Javascript Number formatNumber(decPlaces)

/* Defaults to comma thousands separator , no decimal places */
Number.prototype.formatNumber = function(decPlaces) {
    var n = this,
    decPlaces = isNaN(decPlaces = Math.abs(decPlaces)) ? 0 : decPlaces,
    decSeparator = decPlaces > 0 ? "." : "",
    thouSeparator = ",";
    sign = n < 0 ? "-" : "",
    i = parseInt(n = Math.abs(+n || 0).toFixed(decPlaces)) + "",
    j = (j = i.length) > 3 ? j % 3 : 0;
    return sign + (j ? i.substr(0, j) + thouSeparator : "") + i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + thouSeparator) + (decPlaces ? decSeparator + Math.abs(n - i).toFixed(decPlaces).slice(2) : "");
};