gistfile1.js

var callback = function() {
  //do something
}

...

var someObject = new SomeObjectConstructor();
someObject.on('click', callback);

...
function SomeObjectConstructor() {}

SomeObjectConstructor.prototype.on = function(type, callback, thisValue) {
  var _this = thisValue || this;
  var wrappedCallback = function() {
    callback.call(_this);
  };
  //assume "this._wrappedObject" exists and that it has "addEventListener" method that takes 2 arguments
  this._wrappedObject.addEventListener(type, wrappedCallback);
}

SomeObjectConstructor.prototype.off = function(type, callback) {
  ???
}