1-welcome_controller.rb

class WelcomeController < ApplicationController
  def index
    @foo = "FROM CONTROLLER"
    @user = User.first
  end
end

2-index.html.hbs

<h1>Welcome#index</h1>
<p>Find me in app/views/welcome/index.html.erb</p>
<p>{{foo}}</p>
<p>{{{rails "link_to" user/name user}}}</p>

3-output.textile

4-impl.rb

require 'v8'

class HandlebarsTemplateHandler

  def self.js
    Thread.current[:v8_context] ||= begin
      V8::Context.new do |js|
        js.load(File.join(Rails.root, 'vendor', 'javascripts', 'handlebars.js'))
        js.eval("Templates = {}")
      end
    end
  end

  def self.rails_helper
    Thread.current[:v8_rails_helper] ||= begin
      js.eval(%{
        (function(helper) { 
          var params = Array.prototype.slice.call(arguments, 1);
          var av = this.__context__.fallback.actionview;
          return av[helper].apply(av, params);
        })
      })
    end
  end

  def self.call(template)
    # Here, we're sticking the compiled template somewhere in V8 where
    # we can get back to it
    js.eval(%{Templates["#{template.identifier}"] = Handlebars.compile(#{template.source.inspect}) })

    %{
      js = HandlebarsTemplateHandler.js
      fallback = { :actionview => self, :rails => ::HandlebarsTemplateHandler.rails_helper }
      js.eval("Templates['#{template.identifier}']").call(assigns, fallback)
    }
  end
end

ActionView::Template.register_template_handler(:hbs, HandlebarsTemplateHandler)