gistfile1.js

/*
 Get a warning when jQuery finds no matches for a selector
 Handy for debugging, although I wouldn't want it in production
*/

jQuery.oldFind = jQuery.find;
jQuery.find = function(sel) {
  var result = jQuery.oldFind.apply(jQuery, arguments);
  if(typeof(sel) == "string" && result.length == 0) {
    console.warn("Nothing matches selector: ", sel);
  }
  return result;
}

jQuery(function($) {
  $("div");     // Finds stuff
  $("foo");     // Finds nothing
});