If you like to write custom JS code in your APEX applications or to use lots of 3rd party plugins you should be aware of changes in jQuery and JS libraries in APEX 5.
APEX 5 (at least current version 5.0.0.00.28) uses jQuery 2.1.3 and if you plan to migrate from APEX 4.2 you should check for changes from jQuery version 1.7.1 which is included in APEX 4.2. There are some things that are deprecated and removed like .die() or .live() event handlers.
Here you can find things that are removed:
But APEX development team was one step ahead and they provided us with quick solution to include jQuery Migrate plugin that restores deprecated features and behaviors of jQuery.
Note that, as you can read in jQuery documentation, jQuery 2.x has the same API as jQuery 1.x, but does not support Internet Explorer 6, 7, or 8. APEX development team also thought of that and they put conditional comment to handle this:
Again, don't do that. Rather clean up your code!