[fix] support etag W/ prefix

.changeset/giant-ghosts-give.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] support etag W/ prefix

packages/kit/src/runtime/server/index.js

@@ -69,9 +69,15 @@ export async function respond(incoming, options, state = {}) {
 						if (response.status === 200) {
 							const cache_control = get_single_valued_header(response.headers, 'cache-control');
 							if (!cache_control || !/(no-store|immutable)/.test(cache_control)) {
+								let if_none_match_value = request.headers['if-none-match'];
+								// ignore W/ prefix https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match#directives
+								if (if_none_match_value?.startsWith('W/"')) {
+									if_none_match_value = if_none_match_value.substring(2);
+								}
+
 								const etag = `"${hash(response.body || '')}"`;
 
-								if (request.headers['if-none-match'] === etag) {
+								if (if_none_match_value === etag) {
 									return {
 										status: 304,
 										headers: {},

packages/kit/test/apps/basics/src/routes/etag/_tests.js

@@ -47,4 +47,27 @@ export default function (test) {
 			js: false
 		}
 	);
+
+	test(
+		'support W/ etag prefix',
+		null,
+		async ({ fetch }) => {
+			const r1 = await fetch('/etag/text');
+			const etag = r1.headers.get('etag');
+			assert.ok(!!etag);
+
+			const r2 = await fetch('/etag/text', {
+				headers: etag
+					? {
+							'if-none-match': `W/${etag}`
+					  }
+					: {}
+			});
+
+			assert.equal(r2.status, 304);
+		},
+		{
+			js: false
+		}
+	);
 }