Opal is a ruby to javascript compiler, an implementation of the ruby corelib and stdlib, and associated gems for building fast client side web applications in ruby.
Opal is available as a gem, and can be installed via:
$ gem install opal
Or added to your Gemfile as:
gem 'opal'
At its very core, opal provides a simple method of compiling a string of ruby into javascript that can run on top of the opal runtime, provided by opal.js:
Opal.compile("[1, 2, 3].each { |a| puts a }")
# => "(function() { ... })()"
opal includes sprockets support for compiling Ruby (and ERB) assets, and treating them as first class JavaScript citizens. It works in a similar way to CoffeeScript, where JavaScript files can simply require Ruby sources, and Ruby sources can require JavaScript and other Ruby files.
This relies on the Opal load path. Any gem containing opal code registers that directory to the Opal load path. Opal will then use all Opal load paths when running sprockets instances. For rails applications, opal-rails does this automatically. For building a simple application, we have to do this manually.
Opal uses a load path which works with sprockets to create a set of locations which opal can require files from. If you want to add a directory to this load path, you can add it to the global environment.
In the Opal module, a property paths is used to hold the load paths which
Opal uses to require files from. You can add a directory to this:
Opal.append_path '../my_lib'
Now, any ruby files in this directory can be discovered.
To try and keep ruby relevant in a world where client-side applications are making javascript the primary development platform.
We run opal against the ruby spec as our primary testing setup. We try to make Opal as compatible as possible, whilst also taking into account restrictions of JavaScript when applicable. Opal supports the majority of ruby syntax features, as well as a very large part of the corelib implementation. We support method_missing, modules, classes, instance_exec, blocks, procs and lots lots more. Opal can compile and run RSpec unmodified, as well as self hosting the compiler at runtime.
We are running tests under ruby 3.0.0 conditions, but are mostly compatible with 2.6 level features.
All strings in Opal are immutable because ruby strings just get compiled directly into javascript strings, which are immutable. Wrapping ruby strings as a custom JavaScript object would add a lot of overhead as well as making interaction between ruby and javascript libraries more difficult.