how to learn the primefaces javascript API?
Here is the PrimeFaces 5.3 documentation, in PDF form. (Here are docs for all versions.)
Page 536 documents the parameters for the oncomplete(xhr, status, args) function:
Javascript callback to process when ajax request completes. Takes three arguments, xmlhttprequest, status string and optional arguments provided by RequestContext API.
XMLHttpRequest is the vanilla JS Object that we all know and love.
RequestContext is defined on page 588 of the PrimeFaces 5.3 documentation:
RequestContext is a simple utility that provides useful goodies such as adding parameters to ajax callback functions. RequestContext is available in both ajax and non-ajax requests.