Hello,
my app is heavily based on jQuery.
is there a recommendation on its usage?
pros & cons?
thank you,
Marcel
In general: try to split your logic from your rendering. Heavily based on jQuery reads like the usual frontend ball of mud 
As for jQuery: http://ndugger.github.io/youdontneedjquery/ and https://developers.google.com/web/updates/2015/03/introduction-to-fetch
Almost all of the jQuery’s APIs are now available directly in the DOM and easy to use as well. Since Nextcloud ships jQuery you won’t get that much of a benefit of dropping it but it’s a good idea to future proof things. You may also get performance boosts since jQuery’s selectors are slow (see http://vanilla-js.com/)
Only argument for keeping jQuery apart from familiarity would be IE10 or lower support