total-amount.js

/**
 * Total amount function using module pattern
 */

var TotalAmount = (function () {

    var amount = 0;
    var api = {};

    function isNumber(n) {
        return !isNaN(parseFloat(n)) && isFinite(n);
    }

    api.get = function () {
        var parts = amount.toFixed(2).toString().split('.');
        parts[0] = parts[0].replace(/\B(?=(\d{3})+(?!\d))/g, ',');
        return '$' + parts.join('.');
    };

    api.increment = function (value) {
        if (isNumber(value)) amount += value;
    };

    api.decrement = function (value) {
        if (isNumber(value)) amount -= value;
        if (amount < 0) amount = 0;
    };

    api.set = function (value) {
        if (isNumber(value)) amount = value;
    };

    api.reset = function () {
        amount = 0;
    };

    return api;

})();

console.log(TotalAmount.get()); // $0.00

TotalAmount.increment(329.99);
console.log(TotalAmount.get()); // $329.99

TotalAmount.increment(999);
console.log(TotalAmount.get()); // $1,328.99

TotalAmount.decrement(19.99);
console.log(TotalAmount.get()); // $1,309.00

TotalAmount.reset();
console.log(TotalAmount.get()); // $0.00