Creating Multiple Instances of a Module

I am not sure if you have made a mistake in your post, but you could rewrite the above code as follows

var Test = function() {
  this.counter = 0;
}

Test.prototype.init = function() {
  alert(this.counter);  
}

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


test1.counter = 5;
test2.counter = 10;

test1.init(); // alerts 5

test2.init(); // alerts 10

In your example you don't set the counter to be a property on your Test object/function, rather when you call test1.counter you are essentially setting a new property which didn't exist before, and your init function is not referencing that property.

As dereks answer shows, you seem to have slightly confused the two different patterns between the one my answer follows, and his.

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.

alert(test1.counter); // Alerts 0 ????? It's "0" because you call it before change counter to 5
test1.counter = 5;
alert(test2.counter); // Alerts 0 ????? It's "0" because you call it before change counter to 10
test2.counter = 10;
alert(test1.counter); // Alerts 5 | It's 5 because you call it AFTER change counter to 5

This is what happened:

1. the init() function made a closure on the counter variable, which is defined inside Test scope, holding a reference to it.
2. the return from the Test() function created a new object, with another variable counter, set to value of internal counter.
3. You update that 'another' counter, by setting test1.counter = X, but the init() still holds a reference to original variable.

That's why you see the old value.