Mainly its a 3-liner, but packed in a function it becomes a little more. Feel free to use it for your own:

function thousand_sep(nr) {
    var nr_tmp = nr.toString();
    for (var dots = Math.ceil((nr_tmp.length / 3) - 1); dots > 0; dots--) {
        nr_tmp = nr_tmp.substr(0, nr_tmp.length - (dots * 3)) + '.' +
                 nr_tmp.substring(nr_tmp.length - (dots * 3), nr_tmp.length);
    }
    return (nr_tmp);
}