// JScript File
function valid()
{
    var digit=document.frmCard.txtNo.value.length; 
    var cardNo=document.frmCard.txtNo.value;
    var total=0;
    
    if(digit>=15 && digit<17)
    {					
        if(digit==15)
        {						
            // Credit card no length is 15.						
            for (var i = 0; i < digit; i++) 
            {							
                var ch = cardNo.substring(i, i + 1);							
                var temp=0;							
                if(i%2==0)
                {								
                    temp = ch * 1;								
                    greaternine();
                }
                else
                {
                    temp = ch * 2;
                    greaternine();
                }
                total=total+temp;	
            }
            var modtotal = total % 10;
            if(modtotal==0)  
            {
                alert("Valid credit card.");
            } 
            else 
            {
                alert("Invalid credit card.");
            }						
        }
        else
        {
            // Credit card no length is 16.
            for (var i = 0; i < digit; i++) 
            {							
                var ch = cardNo.substring(i, i + 1);							
                var temp=0;							
                if(i%2!=0)
                {								
                    temp = ch * 1;								
                    greaternine();
                }
                else
                {
                    temp = ch * 2;
                    greaternine();
                }
                total=total+temp;	
            }
            var modtotal = total % 10;
            if(modtotal==0)  
            {
                alert("Valid credit card.");
            } 
            else 
            {
                alert("Invalid credit card.");
            }	
        }
    } 
    else 
    {
        alert("Credit card no is not valid");
    }
    function greaternine()
    {
        if(temp > 9) 
        {
            temp = temp-9;
        }
    }
}