Praise

Opal is truly amazing and it has taken me in a whole new direction.
— @mistergibson from Gitter, on Jan 22 2018
Opal 0.11 is a blessing! Its a giant step forward to ruby on the client! Its just amazing what bugs i find in existing 0.10 code and do keep wondering, "How did this ever work at all?" I am amzed, fantastic 👍👍👍😄
— @janbiedermann from Gitter, on Jan 20 2018
These guys are great. We have a large production app (www.catprint.com) that thanks to opal is 100% RUBY. The UI part is about 17K of opal code, using the hyperstack (www.hyperstack.org) framework.
In practice Opal == MRI Ruby, very reliable and solid.
— @catmando from Gitter, on Dec 1 2018

Overview

Try this code

class User
  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def admin?
    @name == 'Admin'
  end
end

user = User.new('Bob')

# the output will go to your browser's console
puts user
puts user.admin?

Getting Started: Rack

Add rack to your Gemfile.

gem 'rack'

Setup the Opal rack-app in your config.ru as follows:

require 'opal'

run(Opal::SimpleServer.new do |s|
  # the name of the ruby file to load. To use more files they must be required from here (see app)
  s.main = 'application'

  # the directory where the code is (add to opal load path )
  s.append_path 'app'

  # need to set the index explicitly for opal server to pick it up
  s.index_path = 'index.html.erb'
end)

Add a file named hello_world.js.rb to app/ with your hello world:

require 'opal'
require 'native'
$$.alert 'Hello World from Opal!'

Then start the server with bundle exec rackup and visit http://localhost:9292.



Learn more about Opal Sprockets


Getting Started: Rails

Add opal-rails to your Rails app's Gemfile:

gem 'opal-rails'

Rename app/assets/javascripts/application.js to app/assets/javascripts/application.js.rb and replace its contents with this code:

require 'opal'
require 'opal_ujs'
require 'turbolinks'
require_tree '.'

Create a "Hello World" controller:

bin/rails generate controller HelloWorld index

Replace the contents of app/assets/javascripts/hello_world.js.rb with:

Document.ready? do
  Element.find('body').html = '<h1>Hello World from Opal!</h1>'
end

Start the server with bin/rails server and visit: http://localhost:3000/hello_world/index.



Learn more about Opal Rails


Getting Started: Command Line Interface (CLI)

Install opal from Rubygems:

gem install opal

Write this code to hello_world.js.rb:

require 'ostruct'
greeting = OpenStruct.new(type: :Hello, target: :World, source: :Opal)
puts "#{greeting.type} #{greeting.target} from #{greeting.source}!"

Run it with Node.js (assuming it's installed):

opal hello_world.js.rb
# => Hello World from Opal!

Compile it to a JavaScript file:

opal -c hello_world.js.rb > hello_world.js
node hello_world.js
# => Hello World from Opal!

For a full list of supported options see:

opal --help



Learn more about the CLI


Resources