Question

I’m trying to assign a callback dynamically to an Object of mine, I can’t seem to figure out a way to do this while granting this function access to private variables. I’ve listed the relavant code below with comments where I ran into walls.

Object Factory

function makeObj ( o ) {
                function F() {}
                F.prototype = o;
                return new F();
            }

Module

var MODULE = (function(){
   var myMod = {},
       privateVar = "I'm private";
   return myMod;
})();

Various Attempts

---

myMod.someDynamicFunc = function someDynamicFunc(){
    //privateVar === undefined;
    alert( privateVar );
}
myMod.someDynamicFunc();

---

myMod.prototype.someDynamicFunc = function someDynamicFunc(){
    //ERROR: Cannot set property 'someDynamicFunc' of undefined
    alert(privateVar);
}
myMod.someDynamicFunc();

---

In this attempt I tried making a setter in the module object… to no avail.

var MODULE = (function(){
       var myMod = {},
           privateVar = "I'm private";

           myMod.setDynamicFunction = function ( func ){
              if(func !== undefined && typeof func === "function"){
                //Uncaught TypeError: 
                //         Cannot read property 'dynamicFunction' of undefined
                myMod.prototype.dynamicFunction = func;
                //also tried myMod.dynamicFunction = func;
              }
           }
       return myMod;
    })();

var myModule = makeObject( MODULE );

myModule.setDynamicFunction(function(){
   alert(privateVar);
});

myModule.dynamicFunction();

---

Am I just using JavaScript wrong? I’d really like to be able to assign callbacks after the object is initiated. Is this possible?

Answer 1

You can’t access the private variable via a callback function set dynamically (since it can’t be a closure if it’s attached later), but you can set up a system by which you would be able to access the variable:

var MODULE = (function(){
   var myMod = {},
       privateVar = "I'm private";

   myMod.callback = function(fn) {fn(privateVar);};

   return myMod;
})();

var someDynamicFunc = function(param) {alert(param);};
myMod.callback(someDynamicFunc);

Of course, this makes it not really private, since anyone could do this. I don’t see how it would be possible at all for you to have a “private” variable that you access via dynamically attached functions, without allowing anyone else’s dynamically attached functions to have the same privilege (thus making it not really private).