Question

I have begun writing my ‘class’ type JavaScript functions like the Module Pattern or Revealing Module patten. This avoids the use of ‘new’ and allows me to define which functions are public in a single place. However, I have been unable to see how to use and access public data members in this pattern.

e.g. the member ‘id’ below is accessible through myobj.getId(), but myobj.id is undefined.

function setup(){
    var myobj = MyObject();
    myobj.initialize();
    alert(myobj.getId()); // returns 3, as expected
    alert(myobj.id); // returns undefined
}
function MyObject(){
    var id;
    function initialize(){
        //do some stuff that initializes data including...
        id = 3;
    }
    return{
        initialize:initialize,
        id:id,
        getId:function(){ return id;}
    }
}

Is there no way to get myobj.id to work returning the value that was set in initialize()?

Answer 1

The reason is that your variable id is set to undefined by default and it’s type will be undefined as well. Now, because undefined type is a primitive one your assignment in the returned object will be a value assignment, not reference. So obj.id will become undefined in the instance and not the same as the id variable in the constructor function.

Now initialize will change the id in the constructor function, and getId will return the same id, but obj.id will refer to the undefined object property.

function MyObject(){
    // initialize is used as a public function so
    // this will always refer to the current instance
    function initialize(){
        this.id = 3;
    }
    return {
        // no need to define id here it will 
        // report undefined on access by default
        initialize: initialize,
        getId:function(){ return this.id; }
    }
}

Run the whole stuff to see it works as expected.
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