Hijack callback for any function

gistfile1.txt

/*
Takes any function with a callback as last parameter, and allow to add it another callback executed before it. Requires underscore.

Example:

Mongo.Collection.prototype.insert = extendWithCallback(Mongo.Collection.prototype.insert, function() {
  console.log("This logs every time an insert is done on a collection, even though we did not explicitely declared a callback when calling insert");
});

var items = new Mongo.Collection("items");
items.insert(selector, modifier);

*/

Function.prototype.clone = function() {
  var that = this;
  var temp = function temporary() { return that.apply(this, arguments); };
  for (var key in this) {
    if (this.hasOwnProperty(key)) {
      temp[key] = this[key];
    }
  }
  return temp;
};

var extendCallback = function(fn, cb) {
  if (!_.isFunction(fn) || !_.isFunction(cb)) return;
  fn = fn.clone();
  return function() {
    var fnContext = this;
    var fnArgs = _.toArray(arguments);
    var fnCallback = _.last(fnArgs);
    var hasCallback = _.isFunction(fnCallback);
    if (!hasCallback) return;
    fnArgs.pop();
    fnArgs.push(function() {
      var cbContext = this;
      var cbArgs = _.toArray(arguments);
      cb.apply(cbContext, cbArgs);
      fnCallback.apply(cbContext, cbArgs);
    });
    fn.apply(fnContext, fnArgs);
  }
}