Checks the validity of a given Isracard credit card number

isracard.js

function isracardCheck(num) {
    if(num.length < 8 || num.length > 9) return false;
    var diff = "987654321",
        sum = 0,
        a = 0,
        b = 0,
        c = 0;
    if(num.length < 9) num = "0" + num;
    for(var i=1;i<9;i++){
        sum += parseInt(diff.substr(i,1))*parseInt(num.substr(i,1));
    }
    if(sum % 11 == 0) return true;
    else return false;
}