DEV: Defer loading core/plugin/theme JS files (#17063)

This is pre-request work to introduce a splash screen while site assets load.

The only change this commit introduces is that it ensures we add the defer attribute to core/plugin/theme .JS files. This will allow us to insert markup before the browser starts evaluating those scripts later on. It has no visual or functional impact on core.

This will not have any impact on how themes and plugins work. The only exception is themes loading external scripts in the </head> theme field directly via script tags. Everything will work the same but those would need to add the defer attribute if they want to keep the benefits introduced in this PR.

app/assets/javascripts/discourse/app/index.html

@@ -11,8 +11,10 @@
     <bootstrap-content key="before-script-load">
     {{content-for "before-script-load"}}
 
-    <script src="{{rootURL}}assets/vendor.js"></script>
-    <script src="{{rootURL}}assets/discourse.js"></script>
+    <script defer src="{{rootURL}}assets/vendor.js"></script>
+    <script defer src="{{rootURL}}assets/discourse.js"></script>
+
+    <ember-auto-import-scripts defer entrypoint="app"></ember-auto-import-scripts>
 
     <bootstrap-content key="head">
     {{content-for "head"}}
@@ -27,7 +29,7 @@
     <bootstrap-content key="hidden-login-form">
     <bootstrap-content key="preloaded">
 
-    <script src="{{rootURL}}assets/start-discourse.js"></script>
+    <script defer src="{{rootURL}}assets/start-discourse.js"></script>
 
     <bootstrap-content key="body-footer">
     {{content-for "body-footer"}}

app/assets/javascripts/discourse/ember-cli-build.js

@@ -29,6 +29,7 @@ module.exports = function (defaults) {
     },
     autoImport: {
       forbidEval: true,
+      insertScriptsAt: "ember-auto-import-scripts",
     },
     fingerprint: {
       // Handled by Rails asset pipeline

app/assets/javascripts/discourse/lib/bootstrap-json/index.js

@@ -89,7 +89,7 @@ function head(buffer, bootstrap, headers, baseURL) {
     let { admin, staff } = user;
 
     if (staff) {
-      buffer.push(`<script src="${baseURL}assets/admin.js"></script>`);
+      buffer.push(`<script defer src="${baseURL}assets/admin.js"></script>`);
     }
 
     if (admin) {
@@ -98,7 +98,7 @@ function head(buffer, bootstrap, headers, baseURL) {
   }
 
   bootstrap.plugin_js.forEach((src) =>
-    buffer.push(`<script src="${src}"></script>`)
+    buffer.push(`<script defer src="${src}"></script>`)
   );
 
   buffer.push(bootstrap.theme_html.translations);
@@ -108,14 +108,14 @@ function head(buffer, bootstrap, headers, baseURL) {
 }
 
 function localeScript(buffer, bootstrap) {
-  buffer.push(`<script src="${bootstrap.locale_script}"></script>`);
+  buffer.push(`<script defer src="${bootstrap.locale_script}"></script>`);
 }
 
 function beforeScriptLoad(buffer, bootstrap) {
   buffer.push(bootstrap.html.before_script_load);
   localeScript(buffer, bootstrap);
   (bootstrap.extra_locales || []).forEach((l) =>
-    buffer.push(`<script src="${l}"></script>`)
+    buffer.push(`<script defer src="${l}"></script>`)
   );
 }
 

app/assets/javascripts/discourse/tests/index.html

@@ -46,7 +46,9 @@
 
     <script src="/testem.js" integrity=""></script>
     <script src="{{rootURL}}assets/vendor.js"></script>
+    <ember-auto-import-scripts entrypoint="app"></ember-auto-import-scripts>
     <script src="{{rootURL}}assets/test-support.js"></script>
+    <ember-auto-import-scripts entrypoint="tests"></ember-auto-import-scripts>
     <script src="{{rootURL}}assets/discourse.js"></script>
     <script src="{{rootURL}}assets/discourse-markdown.js"></script>
     <script src="{{rootURL}}assets/admin.js"></script>

app/assets/javascripts/discourse/tests/test_starter.js

@@ -1,10 +1,11 @@
 // discourse-skip-module
 
-document.write(
-  '<div id="ember-testing-container"><div id="ember-testing"></div></div>'
-);
-document.write(
-  "<style>#ember-testing-container { position: fixed; background: white; bottom: 0; right: 0; width: 640px; height: 384px; overflow: auto; z-index: 9999; border: 1px solid #ccc; transform: translateZ(0)} #ember-testing { width: 200%; height: 200%; transform: scale(0.5); transform-origin: top left; }</style>"
+document.body.insertAdjacentHTML(
+  "afterbegin",
+  `
+    <div id="ember-testing-container"><div id="ember-testing"></div></div>
+    <style>#ember-testing-container { position: fixed; background: white; bottom: 0; right: 0; width: 640px; height: 384px; overflow: auto; z-index: 9999; border: 1px solid #ccc; transform: translateZ(0)} #ember-testing { width: 200%; height: 200%; transform: scale(0.5); transform-origin: top left; }</style>
+  `
 );
 
 let setupTestsLegacy = require("discourse/tests/setup-tests").setupTestsLegacy;

app/helpers/application_helper.rb

@@ -153,7 +153,7 @@ module ApplicationHelper
   def preload_script_url(url)
     <<~HTML.html_safe
       <link rel="preload" href="#{url}" as="script">
-      <script src="#{url}"></script>
+      <script defer src="#{url}"></script>
     HTML
   end
 

app/models/theme.rb

@@ -154,7 +154,7 @@ class Theme < ActiveRecord::Base
     SvgSprite.expire_cache
   end
 
