through string
    //-------------------------------------------------------------
    for (Idx = LN; Idx >= 0; Idx --){
	Digit = CreditCard.substr(Idx, 1);	  // Isolate character
	if (("0" <= Digit && Digit <= "9")
             || Digit == " " || Digit == "-"){
	    if (Digit != " " && Digit != "-"){ // Skip connector
		Digit -= "0";	  // Remove ASCII bias
		if (Dbl){		  // If in the "double-add" phase
		    Digit += Digit; // Then double first
		    if (Digit > 9){ // Cast nines
			Digit -= 9;
		    }
		}
		Dbl = !Dbl; // Flip doubling flag
		CheckSum += Digit;  // Add to running sum
		if (CheckSum > 9){  // Cast tens
		    CheckSum -= 10; // Same as MOD 10, but faster
		}
	    }
	} else {
	    return(Validity);	// Invalid
	}
    }
    Validity = (CheckSum == 0) ? true : false; // Must sum to 0
  }
  return(Validity);
}