Hardening responsiveness and SEO

hdyc-svelte/src/hooks.server.ts

@@ -13,18 +13,38 @@ const MIME_TYPES: Record<string, string> = {
   '.otf': 'font/otf'
 };
 
+const HTML_CACHE_CONTROL = 'public, max-age=0, must-revalidate';
+const ASSET_404_CACHE_CONTROL = 'no-store';
+
 export const handle: Handle = async ({ event, resolve }) => {
   const response = await resolve(event);
-  if (event.url.pathname.startsWith('/_app/')) {
+  const pathname = event.url.pathname;
+  const contentType = response.headers.get('content-type') ?? '';
+
+  if (pathname.startsWith('/_app/')) {
     const existing = response.headers.get('content-type');
     const hasValidHeader = existing && existing.trim().length > 0;
     if (!hasValidHeader) {
-      const extension = path.extname(event.url.pathname).toLowerCase();
+      const extension = path.extname(pathname).toLowerCase();
       const mime = extension && MIME_TYPES[extension];
       if (mime) {
         response.headers.set('content-type', mime);
       }
     }
+
+    // Missing hashed assets should never be cached; otherwise stale HTML can
+    // keep pointing to already-rotated files long after a deployment.
+    if (response.status >= 400) {
+      response.headers.set('cache-control', ASSET_404_CACHE_CONTROL);
+    }
+
+    return response;
+  }
+
+  // HTML documents should revalidate so they can reference the latest client
+  // bundle hashes after each deployment.
+  if (contentType.includes('text/html')) {
+    response.headers.set('cache-control', HTML_CACHE_CONTROL);
   }
 
   return response;