銀行カード番号アルゴリズム

function checkcard(banknumber) {
        //      
        var last = banknumber.substr(banknumber.length-1,1);
        // 15 18 
        var front_last = banknumber.substr(0,banknumber.length-1);

        var front_arr = new Array();
        //           ( 15 18 )
        for(var i=front_last.length-1;i>-1;i--) {
            // 15 18       
            front_arr.push(front_last.substr(i,1));
        }

        var sum1 = sum2 = sum3 = 0;

        for(var j=0;j<front_arr.length;j++) {
            if((j+1)%2==1) {
                //      
                if(parseInt(front_arr[j])*2<9) {
                    sum1 += parseInt(front_arr[j])*2;
                } else {
                    var str = parseInt(front_arr[j])*2;
                    var str1 = 1;
                    var str2 = str%10;
                    sum2 += str1;
                    sum2 += str2;
                }
            } else {
                //      
                sum3 += parseInt(front_arr[j]);
            }
        }
        var sum = sum1 + sum2 + sum3;
        var luhn = sum%10 == 0?0:10-sum%10;

        if(luhn == parseInt(last)){
            return true;
        }else{
            return false;
        }
    }