test: add test for lazy loading CSS using fetch and document.adoptedS…

…tyleSheets

README.md

@@ -527,7 +527,7 @@ See [boilerplate](https://github.com/webdiscus/webpack-html-scss-boilerplate)
    - [How to inline JS in HTML](#recipe-inline-js)
    - [How to inline SVG, PNG images in HTML](#recipe-inline-image)
    - [How to resolve source assets in an attribute containing JSON value](#recipe-resolve-attr-json)
-   - [How to load CSS file dynamically](#recipe-dynamic-load-css)
+   - [How to load CSS file dynamically](#recipe-dynamic-load-css) (lazy loading CSS)
    - [How to load JS and CSS from `node_modules` in template](#recipe-load-js-css-from-node-modules)
    - [How to import CSS or SCSS from `node_modules` in SCSS](#recipe-import-style-from-node-modules)
    - [How to process a PHP template](#recipe-preprocessor-php)
@@ -2417,7 +2417,7 @@ Possible values:
 
 - `false` - disable minification
 - `true` - enable minification with default options
-- `auto` - in `development` mode disable minification, in `production` mode enable minification with default options,
+- `'auto'` - in `development` mode disable minification, in `production` mode enable minification with default options,
   use [minifyOptions](#option-minify-options) to customize options
 - `{}` - enable minification with custom options, this object are merged with `default options`\
   see [options reference](https://github.com/terser/html-minifier-terser#options-quick-reference)
@@ -2428,7 +2428,7 @@ Possible values:
 
 Type: `Object` Default: `null`
 
-When the [minify](#option-minify) option is set to `auto`, you can configure minification options using the `minifyOptions`.
+When the [minify](#option-minify) option is set to `'auto'` or `true`, you can configure minification options using the `minifyOptions`.
 
 #### [↑ back to contents](#contents)
 
@@ -5209,26 +5209,47 @@ The custom attribute will contains in the generated HTML the resolved output ass
 ## How to load CSS file dynamically
 
 For dynamic file loading, we need the output filename of extracted CSS from a source style file.
-To get the CSS output filename in JavaScript, you can use the `url` query in `require()` function:
+To get the CSS output filename in JavaScript, you can use the `url` query:
 ```js
-const cssFile = require('./style.scss?url');
+import cssUrl from './style.scss?url';
+// - OR -
+const cssUrl = require('./style.scss?url');
 ```
 Where the `./style.scss` is the source SCSS file relative to the JavaScript file.
 
-To load a CSS file dynamically, you can use the function:
+To load a CSS file dynamically, you can use following function:
 ```js
-function loadCSS(file) {
+import cssUrl from './style.scss?url';
+
+function loadCSS(url) {
   const style = document.createElement('link');
-  style.href = file;
+  style.href = url;
   style.rel = 'stylesheet';
   document.head.appendChild(style);
 }
 
-const cssFile = require('./style.scss?url');
-loadCSS(cssFile);
+loadCSS(cssUrl);
 ``` 
 
-The CSS will be extracted into separate file and the `cssFile` variable will contains the CSS output filename. 
+The CSS will be extracted into separate file and the `cssUrl` variable will contains the CSS output filename.
+
+Since 2023, many browsers support the modern way to add the stylesheets into DOM without creating the `link` tag.
+
+```js
+import cssUrl from './style.scss?url';
+
+async function loadCSS(url) {
+  const response = await fetch(url);
+  const css = await response.text();
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync(css);
+  document.adoptedStyleSheets = [sheet];
+}
+
+loadCSS(cssUrl);
+```
+
+See the [browser compatibility](https://developer.mozilla.org/en-US/docs/Web/API/Document/adoptedStyleSheets#browser_compatibility).
 
 #### [↑ back to contents](#contents)
 

test/cases/entry-multiple-chunks-same-filename/webpack.config.js

@@ -11,10 +11,10 @@ module.exports = {
   entry: {
     // Test case multiple chunks with same filename:
     // - the `main.css` file is defined here, in entry point
-    // - the same `main.css` file is defined in `index.pug`
+    // - the same `main.css` file is defined in `index.html`
 
     // Note: this use case has no sense and should not be used!
-    // Specify all scripts and styles directly in Pug.
+    // Specify all scripts and styles directly in HTML.
 
