Example of how JS async works for a student

js.rb

class JavaScript
  def initialize
    @work = []
    @background_worker_count = 0
  end

  def run(&code)
    @work << code
    while @work.any? || @background_worker_count > 0
      instance_eval &@work.shift if @work.any?
    end
  end
  
  def set_timeout(milliseconds, &callback)
    @background_worker_count += 1
    Thread.new do
      sleep milliseconds/1000.0
      @work << callback
      @background_worker_count -= 1
    end
  end
end

JavaScript.new.run do
  puts "first: #{Time.now.strftime '%S'}"

  set_timeout 2000 do
    puts "second #{Time.now.strftime '%S'}"
  end

  puts "third #{Time.now.strftime '%S'}"

  set_timeout 1000 do
    puts "fourth #{Time.now.strftime '%S'}"
  end

  # sleep 3
end

# >> first: 22
# >> third 22
# >> fourth 23
# >> second 24