Helper to load from Google CDN but with fallbacks

application_helper.rb

def javascript_fallback primary, fallback, test
  html = javascript_include_tag( primary )
  html << "\n" << content_tag(:script, :type => "text/javascript") do
    %Q{
      if (#{test}) {
        document.write(unescape("%3Cscript src='#{fallback}' type='text/javascript'%3E%3C/script%3E"));
      }
    }.gsub(/^ {6}/, '')
  end
  html
end

application_helper_test.rb

test "javascript_fallback should work" do
  helper_html = javascript_fallback "http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js", "/javascripts/jquery-1.4.3.min.js", "typeof jQuery == 'undefined'"
  raw_html = %q{
    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js" type="text/javascript"></script>
    <script type="text/javascript">
      if (typeof jQuery == 'undefined') {
        document.write(unescape("%3Cscript src='/javascripts/jquery-1.4.3.min.js' type='text/javascript'%3E%3C/script%3E"));
      }
    </script>
  }.gsub(/^ {6}/, '').strip
  
  assert_equal(raw_html, helper_html)
end

layout.html.haml

/ Usage:
= javascript_fallback "http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js", "/javascripts/jquery-1.4.3.min.js", "typeof jQuery == 'undefined'"
= javascript_fallback "http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.6/jquery-ui.min.js", "/javascripts/jquery-ui-1.8.6.min.js", "!jQuery.ui"