There’s yet another reason to master more than one JavaScript library: you can use some of them together! Since MooTools is prototype-based and jQuery is not, jQuery and MooTools may be used together on the same page.
Many JavaScript libraries use $ as a function or variable name, just as jQuery does. In jQuery’s case, $ is just an alias for jQuery, so all functionality is available without using $. If we need to use another JavaScript library alongside jQuery, we can return control of $ back to the other library with a call to jQuery.noConflict();
see example here :
jQuery.noConflict();
(function($){
//code goes here(here is your jQuery code)
// here $ is now jQuery
})(jQuery);
// here $ is not jQuery
(function($){
//code goes here(here is your jQuery code)
// here $ is now jQuery
})(jQuery);
// here $ is not jQuery