example.js

// A specific controller and action
$S.init('users.index', function() {});

// All actions in Users
$S.init('users.*', function() {});

// All new actions in all controllers
$S.init('*.new', function() {});

// Everything all the time
$S.init('*.*', function() {});

// Multiple selectors (can be as many as you need)
$S.init('users.index', 'admins.index', function() {});

loader.js

/* 
 * LOADER
 * Executes a function based on the section -- indicated by the body
 * id -- and the page -- indicated by the body class. This allows us to
 * conditionally execute JS on a page without having to manually check for the
 * existence of elements. 
 */
var $S = {
  _inits: {},
  init: function() {
    var args = Array.prototype.slice.call(arguments);
    args.length > 2 ? this._forPages(args) : this._forPage(args[0], args[1]);
  },
  _forPage: function(section, fun) {
    var parts = section.split('.');
    if (parts[0] === '') {
      var section = parts[1];
      var page = '*';
    }
    else {
      var section = parts[0];
      var page = parts[1];
    }
    var entry = this._inits[section] || (this._inits[section] = {});
    entry[page] || (entry[page] = []);
    entry[page].push(fun);
  },
  _forPages: function(args) {
    var fun = args.pop();
    for (var i = 0; i < args.length; i++) { this._forPage(args[i], fun); }
  },
  load: function() {
    var body = $(document.body);
    var section = body.attr('id');
    var page = body.attr('class');
    this._fire(section, page);
    this._fire('*', page);
  },
  _fire: function(section, page) {
    var entry = $S._inits[section];
    if (entry) {
      var all = entry['*'], per = entry[page];
      if (all) for (var i = 0; i < all.length; i++) { all[i](); }
      if (per) for (var j = 0; j < per.length; j++) { per[j](); }
    }
  }
};

jQuery($.proxy($S, 'load'));