Format number in javascript

formatNumber.js

Number.prototype.formatNumber = function (decimalPlaces, thousandsSeparator, decimalSeparator) {
    decimalPlaces = isNaN(decimalPlaces = Math.abs(decimalPlaces)) ? 2 : decimalPlaces;
    decimalSeparator = decimalSeparator == undefined ? "." : decimalSeparator;
    thousandsSeparator = thousandsSeparator == undefined ? "," : thousandsSeparator;

    var number =  this.toFixed(decimalPlaces);
    if (decimalPlaces) {
        var i = number.substr(0, number.length - (decimalPlaces + 1));
        var j = decimalSeparator + number.substr(-decimalPlaces);
    } else {
        i = number;
        j = '';
    }

    function reverse(str) {
        var result = '';
        for (var l = str.length - 1; l >= 0; l--) {
            result += str.charAt(l);
        }
        return result;
    }

    if (parseInt(i)) {
        i = reverse(reverse(i).replace(/(\d{3})(?=\d)/g, "$1" + thousandsSeparator));
    }

    return i + j;
};