When a Rails controller action should handle both HTML and JSON responses, do not use request.xhr? to decide that. Use respond_to.

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I've too often seen code like this:

def show
  # ...
  if request.xhr?
    render json: @user.as_json
  else
     # renders default HTML view
  end
end

This is just plain wrong. Web browsers often fetch JSON via XHR, but they (should) also send the correct Accept HTTP header to tell the server the data they expect to receive.

If you say request.xhr? as a means for "wants JSON" you are locking out any client applications that correctly send Accept: application/json as a "regular" (non-XHR) request. And you are messing up responses for XHRs that want to load something else (like HTML views in Unpoly Show archive.org snapshot ).

Your actions should choose the response format via the respond_to method:

def show
  # ...
  respond_to do |format|
    format.html # renders default HTML view
    format.json { render json: @user.as_json }
  end
end

If your JSON is a bit more complex than the example above, you should put it into its own view (e.g. show.json.jbuilder). You can then refer to both views in your controller:

def show
  # ...
  respond_to :html, :json
end