UJS translation of ugly RJS problem posted to Devchix that I responded to on 2010-04-10.

controller_snippet.rb

  # PUT /users/1
  def update        
    @user = @session_user
    respond_to do |format|
      if @user.update_attributes(params[:user])
        format.js do
          render :json => {:status => 'success', :html => (render_to_string(:partial => 'display_avatar', :locals => { :user => @user })) }
        end
        format.html do 
          flash[:notice] = 'The resource was updated'
          redirect_to your_resources_url
        end
      else
        format.js do
          render :json => {:status => 'error', :html => (render_to_string(:partial => 'avatar_errors', :locals => { :user => @user })) }
        end
        format.html do
          flash[:notice] = 'There were errors in the form'
          render :action => 'edit'
        end
      end
    end
  end

# replacing original code of:
#    render :update do |page|
#      if @user.update_attributes(params[:user]) 
#        page.hide "upload-avatar"
#        page.replace_html "avatar-error", ""
#        page.replace "avatar", :partial => "display_avatar", :locals => {:user => @user}
#      else                        
#        page.insert_html :before, "disclaimer", :partial => "avatar_errors", :locals => {:user => @user}
#      end
#    end

javascript_snippet.js

(function ($) {
  $(document).ready(function () {
    // you can just use $('selector_for_editor_form').submit(...) instead if the form isn't 
    // dynamically added to the DOM and available to bind events to on page ready event.
    // This can also be turned very easily into a jQuery plugin so you would only need to do:
    // $('selector_for_edit_form').ajaxSubmit() or something like that.
    $('selector_for_edit_form').live('submit', function(e) {
      var serializedData = $(this).serialize();
      var actionUrl = $(this).attr('action');
      $.getJSON(actionUrl, serializedData, MY_APP.users.editResponse)
      return false;
    });
  });

  // Assumes your application has a namespace that you can add your own code to called MY_APP.users:
  MY_APP.users.handleEditResponse = function (data, statusText) {
    var status = data['status'];
    var html = data['html'];
    if ('success' === status) {
      $('#upload-avatar').hide();
      $('#avatar-error').html('');
      $('#avatar').html(html);
    } else {
      // perhaps this is what you really want?
      $('#avatar-error').html(html);
      // if not then take the above line out and just use:
      $('#disclaimer').before(html);
    }
  };
}) (jQuery);