From 82327a73ea6b7936031c6b0ef889d868f5773882 Mon Sep 17 00:00:00 2001
From: Charles Lyding <19598772+clydin@users.noreply.github.com>
Date: Mon, 28 Oct 2024 13:20:31 -0400
Subject: [PATCH] refactor(@angular/build): use ETags for dev-server resource
 requests

When using the development server with the application build system,
resource files managed by the build system will now using ETag headers
to avoid resending content that has not changed since the last request.
This includes such content as global stylesheet files, image/font files
referenced in stylesheets, and other files managed by the bundler.
---
 .../src/tools/vite/middlewares/assets-middleware.ts    | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/packages/angular/build/src/tools/vite/middlewares/assets-middleware.ts b/packages/angular/build/src/tools/vite/middlewares/assets-middleware.ts
index 0fd95baeea30..30f20fd2f4d7 100644
--- a/packages/angular/build/src/tools/vite/middlewares/assets-middleware.ts
+++ b/packages/angular/build/src/tools/vite/middlewares/assets-middleware.ts
@@ -119,11 +119,21 @@ export function createAngularAssetsMiddleware(
           }
         }
 
+        // Avoid resending the content if it has not changed since last request
+        const etag = `W/"${outputFile.contents.byteLength}-${outputFile.hash}"`;
+        if (req.headers['if-none-match'] === etag) {
+          res.statusCode = 304;
+          res.end();
+
+          return;
+        }
+
         const mimeType = lookupMimeType(extension);
         if (mimeType) {
           res.setHeader('Content-Type', mimeType);
         }
         res.setHeader('Cache-Control', 'no-cache');
+        res.setHeader('ETag', etag);
         res.end(data);
 
         return;