JS Format a number with grouped thousands

function number_format(number, decimals, dec_point, thousands_sep) {

    var i, j, kw, kd, km;

    // input sanitation & defaults
    if (isNaN(decimals = Math.abs(decimals))) {
        decimals = 2;
    }
    if (dec_point == undefined) {
        dec_point = ",";
    }
    if (thousands_sep == undefined) {
        thousands_sep = ".";
    }

    i = parseInt(number = (+number || 0).toFixed(decimals)) + "";

    if ((j = i.length) > 3) {
        j = j % 3;
    } else {
        j = 0;
    }

    km = (j ? i.substr(0, j) + thousands_sep : "");
    kw = i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + thousands_sep);
    kd = (decimals ? dec_point + Math.abs(number - i).toFixed(decimals).replace(/-/, 0).slice(2) : "");

    return km + kw + kd;
}