Stimulus.js without a build system, but with babel, Sinatra

app.rb

#...
get '/' do
  haml :index, layout: false
end
#...

hello_controller.js

//app-js-src/es6/stimulus/hello_controller.js
(() => {
  stimulus.register("hello", class extends Stimulus.Controller {
    static get targets() {
      return [ "name" ]
    }

    greet() {
      console.log(`Hello, ${this.name}!`)
    }

    get name() {
      return this.nameTarget.value
    }
  })
})()

index.haml

# views/index.haml
!!!
%html
  %head
    %meta{:content => "text/html; charset=UTF-8", "http-equiv" => "Content-Type"}/
    %meta{:charset => "utf-8"}/
    %script{:src => "/javascript/src/ext/stimulus.umd.js"}
    %script{:src => "/javascript/src/lib/stimulus/init.js"}
    %script{:src => "/javascript/src/lib/stimulus/hello_controller.js"}
  %body
    %div{"data-controller" => "hello"}
      %input{"data-target" => "hello.name", :type => "text"}/
      %button{"data-action" => "click->hello#greet"} Greet	

init.js

//app-js-src/es6/stimulus/init.js
(function () {
  if (!("stimulus" in window)) {
    window.stimulus = Stimulus.Application.start();
  }
})();

rakefile.rb

#...
  desc 'Transpile javascript assets into bundles'
  task :transpile do
    require 'babel/transpiler'
    options = { blacklist: ['useStrict'], comments: false }
    srcfiles = Dir.glob('./app-js-src/es6/**/*.js')
    srcfiles.map { |filepath| File.new(filepath) }.each do |srcfile|
      es5_code = Babel::Transpiler.transform(srcfile.read, options)['code']
      newpath = srcfile.path.split('/')[3..-1].join('/')
      dirname = File.dirname("public/javascript/src/lib/#{newpath}")
      unless File.directory?(dirname)
        FileUtils.mkdir_p(dirname)
      end
      out_file = File.new("public/javascript/src/lib/#{newpath}", 'w')
      out_file.puts(es5_code)
      out_file.close
      puts "TRANSPILED TO #{out_file.path}"
    end
  end

  task default: %i[clean transpile]
end

#...

Note to self: Define target names in your controller class using the static targets array:

// controllers/search_controller.js
import { Controller } from "stimulus"

export default class extends Controller {
static targets = [ "query", "errorMessage", "results" ]
// …
}
Note: You may need to enable support in your JavaScript environment for the static class properties standard (see @babel/plugin-proposal-class-properties).