Creating Multiple Instances of a Module

I thought I was starting to understand JavaScript quite well but clearly not. Let me explain my problem with an example. First I have the following module defined:

var Test = function() {
    var counter = 0;

    function init() {
        alert(counter);
    }

    return {
        counter: counter,
        init: init
    }
};

I then create 2 instances:

var test1 = new Test();
var test2 = new Test();

Now I update the counter variable (as it is public) and do some alerts. So far so good.

alert(test1.counter); // Alerts 0
test1.counter = 5;
alert(test2.counter); // Alerts 0
test2.counter = 10;
alert(test1.counter); // Alerts 5

Now finally I say the following:

test1.init(); // Alerts 0
test2.init(); // Alerts 0

This is the bit I don't understand. Why does this alert 0? I thought the first alert would be 5 and the second 10.

I'd appreciate if someone could explain how the above could works or point me in the right direction. Thanks

It stays 0 is because you are not changing the variable inside Test, you are changing the object returned by the function. counter is kept "private" and only a function in Test can access it.

var Test = function() {
    var counter= 0;

    function init() {
            alert(counter);
    }
    function changeNum(n){
        counter = n;            //add a function inside `Test` so that it can
    }                           //access the variable

    return {
        counter: counter,
        init: init,
        changeNum: changeNum
    }
};

Now it will work: http://jsfiddle.net/DerekL/pP284/

var test1 = new Test();
alert(test1.counter);           //0
test1.init();                   //0
test1.changeNum(5);
alert(test1.counter);           //5
test1.init();                   //5

For more information, see JavaScript Closures.