How can I clear out the Rails flash object after responding to an Ajax request?

Rahul's answer can be expressed more succinctly in application_controller.rb or any other controller as:

after_action -> { flash.discard }, if: -> { request.xhr? }

This takes advantage of ActionController's handling of lambdas and conditional filters.

For cleaning all flashes use flash.clear instead of flash.discard

https://api.rubyonrails.org/v5.1.7/classes/ActionDispatch/Flash/FlashHash.html#method-i-clear