Respond with 422 for non-GET JS requests with errors

Much like HTML responses, JS will respond with a 422 Unprocessable
Entity for non-GET requests if the given resource contains errors, so it
is consistent with all other formats going forward.

Closes #159.

CHANGELOG.md

@@ -1,6 +1,6 @@
 ## Unreleased
 
-* Responding to an `HTML` request that has errors on the resource now sets the status to `422 Unprocessable Entity`. (instead of the default of `200 OK`.) This makes it more consistent with other statuses more commonly used in APIs (JSON/XML for example), and works by default with Turbo/Hotwire. Note that this change may break your application if you're relying on the previous 2xx status to handle error cases.
+* Responding to an `HTML` or `JS` request that has errors on the resource now sets the status to `422 Unprocessable Entity`. (instead of the default of `200 OK`.) This makes it more consistent with other statuses more commonly used in APIs (JSON/XML for example), and works by default with Turbo/Hotwire which expects a 422 on form error HTML responses. Note that this change may break your application if you're relying on the previous 2xx status to handle error cases.
 * Add support for Ruby 3.0, drop support for Ruby < 2.5.
 * Add support for Rails 6.1, drop support for Rails < 5.2.
 * Move CI to GitHub Actions.

lib/action_controller/responder.rb

@@ -236,6 +236,8 @@ def resource_location
     def default_render
       if @default_response
         @default_response.call(options)
+      elsif !get? && has_errors?
+        controller.render(options.merge(status: :unprocessable_entity))
       else
         controller.render(options)
       end

test/action_controller/respond_with_test.rb

@@ -170,6 +170,17 @@ def test_using_resource_with_js_simply_tries_to_render_the_template
     get :using_resource
     assert_equal "text/javascript", @response.media_type
     assert_equal "alert(\"Hi\");", @response.body
+    assert_equal 200, @response.status
+  end
+
+  def test_using_resource_for_post_with_js_renders_the_template_and_yields_unprocessable_entity_on_failure
+    @request.accept = "text/javascript"
+    errors = { name: :invalid }
+    Customer.any_instance.stubs(:errors).returns(errors)
+    post :using_resource
+    assert_equal "text/javascript", @response.media_type
+    assert_equal "alert(\"Hi\");", @response.body
+    assert_equal 422, @response.status
   end
 
   def test_using_hash_resource_with_js_raises_an_error_if_template_cant_be_found