angular · alan-agius4 · Oct 2, 2024 · Sep 30, 2024
diff --git a/packages/angular/build/src/utils/index-file/index-html-generator.ts b/packages/angular/build/src/utils/index-file/index-html-generator.ts
@@ -14,6 +14,7 @@ import { addEventDispatchContract } from './add-event-dispatch-contract';
 import { CrossOriginValue, Entrypoint, FileInfo, augmentIndexHtml } from './augment-index-html';
 import { InlineCriticalCssProcessor } from './inline-critical-css';
 import { InlineFontsProcessor } from './inline-fonts';
+import { addNgcmAttribute } from './ngcm-attribute';
 import { addNonce } from './nonce';
 
 type IndexHtmlGeneratorPlugin = (
@@ -82,6 +83,7 @@ export class IndexHtmlGenerator {
 
     // SSR plugins
     if (options.generateDedicatedSSRContent) {
+      this.csrPlugins.push(addNgcmAttributePlugin());
       this.ssrPlugins.push(addEventDispatchContractPlugin(), addNoncePlugin());
     }
   }
@@ -203,3 +205,7 @@ function postTransformPlugin({ options }: IndexHtmlGenerator): IndexHtmlGenerato
 function addEventDispatchContractPlugin(): IndexHtmlGeneratorPlugin {
   return (html) => addEventDispatchContract(html);
 }
+
+function addNgcmAttributePlugin(): IndexHtmlGeneratorPlugin {
+  return (html) => addNgcmAttribute(html);
+}
diff --git a/packages/angular/build/src/utils/index-file/ngcm-attribute.ts b/packages/angular/build/src/utils/index-file/ngcm-attribute.ts
@@ -0,0 +1,42 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { htmlRewritingStream } from './html-rewriting-stream';
+
+/**
+ * Defines a name of an attribute that is added to the `<body>` tag
+ * in the `index.html` file in case a given route was configured
+ * with `RenderMode.Client`. 'cm' is an abbreviation for "Client Mode".
+ *
+ * @see https://github.com/angular/angular/pull/58004
+ */
+const CLIENT_RENDER_MODE_FLAG = 'ngcm';
+
+/**
+ * Transforms the provided HTML by adding the `ngcm` attribute to the `<body>` tag.
+ * This is used in the client-side rendered (CSR) version of `index.html` to prevent hydration warnings.
+ *
+ * @param html The HTML markup to be transformed.
+ * @returns A promise that resolves to the transformed HTML string with the necessary modifications.
+ */
+export async function addNgcmAttribute(html: string): Promise<string> {
+  const { rewriter, transformedContent } = await htmlRewritingStream(html);
+
+  rewriter.on('startTag', (tag) => {
+    if (
+      tag.tagName === 'body' &&
+      !tag.attrs.some((attr) => attr.name === CLIENT_RENDER_MODE_FLAG)
+    ) {
+      tag.attrs.push({ name: CLIENT_RENDER_MODE_FLAG, value: '' });
+    }
+
+    rewriter.emitStartTag(tag);
+  });
+
+  return transformedContent();
+}
diff --git a/packages/angular/build/src/utils/index-file/ngcm-attribute_spec.ts b/packages/angular/build/src/utils/index-file/ngcm-attribute_spec.ts
@@ -0,0 +1,33 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { addNgcmAttribute } from './ngcm-attribute';
+
+describe('addNgcmAttribute', () => {
+  it('should add the ngcm attribute to the <body> tag', async () => {
+    const result = await addNgcmAttribute(`
+      <html>
+        <head></head>
+        <body><p>hello world!</p></body>
+      </html>
+    `);
+
+    expect(result).toContain('<body ngcm=""><p>hello world!</p></body>');
+  });
+
+  it('should not override an existing ngcm attribute', async () => {
+    const result = await addNgcmAttribute(`
+      <html>
+        <head></head>
+        <body ngcm="foo"><p>hello world!</p></body>
+      </html>
+    `);
+
+    expect(result).toContain('<body ngcm="foo"><p>hello world!</p></body>');
+  });
+});