既存のJavaScript app/assets/javascripts/ と
Importmapに対応したJavascript app/javascript/ の共存、その備忘録になります。

ここでは、js_compressor にTerserを利用しています。

Gemfile

Gemfile

...

# The original asset pipeline for Rails [https://github.com/rails/sprockets-rails]
gem "sprockets-rails"

# Use JavaScript with ESM import maps [https://github.com/rails/importmap-rails]
gem "importmap-rails"

# Terser minifies JavaScript files by wrapping TerserJS to be accessible in Ruby [https://rubygems.org/gems/terser]
gem 'terser', '~> 1.1', '>= 1.1.8'

...

environment

config/environments/production.rb

# Compress JavaScripts and CSS.
config.assets.js_compressor = Terser.new(compress: { drop_console: true })

Javascript

application.js リネーム

app/assets/javascripts/, app/javascript/ 2箇所にapplication.js ファイルが存在するとエラーとなるため、application_old.js にリネームしています。

require 指定

require_directory, require_tree 等、環境に合わせて調整してください。

app/assets/javascripts/application_old.js

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file. JavaScript code in this file should be added after the last require_* statement.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//

//= require_directory .

app/javascript/application.js

// Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails

import Rails from "@rails/ujs";
Rails.start();

Layout

app/views/layouts/application.html.erb

<!DOCTYPE html>
<html>
  <head>
    <title></title>
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <%= csrf_meta_tags %>
    <%= csp_meta_tag %>

    <%= stylesheet_link_tag 'application', media: 'all' %>
-   <%= javascript_include_tag 'application' %>
+   <%= javascript_include_tag 'application_old' %>
+   <%= javascript_importmap_tags %>
  </head>

  <body>
    <%= yield %>
  </body>
</html>

最後に

同じケースにTryしている方のヒントになれば幸いです。
もっと良い方法が見つかりましたらアップデートしていきたいと思います。