Generic Javascript proxy for Firebase

Question

I'm using a proxy class as the data I have is a reference to a Firebase location that stores my object but I want to act as if I have the object itself. I've got something that works fine but I would like to improve it, the key criteria being to reduce repetition. I suspect something is possible by inspecting the Map class and using apply() but I don't know quite how to do that (or if there is a better solution).

I think it would also be useful if the solution could be generalised to support any class, not just the Map class.

var Map = function() {
  ...
};

var MapProxy = function(mapRef) {
  this.mapRef = mapRef;
};

Map.prototype.addToken = function(portrait, newLocation) {
  ...
};

Map.prototype.removeToken = function(token) {
  ...
};

Map.prototype.moveToken = function(token, newLocation) {
  ...
};

MapProxy.prototype.addToken = function(portrait, newLocation) {
  var mapRef = this.mapRef;

  mapRef.once('value', function(data) {
    var map = new Map();
    map.init(mapRef, data.val());

    map.addToken(portrait, newLocation);
  });
};

MapProxy.prototype.removeToken = function(token) {
  var mapRef = this.mapRef;

  mapRef.once('value', function(data) {
    var map = new Map();
    map.init(mapRef, data.val());

    map.removeToken(token);
  });
};

MapProxy.prototype.moveToken = function(token, newLocation) {
  var mapRef = this.mapRef;

  mapRef.once('value', function(data) {
    var map = new Map();
    map.init(mapRef, data.val());

    map.moveToken(token, newLocation);
  });
};

var mapProxy = new MapProxy(mapRef);    

Answer 1

Think I solved it myself in the end.

var FirebaseProxy = function(classToProxy, firebaseRef) {
  var key,
      self = this;

  self.proxy = classToProxy;
  self.firebaseRef = firebaseRef;

  for (key in self.proxy.prototype) {
    if (typeof self.proxy.prototype[key] === 'function') {
      (function(inner_key) {
        self[inner_key] = function ()
        {
          var args = arguments;

          self.firebaseRef.once('value', function(data) {
            var proxiedInstance = new self.proxy();

            if (typeof proxiedInstance.init === 'function') {
              proxiedInstance.init(self.firebaseRef, data.val());
            }

            proxiedInstance[inner_key].apply(proxiedInstance, args);
          });
        } 
      })(key);        
    }
  }
}