How does the JavaScript variable change function work for variables inside a private IIFE namespace?

This is a piggyback from another question I asked. This was solved in the comments, so I am not looking for an answer to it. Here's where I would like to know about variable scopes:

 var JOHNNY = (function()
    {
        var module = {};
        function getData(id, callback){
                var xhr = new XMLHttpRequest();
                var url = "http://someurl/api/5";

                xhr.onreadystatechange = function () {
                    if (xhr.readyState == 4 && (xhr.status == 200)) {
                        callback(xhr.response);
                    }
                };

                xhr.open("GET", url, true);
                xhr.responseType = 'json';
                xhr.send(null);

        }
        function myCallBack(data){
            return data;  //console.log works;
        }
        module.getStuff=function(id){
            return getData(5, myCallBack);  //just hardcoded id here
        }
        return module;
    })();

      

        
        
        
      

    

I know why this doesn't work due to asynchronous communication. So I get this,

JOHNNY.getStuff(5);
undefined

      

        
        
        
      

    

Again, I know why this is the case. This is not the part I am asking for (the asynchronous part).

My question is about this part:

I can get data if I do this,

function myCallBack(data){
    module.test = data;
}

      

        
        
        
      

    

and then

  JOHNNY.test;  
//has the answer, 5,; this assumes the callback has been called for the questions sake.

      

        
        
        
      

    

The callback assigns the data to the var module

in the callback myCallBack

and I won't rely on the instant response, it is unreliable, etc.

What I am not getting is how the variable module

in the private namespace is updated after I have called this function and the link to JOHNNY has already been created.

In the console I have already made my call,

JOHNNY.getStuff (5);

did not return module

to IIFE? module

Does variable not require using prototype here to update all cases?