Hot code reloading for Opal apps, includes React.rb support
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spec-opal [fixes #9] make alerts customizable Mar 29, 2017
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opal-hot-reloader is a hot reloader for Opal. It has built in react.rb support and can be extended to support an arbitrary hook to be run after code is evaluted. It watches directories specified and when a file is modified it pushes the change via websocket to the client. opal-hot-reloader reloader will reload the following without reloading the whole page and destroying any state the page has.

  • opal code
  • css (currently supporting Rack::Sass:Place and Rails asset pipeline)


Add this line to your application's Gemfile:

gem 'opal_hot_reloader'

And then execute:

$ bundle

Or install it yourself as:

$ gem install opal_hot_reloader


Server Setup

NOTE: fixed for Opal 0.10 as of commit d2dc849 - repull if not working for 0.10.0

After adding gem "opal_hot_loader" to your gemfile, you must start the server-side part. This will allow websocket connections, and whenever a file is changed it will send it via the socket to listening clients.

To start the server-side of the hotloader:

opal-hot-reloader -p 25222 -d dir1,dir2,dir3

Usage: opal-hot-reloader [options]
    -p, --port [INTEGER]             port to run on, defaults to 25222
    -d, --directories x,y,z          comma separated directories to watch

For a react.rb Rails app, opal-hot-reloader automatically includes app/assets/javascripts,app/views/components if they exist

Example adding 2 directories

opal-hot-reloader -d app/js,app/client/components

You may consider using foreman and starting the Rails server and hot reloader at the same time. If you are doing react.rb development w/Rails, you may already be doing so with the Rails server and webpack.

Client Setup

Require in an opal file (for opal-rails apps application.js.rb is a good place) and start listening for changes:

Note: OpalHotReloader.listen() deprecation

OpalHotReloader.listen() used to take a 2nd Boolean parameter to signify a reactrb app. This is deprecated and no longer needed.

require 'opal_hot_reloader'

# @param port [Integer] opal hot reloader port to connect to. Defaults to 25222 to match opal-hot-loader default

If you are using the default port then you can just call:



Reload Error Alert

By default, if there is an error hot loading code, opal_hot_reloader will present an alert of the error. The following options can be used to turn the alerts on and off:

OpalHotReloader.alerts_on!  # turns on alerts
OpalHotReloader.alerts_off! # turns off alerts

This will open up a websocket from the client to the server on the given port. The server-side should already be running.


Run only in development on Rails

One way to run this only in development on Rails is to change your application.js.rb to application.js.rb.erb and add this to the bottom of the file

<% if Rails.env.development? %>
    require 'opal_hot_reloader'
<% end %>


Some of you might be asking? Why do this, isn't this reinventing the wheel by programs like webpack, etc.? I should mention that reinventing the wheel seems happens all the time in the Javascript world.

Yes and no. opal-hot-reloader is an "All Ruby(Opal)", self contained system, so if you're doing any kind of Opal frontend/Ruby backend webserver type of project, you will be able to just drop in opal-hot-reloader and it will work out of the box without having install/configure webpack or similar.

I believe it will be most advantageous for Opal to be able to straddle a hybrid approach where:

  • With Opal and Rails, we use the existing mechanism, sprockets, to serve up all the things it does in the "normal" Rails ecosystem. I.e we want to work with the system. We want all the perks of Ruby and Rails without have to hand cobble it ourselves
  • We use webpack or similar for being a "1st class JS citizen". This gives us access to all the frontend assets in npm, we want all those options and perks.

While I do favor moving as much Javascript to webpack, following suit to React.js's lead, I see an "all webpack solution" for Opal apps being only one of a few permutations, and not particularly appealing to most Rails programmers - who I think is the largest demographic likely to want to do Opal programming.

While we wait for the other approaches to evolve and get implemented this solution is here and works now. It works with an "All Ruby" system, it works with a Rails app that is using webpack to provide react.js components to react.rb.


  • Bring the benefits of "leading edge web development" to All Ruby (via Opal) full stack development. One of my efforts to be more like Einstein in “Opening up yet another fragment of the frontier of beauty” - i.e. share the joy.
  • Batteries included out of the box - make it (increasingingly) easy for Rubyists to enjoy the previous goal. This is a manifestation of the "Ruby Way" of making the programmer happy
  • Try to add the least amount of additional dependencies to projects it's used in



After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.