Add 103 Early Hints test (#609)

* Add 103 Early Hints test

* Update test/e2e/onTTFB-test.js

* Update test/e2e/onTTFB-test.js

Author: tunetheweb

test/e2e/onTTFB-test.js

@@ -398,6 +398,32 @@ describe('onTTFB()', async function () {
       assert.strictEqual(ttfb.attribution.requestDuration, 0);
       assert.strictEqual(ttfb.attribution.navigationEntry, undefined);
     });
+
+    it('reports the correct value for Early Hints', async function () {
+      await navigateTo('/test/ttfb?earlyHintsDelay=50&attribution=1');
+
+      const ttfb = await getTTFBBeacon();
+
+      if ('finalResponseHeadersStart' in ttfb.attribution.navigationEntry) {
+        assert.strictEqual(
+          ttfb.value,
+          ttfb.attribution.navigationEntry.responseStart,
+        );
+        assert.strictEqual(
+          ttfb.value,
+          ttfb.attribution.navigationEntry.firstInterimResponseStart,
+        );
+        assert(
+          ttfb.value <
+            ttfb.attribution.navigationEntry.finalResponseHeadersStart,
+        );
+      } else {
+        assert.strictEqual(
+          ttfb.value,
+          ttfb.attribution.navigationEntry.responseStart,
+        );
+      }
+    });
   });
 });
 

test/server.js

@@ -39,6 +39,19 @@ app.use((req, res, next) => {
   }
 });
 
+// Allow the use of a `earlyHintsDelay` query param to delay any response
+// after sending an early hints
+app.use((req, res, next) => {
+  if (req.query && req.query.earlyHintsDelay) {
+    res.writeEarlyHints({
+      'link': '</styles.css>; rel=preload; as=style',
+    });
+    setTimeout(next, req.query.earlyHintsDelay);
+  } else {
+    next();
+  }
+});
+
 // Add a "collect" endpoint to simulate analytics beacons.
 app.post('/collect', bodyParser.text(), (req, res) => {
   // Uncomment to log the metric when manually testing.