     index: './src/index.html',
 

test/cases/js-import-css-lazy-url-fetch/expected/index.html

@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <title>Test</title>
+  <link href="favicon.edda23bf.png" rel="icon">
+  <link href="style.7f152e84.css" rel="stylesheet" />
+  <script src="main.js" defer></script>
+</head>
+<body>
+  <h1>Hello World!</h1>
+  <p class="lazy">Lazy load CSS</p>
+</body>
+</html>

test/cases/js-import-css-lazy-url-fetch/expected/style-dynamic.1f7039c8.css

@@ -0,0 +1 @@
+/* extracted by HTMLBundler CSSLoader */.lazy{color:green}

test/cases/js-import-css-lazy-url-fetch/expected/style.7f152e84.css

@@ -0,0 +1 @@
+h1{color:red}

test/cases/js-import-css-lazy-url-fetch/src/index.html

@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <title>Test</title>
+  <link href="@images/favicons/favicon.png" rel="icon">
+  <link href="./style.scss" rel="stylesheet" />
+  <script src="main.js" defer></script>
+</head>
+<body>
+  <h1>Hello World!</h1>
+  <p class="lazy">Lazy load CSS</p>
+</body>
+</html>

test/cases/js-import-css-lazy-url-fetch/src/main.js

@@ -0,0 +1,40 @@
+import cssUrl from './style-dynamic.scss?url';
+
+// universal way
+function lazyLoad(url) {
+  const style = document.createElement('link');
+  style.rel = 'stylesheet';
+  style.href = url;
+  document.head.appendChild(style);
+  console.log('lazyLoad: ', { cssUrl });
+}
+
+// Using fetch and document.adoptedStyleSheets
+// Browser compatibility (since early 2023):
+// https://developer.mozilla.org/en-US/docs/Web/API/Document/adoptedStyleSheets#browser_compatibility
+
+// modern way, using promise
+function lazyLoad2(url) {
+  fetch(url).then((response) => {
+    response.text().then((css) => {
+      const sheet = new CSSStyleSheet();
+      sheet.replaceSync(css);
+      document.adoptedStyleSheets = [sheet];
+      console.log('lazyLoad2: ', { cssUrl });
+    });
+  });
+}
+
+// modern way, using async await
+async function lazyLoad3(url) {
+  const response = await fetch(url);
+  const css = await response.text();
+  const sheet = new CSSStyleSheet();
+  sheet.replaceSync(css);
+  document.adoptedStyleSheets = [sheet];
+  console.log('lazyLoad3: ', { cssUrl });
+}
+
+//lazyLoad(cssUrl);
+//lazyLoad2(cssUrl);
+lazyLoad3(cssUrl);

test/cases/js-import-css-lazy-url-fetch/src/style-dynamic.scss

@@ -0,0 +1,4 @@
+$color: green;
+.lazy {
+  color: $color;
+}

test/cases/js-import-css-lazy-url-fetch/src/style.scss

@@ -0,0 +1,4 @@
+$color: red;
+h1 {
+  color: $color;
+}

test/cases/js-import-css-lazy-url-fetch/webpack.config.js

@@ -0,0 +1,47 @@
+const path = require('path');
+const HtmlBundlerPlugin = require('@test/html-bundler-webpack-plugin');
+
+module.exports = {
+  mode: 'production',
+
+  output: {
+    path: path.join(__dirname, 'dist/'),
+  },
+
+  resolve: {
+    alias: {
+      '@images': path.join(__dirname, '../../fixtures/images'),
+    },
+  },
+
+  plugins: [
+    new HtmlBundlerPlugin({
+      entry: {
+        index: './src/index.html',
+      },
+      css: {
+        filename: '[name].[contenthash:8].css',
+      },
+      // test disable - OK
+      preprocessor: false,
+      // test output of lazy loaded CSS file
+      verbose: true,
+    }),
+  ],
+
+  module: {
+    rules: [
+      {
+        test: /\.s?css$/,
+        use: ['css-loader', 'sass-loader'],
+      },
+      {
+        test: /\.(png|jpe?g|ico|svg)$/,
+        type: 'asset/resource',
+        generator: {
+          filename: '[name].[hash:8][ext]',
+        },
+      },
+    ],
+  },
+};

test/integration.test.js

@@ -335,8 +335,8 @@ describe('import styles in JavaScript', () => {
 });
 
 describe('import lazy styles in JavaScript', () => {
-  // TODO: yet experimental, undocumented
   test('lazy url in js', () => compareFiles('js-import-css-lazy-url'));
+  test('lazy load CSS in js using fetch', () => compareFiles('js-import-css-lazy-url-fetch'));
 });
 
 describe('CSS source map', () => {