Credit card number checker

gistfile1.js

function fnValidCreditCardNumber( objFormField )
{
		var		nSum = 0;
		var		nMultiplier = 1;
		var		nIndex;
		var		sDigit;
		var		nProduct;
		var		sCreditCardAccountNumber	= objFormField.value;
		var		nLength						= objFormField.value.length;
	
		if( ( nLength < 13 ) || ( sCreditCardAccountNumber.length > 19 ) )
			return(false);
		for( nIndex = 0; nIndex < nLength; ++nIndex )
		{
			sDigit		= sCreditCardAccountNumber.substring( (nLength-nIndex-1) , (nLength-nIndex) );
			nProduct	= parseInt(sDigit,10) * nMultiplier;
	
			if( nProduct >= 10 )	nSum += ( nProduct % 10) + 1;
			else 					nSum += nProduct;


			if( nMultiplier == 1 )	++nMultiplier;
			else  					--nMultiplier;
		}
	
		if( ( nSum % 10) == 0 )		return(true);
		else						return(false);
}