I know a little bit about how to create AJAX request using Javascript. But I want to understand how to handle AJAX request using pipeline in Enfinity. Is there any special treatment? Can I just receive the connection using the standard pipeline and standard pipeline url encoding using url() or urlex() function? How if I want to return a value? Do I need special treatment? Or I just can return it as usual page using template?
From server point of view there is no difference between an "ordinary" and an AJAX request. There is no special treatment. Calling a view pipeline and rendering an ISML template in the interaction node is perfectly fine. If you return a JSON as the AJAX response you should mark this in the template, e.g.:
<iscontent type="text/json" charset="UTF-8" compact="true" templatemarker="false">