You’re asking for zero padding? Not really rounding. You’ll have to convert it to a string since numbers don’t make sense with leading zeros. Something like this…

function pad(num, size) {
    var s = num+"";
    while (s.length < size) s = "0" + s;
    return s;
}

Or if you know you'd never be using more than X number of zeros this might be better. This assumes you'd never want more than 10 digits.

function pad(num, size) {
    var s = "000000000" + num;
    return s.substr(s.length-size);
}