format_number.js

function format_number(n,prec,dec,sep) {
    // Original by Luke Smith (http://lucassmith.name)

    n    = !isFinite(+n) ? 0 : +n;
    prec = !isFinite(+prec) ? 2 : Math.abs(prec);
    sep  = sep == undefined ? ',' : sep;

    var s   = n.toFixed(prec),
        abs = Math.abs(n).toFixed(prec),
        _, i;

    if (abs > 1000) {
        _  = abs.split(/\D/);
        i  = _[0].length % 3 || 3;

        _[0] = s.slice(0,i + (n < 0)) +
              _[0].slice(i).replace(/(\d{3})/g, sep+'$1');

        s = _.join(dec || '.');
    }

    return s;
}