behaviors.js

// An Unobtrusive Javascript (UJS) driver based on explicit behavior definitions. Just
// put a "data-behaviors" attribute on your view elements, and then assign callbacks
// for those named behaviors via Behaviors.add.

var Behaviors = {
  add: function(trigger, behavior, handler) {
    document.observe(trigger, function(event) {
      var element = event.findElement("*[data-behaviors~=" + behavior + "]");
      if (element) handler(element, event);
    });
  }
};

/*
EXAMPLE

-- step 1
erb:
<%= link_to "Add an alias", "#", "data-behaviors" => "add-alias" %>

html:
<a data-behaviors="add-alias" href="#">Add an alias</a>

N.B.: Elements can have multiple behaviors, too: just give a space-delimited 
list of behavior names in the “data-behaviors” attribute.

-- step 2

Behaviors.add("click", "add-alias", function(element, event) {
  // ...
});

sources:
http://weblog.jamisbuck.org/2010/3/2/unobtrusive-yet-explicit
http://prototypejs.org/api/event/observe
*/