-  BASE_COMPILER_VERSION = 55
+  BASE_COMPILER_VERSION = 56
   def self.compiler_version
     get_set_cache "compiler_version" do
       dependencies = [
@@ -391,7 +391,7 @@ class Theme < ActiveRecord::Base
       end
       caches = JavascriptCache.where(theme_id: theme_ids)
       caches = caches.sort_by { |cache| theme_ids.index(cache.theme_id) }
-      return caches.map { |c| "<script src='#{c.url}' data-theme-id='#{c.theme_id}'></script>" }.join("\n")
+      return caches.map { |c| "<script defer src='#{c.url}' data-theme-id='#{c.theme_id}'></script>" }.join("\n")
     end
     list_baked_fields(theme_ids, target, name).map { |f| f.value_baked || f.value }.join("\n")
   end

app/models/theme_field.rb

@@ -149,7 +149,7 @@ class ThemeField < ActiveRecord::Base
     javascript_cache.content = js_compiler.content
     javascript_cache.save!
 
-    doc.add_child("<script src='#{javascript_cache.url}' data-theme-id='#{theme_id}'></script>") if javascript_cache.content.present?
+    doc.add_child("<script defer src='#{javascript_cache.url}' data-theme-id='#{theme_id}'></script>") if javascript_cache.content.present?
     [doc.to_s, errors&.join("\n")]
   end
 
@@ -265,7 +265,7 @@ class ThemeField < ActiveRecord::Base
     javascript_cache.content = js_compiler.content
     javascript_cache.save!
     doc = ""
-    doc = "<script src='#{javascript_cache.url}' data-theme-id='#{theme_id}'></script>" if javascript_cache.content.present?
+    doc = "<script defer src='#{javascript_cache.url}' data-theme-id='#{theme_id}'></script>" if javascript_cache.content.present?
     [doc, errors&.join("\n")]
   end
 

app/views/layouts/application.html.erb

@@ -112,11 +112,11 @@
 
     <div class="hidden" id="data-preloaded" data-preloaded="<%= preloaded_json %>"></div>
 
-    <script src="<%= script_asset_path "start-discourse" %>"></script>
+    <script defer src="<%= script_asset_path "start-discourse" %>"></script>
 
     <%= yield :data %>
 
-    <script src="<%= script_asset_path "browser-update" %>"></script>
+    <script defer src="<%= script_asset_path "browser-update" %>"></script>
 
     <%- unless customization_disabled? %>
       <%= theme_lookup("body_tag") %>

spec/helpers/application_helper_spec.rb

@@ -7,7 +7,7 @@ describe ApplicationHelper do
     def preload_link(url)
       <<~HTML
           <link rel="preload" href="#{url}" as="script">
-          <script src="#{url}"></script>
+          <script defer src="#{url}"></script>
       HTML
     end
 

spec/models/theme_field_spec.rb

@@ -75,7 +75,7 @@ describe ThemeField do
 
     theme_field = ThemeField.create!(theme_id: 1, target_id: 0, name: "header", value: html)
     theme_field.ensure_baked!
-    expect(theme_field.value_baked).to include("<script src=\"#{theme_field.javascript_cache.url}\" data-theme-id=\"1\"></script>")
+    expect(theme_field.value_baked).to include("<script defer=\"\" src=\"#{theme_field.javascript_cache.url}\" data-theme-id=\"1\"></script>")
     expect(theme_field.value_baked).to include("external-script.js")
     expect(theme_field.value_baked).to include('<script type="text/template"')
     expect(theme_field.javascript_cache.content).to include('a = "inline discourse plugin"')
@@ -110,7 +110,7 @@ HTML
     field = ThemeField.create!(theme_id: 1, target_id: 0, name: "header", value: html)
     field.ensure_baked!
     expect(field.error).not_to eq(nil)
-    expect(field.value_baked).to include("<script src=\"#{field.javascript_cache.url}\" data-theme-id=\"1\"></script>")
+    expect(field.value_baked).to include("<script defer=\"\" src=\"#{field.javascript_cache.url}\" data-theme-id=\"1\"></script>")
     expect(field.javascript_cache.content).to include("Theme Transpilation Error:")
 
     field.update!(value: '')
@@ -130,7 +130,7 @@ HTML
     theme_field.ensure_baked!
     javascript_cache = theme_field.javascript_cache
 
-    expect(theme_field.value_baked).to include("<script src=\"#{javascript_cache.url}\" data-theme-id=\"1\"></script>")
+    expect(theme_field.value_baked).to include("<script defer=\"\" src=\"#{javascript_cache.url}\" data-theme-id=\"1\"></script>")
     expect(javascript_cache.content).to include("testing-div")
     expect(javascript_cache.content).to include("string_setting")
     expect(javascript_cache.content).to include("test text \\\" 123!")
@@ -378,7 +378,7 @@ HTML
     describe "javascript cache" do
       it "is generated correctly" do
         fr1.ensure_baked!
-        expect(fr1.value_baked).to include("<script src='#{fr1.javascript_cache.url}' data-theme-id='#{fr1.theme_id}'></script>")
+        expect(fr1.value_baked).to include("<script defer src='#{fr1.javascript_cache.url}' data-theme-id='#{fr1.theme_id}'></script>")
         expect(fr1.javascript_cache.content).to include("bonjourworld")
         expect(fr1.javascript_cache.content).to include("helloworld")
         expect(fr1.javascript_cache.content).to include("enval1")