opal makes it easy to build static opal applications. Opal uses internal load paths to make it easy to handle resolving requirements during building. opal forms the basis of rails support, so anything you can do there, you can do standalone as well.
First, install opal adding them to a Gemfile:
# Gemfile
gem "opal", "~> 0.7.0"
Next, we want to add our main app code. Keep all opal code inside app/ directory, and edit app/application.rb:
# app/application.rb
require "opal"
puts "Wow, running opal!"
You will notice the require "opal" line which will automatically include the opal runtime and corelib into our output, giving us access to the puts() method.
To build this, we need the rake task to add our app/ directory to the load path, and then to build our target file application.rb which will be found because it is inside our added load path.
# Rakefile
require 'opal'
desc "Build our app to build.js"
task :build do
Opal.append_path "app"
File.binwrite "build.js", Opal::Builder.build("application").to_s
end
Now, if you run rake build you will get the build.js output file with our application compiled, with the opal runtime included as well.
To run the application, lets create a very simple html file:
<!DOCTYPE html>
<html>
<head>
<script src="build.js"></script>
</head>
<body>
</body>
</html>
Now, open this html file and check the browsers console. You should see our message printed in the console.