Conditional javascript require in the asset pipeline

I'm struggling with the asset pipeline. I'm loading dojo from Google CDN putting this in my template:

= javascript_include_tag 'http://ajax.googleapis.com/ajax/libs/dojo/1.6.1/dojo/dojo.xd.js', :'data-dojo-config' => %Q(dojoBlankHtmlUrl:'/blank.html', baseUrl: 'assets/', modulePaths: {custom: 'javascripts/modules'})

I just want a fallback to a local version if running locally or if the CDN is down. I thought of doing this:

script typeof(dojo) === "undefined" && document.write(unescape('%3Cscript src="js/libs/dojo-1.6.1.min.js"%3E%3C/script%3E'));

But I don't like it as it works out of the asset pipeline. I want to keep dojo in vendors/assets/javascripts/dojo. How can I get the fallback to be served by the asset pipeline.

Is there a way do declare conditional require in the asset pipeline. What I want is to run some javascript tests, and depending on the result serve a file.

Thanks

Solution

Thanks Richard!

I don't want to have yepnope to load one library. It would be overkill imo. Here is the solution I came up with, based on your help (written in slim):

1/ In vendors/assets/javascripts/, I have my dojo.js.

2/ In config/application.rb:

# Precompile these assets files
config.assets.precompile += ['dojo.js']

3/ In the template:

= javascript_include_tag "http://ajax.googleapis.com/ajax/libs/dojo/#{Settings.dojoVersion}/dojo/dojo.xd.js", :'data-dojo-config' => %Q(dojoBlankHtmlUrl:'/blank.html', baseUrl: 'assets/', modulePaths: {custom: 'javascripts/modules'})
script = "typeof(dojo) === \"undefined\" && document.write(unescape('%3Cscript src=\"#{asset_path('dojo')}\"%3E%3C/script%3E'));".html_safe

I also posted on the Rails Google Group to request the addition of two options to the javascript_include_tag, :test and :local that would take care of all the work. We'll see.