Show Rails error page for a morph reload

app/assets/javascripts/hotwire_spark.js

@@ -533,6 +533,12 @@ var HotwireSpark = (function () {
       reload: Date.now()
     });
   }
+  class ResponseError extends Error {
+    constructor(message, response) {
+      super(message);
+      this.response = response;
+    }
+  }
   async function reloadHtmlDocument() {
     let currentUrl = cacheBustedUrl(urlWithParams(window.location.href, {
       hotwire_spark: "true"
@@ -542,12 +548,14 @@ var HotwireSpark = (function () {
         "Accept": "text/html"
       }
     });
-    if (!response.ok) {
-      throw new Error(`${response.status} when fetching ${currentUrl}`);
-    }
     const fetchedHTML = await response.text();
     const parser = new DOMParser();
-    return parser.parseFromString(fetchedHTML, "text/html");
+    const parsedResult = parser.parseFromString(fetchedHTML, "text/html");
+    if (response.ok) {
+      return parsedResult;
+    } else {
+      throw new ResponseError(`${response.status} when fetching ${currentUrl}`, parsedResult);
+    }
   }
   function getConfigurationProperty(name) {
     return document.querySelector(`meta[name="hotwire-spark:${name}"]`)?.content;
@@ -1490,12 +1498,23 @@ var HotwireSpark = (function () {
     }
     async #reloadHtml() {
       log("Reload html with morph...");
-      const reloadedDocument = await reloadHtmlDocument();
-      this.#updateBody(reloadedDocument.body);
-      return reloadedDocument;
+      const {
+        documentElement
+      } = await this.#reloadHtmlDocument();
+      return this.#updateDocument(documentElement);
+    }
+    #updateDocument(newDocument) {
+      Idiomorph.morph(document.documentElement, newDocument);
+      return newDocument;
     }
-    #updateBody(newBody) {
-      Idiomorph.morph(document.body, newBody);
+    async #reloadHtmlDocument() {
+      try {
+        return await reloadHtmlDocument();
+      } catch ({
+        response
+      }) {
+        return response;
+      }
     }
     async #reloadStimulus() {
       await StimulusReloader.reloadAll();

app/javascript/hotwire/spark/helpers.js

@@ -18,17 +18,27 @@ export function cacheBustedUrl(urlString) {
   return urlWithParams(urlString, { reload: Date.now() })
 }
 
+class ResponseError extends Error {
+  constructor(message, response) {
+    super(message)
+    this.response = response
+  }
+}
+
 export async function reloadHtmlDocument() {
   let currentUrl = cacheBustedUrl(urlWithParams(window.location.href, { hotwire_spark: "true" }))
   const response = await fetch(currentUrl, { headers: { "Accept": "text/html" }})
 
-  if (!response.ok) {
-    throw new Error(`${response.status} when fetching ${currentUrl}`)
-  }
 
   const fetchedHTML = await response.text()
   const parser = new DOMParser()
-  return parser.parseFromString(fetchedHTML, "text/html")
+  const parsedResult = parser.parseFromString(fetchedHTML, "text/html")
+
+  if (response.ok) {
+    return parsedResult
+  } else {
+    throw new ResponseError(`${response.status} when fetching ${currentUrl}`, parsedResult)
+  }
 }
 
 export function getConfigurationProperty(name) {

app/javascript/hotwire/spark/reloaders/morph_html_reloader.js

@@ -16,13 +16,21 @@ export class MorphHtmlReloader {
   async #reloadHtml() {
     log("Reload html with morph...")
 
-    const reloadedDocument = await reloadHtmlDocument()
-    this.#updateBody(reloadedDocument.body)
-    return reloadedDocument
+    const { documentElement } = await this.#reloadHtmlDocument()
+    return this.#updateDocument(documentElement)
   }
 
-  #updateBody(newBody) {
-    Idiomorph.morph(document.body, newBody)
+  #updateDocument(newDocument) {
+    Idiomorph.morph(document.documentElement, newDocument)
+    return newDocument
+  }
+
+  async #reloadHtmlDocument() {
+    try {
+      return await reloadHtmlDocument()
+    } catch ({ response }) {
+      return response
+    }
   }
 
   async #reloadStimulus() {

test/dummy/config/environments/test.rb

@@ -25,7 +25,7 @@
   config.cache_store = :null_store
 
   # Render exception templates for rescuable exceptions and raise for other exceptions.
-  config.action_dispatch.show_exceptions = :rescuable
+  config.action_dispatch.show_exceptions = true
 
   # Disable request forgery protection in test environment.
   config.action_controller.allow_forgery_protection = false

test/html_reload_test.rb

@@ -0,0 +1,12 @@
+require "application_system_test_case"
+
+class HtmlReloadTest < ApplicationSystemTestCase
+  test "error page is displayed" do
+    visit root_path
+    assert_no_text "ZeroDivisionError"
+
+    edit_file "app/views/home/show.html.erb", replace: "_REPLACE_", with: "<%= 1 / 0 %>"
+
+    assert_text "ZeroDivisionError"
+  end
+end

test/morph_html_reload_test.rb → test/html_reload_test/morph_html_reload_test.rb

@@ -1,6 +1,6 @@
 require "application_system_test_case"
 
-class MorphHtmlReloadTest < ApplicationSystemTestCase
+class MorphHtmlReloadTest < HtmlReloadTest
   setup do
     Hotwire::Spark.html_reload_method = :morph
   end

test/replace_html_reload_test.rb → test/html_reload_test/replace_html_reload_test.rb

@@ -1,6 +1,6 @@
 require "application_system_test_case"
 
-class ReplaceHtmlReloadTest < ApplicationSystemTestCase
+class ReplaceHtmlReloadTest < HtmlReloadTest
   setup do
     Hotwire::Spark.html_reload_method = :replace
   end