Published: 04/11/2016

One of the things that I feel is really missing from the JavaScript programming language is the option of private data and private behavior (methods) to an object. I never could get data that an outside caller couldn’t see. That is until I discovered the JavaScript closure.

A closure is typically a function. You can control scope, by returning an object containing data and public instances. What does one look like?



On line 1, we declare an anonymous function and on line 3, we end it and then immediately invoke it. We don’t return any data, we return undefined by default. What’s interesting is that you can pass arguments so that the variables that you use within the scope are local:


})($, _);

As you can see we’re now passing in a jQuery block and an underscore block, no longer do they have to be globals.

To expose data you can return specific data or an object:


var myPrivateData = “randomSocialSecurityNumber”;

return {


socialSecurityNumber: myPrivateData



Here’s a full example with public and private data:

var App = window.App; // get a global

var id = submissionID;

var payRoll = (function($,_, App, id){

var user = App.collection.model(id);

var timeSheet = App.collection.timeSheets.model({userid: id});

var grossPay = (function(timesheet){

return timesheet.rate * timesheet.hours;


var takeHomeRate = (function(user){

return 1 – user.taxRate;


return {

hourlyRate: timeSheet.rate,

hoursWorked: timeSheet.hours,

grossPay: grossPay,

netPay: grossPay * takeHomeRate,


firstName: user.firstName,

lastName: user.lastName,


})($, _, App, id);

So for our payroll object, if we wanted to access the takeHomeRate, we can’t because it’s completely private. We can’t say payRoll.takeHomeRate. But we can say payRoll.grossPay, because it is data we expose by returning it. If we don’t return it, won’t be public. Which means that now we can have an assortment of private attributes to an object that has no exposure, creating a contract by which you can modify the internal structure of JavaScript objects without changing the contract and breaking future users.