Format Number as Thousands Currency and Reverse Formate into Numbers in JavaScript

<p id="p1"></p>   

var number = "1234567890.77";

document.getElementById("p1").innerHTML= formatThousands(number);

function formatThousands(number) {

 var number = '' + number;

        var array_n = number.split('.');

        if (array_n.length > 1) {

            var s = array_n[0];

            var d = array_n[1];

            if (array_n[0].length > 3) {

                var i = s.length;

                var r = '';

                var var_2 = i - 3;

                if (s.length > 3) {

                    while ((i -= 3) > 0) {

                        r = ',' + s.substr(i, 3) + r;

                        break;

                    }

                    while ((i -= 2) > 0) {

                        r = ',' + s.substr(i, 2) + r;

                    }

                    return s.substr(0, i + 2) + r + "." + d;

                }

            }

            else {

                return array_n[0] + "." + d;

            }

        }

        else {

            return array_n[0] + "." + "00";

        }

    }

    function reverse_formatThousands(amount) {

        var array_amount = "";

        var array_num = "";

        var var_decimal = "";

        var var_temp_num = "";

        var amount= '' + amount;

        array_amount = amount.split(".");

        if (array_amount.length > 1) {

            var_decimal = array_amount[1];

            array_num = array_amount[0].split(",");

            for (var i = 0; i < array_num.length; i++) {

                var_temp_num += array_num[i];

            }

            return (var_temp_num + "." + var_decimal);

        }

        else {

            array_num = array_amount[0].split(",");

            for (var i = 0; i < array_num.length; i++) {

                var_temp_num += array_num[i];

            }

            return var_temp_num;

        }

    }