Using Opal with Sinatra

Add Opal to your Gemfile (or install using gem):

# Gemfile
gem 'sinatra'
gem 'opal', '~> 0.6.2'

Opal uses sprockets as its default build system, so the asset-pipeline from rails can be mimicked here to map all ruby assets in the /assets path to be compiled using opal.

Basic Application

require 'opal'
require 'sinatra'

opal = {|s|
  s.append_path 'app'
  s.main = 'application'

map opal.source_maps.prefix do
  run opal.source_maps

map '/assets' do
  run opal.sprockets

get '/' do
    <!doctype html>
        <script src="/assets/application.js"></script>

run Sinatra::Application

This creates a simple sprockets instance under the /assets path. Opal uses a set of load paths to compile assets using sprockets. The Opal::Environment instance is a simple subclass of Sprockets::Environment with all the custom opal paths added automatically.

This env object includes all the opal corelib and stdlib paths. To add any custom application directories, you must add them to the load path using env.append_path. You can now add an app/application.rb file into this added path with some basic content:

# app/application.rb
require 'opal'

puts "wow, running ruby!"

It is necessary to require the opal corelib (seen in the require call) above. This just makes the Opal runtime and corelib available. Then it is possible to use all the corelib methods and classes, e.g. Kernel#puts as seen above.

Running Application

As this is just a simple sinatra application, you can run it:

$ bundle exec rackup

And point your browser towards http://localhost:9292/ and view the browser debug console. You should see this message printed.