Question

Say I have the following modules, split across multiple files both capable of extending skillet:

File1.js:

(function(){
 var privateVar1 = 0;

 var privateFunction1 = function() {
  //function definiton
 };

 skillet.fry() = function() {
  //fry it
  //matchbox.light();
 };

})(window.skillet = window.skillet || {});

File2.js:

(function(){
 var privateVar2 = 0;

 var privateFunction2 = function() {
  //some private function 
 };

 skillet.grillIt = function() {
  //grill It
  //matchbox.strike(); <-- Shared with File1.js
 };

})(window.skillet = window.skillet || {});

Is it possible to have a shared variable/object like matchbox be sharable by the two modules without being bound to window.matchbox or window.skillet.matchbox? I.e. the visibility of matchbox should only be to File1.js and File2.js and must not be accessible elsewhere. I doubt if it’s possible, but is there a way to achieve such a behavior in JavaScript? If not, what’s the best practice to use in this regard?

(It’s more like having a shared event-bus among a set of related modules without exposing that bus globally)

Answer 1

Nope.

“private” variables work in JS only because of the scope that the function was declared in. There is no way to share that scope with a function declared in an entirely different scope. Scope is an unchangeable property of functions, once they are created.

This is why this sort of thing is usually done with _foo style properties.

skillet._matchbox = { strike: function() { ... } };

The underscore prefix is convention for “internal” and serves as a hint not to mess with it.

---

You could also get creative with how you pass matchbox around though, though in all cases it will mean providing a way to get matchbox out it’s original scope. Like perhaps, makes a skillet.extend method that passes the matchbox to it’s argument?

(function() {
  var matchbox = { strike: function() { ... } }
  window.skillet = {
    extend: function(fn) {
      fn(matchbox);
    }
  };
})();

skillet.extend(function(matchbox) {
  var privateVar2 = 0;
  var privateFunction2 = function() {};

  skillet.grillIt = function() {
    //grill It
    matchbox.strike();
  };
}

Which allows you to use matchbox outside it’s original scope in a controlled way. But it also allows anyone to get matchbox that maybe shouldn’t.

var stolenMatchbox;
skillet.extend(function(matchbox) {
  stolenMatchbox = matchbox;
});
while (stolenMatchbox.count > 0) { stolenMatchbox.strike(); }
alert("Now you are outta matches, sucker!");