Use ES module syntax for application.js.tt and docs

This change swaps the CommonJS require() syntax in the Webpacker
application.js pack template file and in documentation examples with ES
module import syntax.

Benefits of this change include:

Provides continuity with the larger frontend community: Arguably, one of
the main draws in adopting Webpacker is its integration with Babel to
support ES module syntax. For a fresh Rails install with Webpacker, the
application.js file will be the first impression most Rails developers
have with webpack and Webpacker.  Most of the recent documentation and
examples they will find online for using other libraries will be based
on ES module syntax.

Reduces confusion: Developers commonly add ES imports to their
application.js pack, typically by following online examples, which means
mixing require() and import statements in a single file. This leads to
confusion and unnecessary friction about differences between require()
and import.

Embraces browser-friendliness: The ES module syntax forward-looking and
is meant to be supported in browsers. On the other hand, require()
syntax is synchronous by design and not browser-supported as CommonJS
originally was adopted in Node.js for server-side JavaScript. That
webpack supports require() syntax is merely a convenience.

Encourages best practices regarding optimization: webpack can statically
analyze ES modules and "tree-shake", i.e., strip out unused exports from
the final build (given certain conditions are met, including
`sideEffects: false` designation in package.json).

actionview/app/assets/javascripts/README.md

@@ -40,7 +40,8 @@ In a conventional Rails application that uses the asset pipeline, require `rails
 If you're using the Webpacker gem or some other JavaScript bundler, add the following to your main JS file:
 
 ```javascript
-require("@rails/ujs").start()
+import Rails from "@rails/ujs"
+Rails.start()
 ```
 
 ## How to run tests

activestorage/README.md

@@ -151,7 +151,8 @@ Active Storage, with its included JavaScript library, supports uploading directl
     ```
     Using the npm package:
     ```js
-    require("@rails/activestorage").start()
+    import * as ActiveStorage from "@rails/activestorage"
+    ActiveStorage.start()
     ```
 2. Annotate file inputs with the direct upload URL.
 

guides/source/active_storage_overview.md

@@ -578,7 +578,8 @@ directly from the client to the cloud.
     Using the npm package:
 
     ```js
-    require("@rails/activestorage").start()
+    import * as ActiveStorage from "@rails/activestorage"
+    ActiveStorage.start()
     ```
 
 2. Annotate file inputs with the direct upload URL.

railties/lib/rails/generators/rails/app/templates/app/javascript/packs/application.js.tt

@@ -3,17 +3,24 @@
 // a relevant structure within app/javascript and only use these pack files to reference
 // that code so it'll be compiled.
 
-require("@rails/ujs").start()
+import Rails from "@rails/ujs"
 <%- unless options[:skip_turbolinks] -%>
-require("turbolinks").start()
+import Turbolinks from "turbolinks"
 <%- end -%>
 <%- unless skip_active_storage? -%>
-require("@rails/activestorage").start()
+import * as ActiveStorage from "@rails/activestorage"
 <%- end -%>
 <%- unless options[:skip_action_cable] -%>
-require("channels")
+import "channels"
 <%- end -%>
 
+Rails.start()
+<%- unless options[:skip_turbolinks] -%>
+Turbolinks.start()
+<%- end -%>
+<%- unless skip_active_storage? -%>
+ActiveStorage.start()
+<%- end -%>
 
 // Uncomment to copy all static images under ../images to the output folder and reference
 // them with the image_pack_tag helper in views (e.g <%%= image_pack_tag 'rails.png' %>)

railties/test/application/rake_test.rb

@@ -157,7 +157,7 @@ class Hello
     end
 
     def test_code_statistics_sanity
-      assert_match "Code LOC: 29     Test LOC: 3     Code to Test Ratio: 1:0.1",
+      assert_match "Code LOC: 32     Test LOC: 3     Code to Test Ratio: 1:0.1",
         rails("stats")
     end
 

railties/test/generators/shared_generator_tests.rb

@@ -203,7 +203,8 @@ def test_generator_for_active_storage
 
     unless generator_class.name == "Rails::Generators::PluginGenerator"
       assert_file "#{application_path}/app/javascript/packs/application.js" do |content|
-        assert_match(/^require\("@rails\/activestorage"\)\.start\(\)/, content)
+        assert_match(/^import \* as ActiveStorage from "@rails\/activestorage"/, content)
+        assert_match(/^ActiveStorage.start\(\)/, content)
       end
     end
 
@@ -264,7 +265,7 @@ def test_generator_does_not_generate_active_storage_contents_if_skip_active_reco
     assert_file "#{application_path}/config/application.rb", /#\s+require\s+["']active_storage\/engine["']/
 
     assert_file "#{application_path}/app/javascript/packs/application.js" do |content|
-      assert_no_match(/^require\("@rails\/activestorage"\)\.start\(\)/, content)
+      assert_no_match(/activestorage/i, content)
     end
 
     assert_file "#{application_path}/config/environments/development.rb" do |content|