Formatting numbers in Javascript with correct rounding. Binding as Prototype to number.

numberFormat.js

//Plain method
var numberFormat = function (number, decPlaces, decSeparator, thouSeparator) {
    decPlaces = isNaN(decPlaces = Math.abs(decPlaces)) ? 2 : decPlaces;
    decSeparator = decSeparator == undefined ? "." : decSeparator;
    thouSeparator = thouSeparator == undefined ? "," : thouSeparator;

    var roundAndReduceDecimals = function (num, length) {
        return Math.round(num * Math.pow(10, length)) / Math.pow(10, length);
    };

    var format = function(n, decPlaces, decSeparator, thouSeparator){
        var sign = n < 0 ? "-" : "",
            i = parseInt(n) + "",
            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) : "");
    };

    return format(roundAndReduceDecimals(number, decPlaces), decPlaces, decSeparator, thouSeparator);
};

//prototyping to Number object
Number.prototype.formatNumber = function(decPlaces, decSeparator, thouSeparator){
    return numberFormat(this, decPlaces, decSeparator, thouSeparator);
};

//example:

var n = 198.6859;
n.formatNumber(2, ',', '.');
//n = 198,69