Review feedback

CHANGELOG.md

@@ -1,11 +1,14 @@
 # Changelog
 
+### v5.0.0-rc.1 (???)
+
+- **[BREAKING]** Add interoperable TTFB to measure Early Hints consistently ([#566](https://github.com/GoogleChrome/web-vitals/pull/566))
+
 ### v5.0.0-rc.0 (2024-10-03)
 
 - **[BREAKING]** Remove the deprecated `onFID()` function ([#519](https://github.com/GoogleChrome/web-vitals/pull/519))
 - **[BREAKING]** Change browser support policy to Baseline Widely Available ([#525](https://github.com/GoogleChrome/web-vitals/pull/525))
 - **[BREAKING]** Sort the classes that appear in attribution selectors to reduce cardinality ([#518](https://github.com/GoogleChrome/web-vitals/pull/518))
-- **[BREAKING]** Add interoperable TTFB to measure Early Hints consistently ([#566](https://github.com/GoogleChrome/web-vitals/pull/566))
 - Cap INP breakdowns to INP duration ([#528](https://github.com/GoogleChrome/web-vitals/pull/528))
 - Cap LCP load duration to LCP time ([#527](https://github.com/GoogleChrome/web-vitals/pull/527))
 

src/attribution/onLCP.ts

@@ -47,10 +47,11 @@ const attributeLCP = (metric: LCPMetric): LCPMetricWithAttribution => {
 
       const ttfb = Math.max(
         0,
-        // From Chrome 115 until, Chrome reported responseStart as the document
-        // bytes, rather than Early Hint bytes. Prefer the Early Hint bytes
-        // (firstInterimResponseStart) for consistency with other browers, if
-        // non-zero
+        // From Chrome 115 until 133, Chrome reported responseStart as the
+        // document bytes, rather than Early Hint bytes. Prefer the Early Hint
+        // bytes (firstInterimResponseStart) for consistency with other
+        // browers, but only if non-zero (so use || rather than ??) as zero
+        // indicates no early hints.
         navigationEntry.firstInterimResponseStart ||
           navigationEntry.responseStart - activationStart,
       );

src/onTTFB.ts

@@ -71,10 +71,11 @@ export const onTTFB = (
     const navigationEntry = getNavigationEntry();
 
     if (navigationEntry) {
-      // From Chrome 115 until, Chrome reported responseStart as the document
-      // bytes, rather than Early Hint bytes. Prefer the Early Hint bytes
-      // (firstInterimResponseStart) for consistency with other browers, if
-      // non-zero
+      // From Chrome 115 until 133, Chrome reported responseStart as the
+      // document bytes, rather than Early Hint bytes. Prefer the Early Hint
+      // bytes (firstInterimResponseStart) for consistency with other
+      // browers, but only if non-zero (so use || rather than ??) as zero
+      // indicates no early hints.
       const responseStart =
         navigationEntry.firstInterimResponseStart ||
         navigationEntry.responseStart;

src/types.ts

@@ -56,7 +56,7 @@ declare global {
   interface PerformanceNavigationTiming {
     // https://wicg.github.io/nav-speculation/prerendering.html#performance-navigation-timing-extension
     activationStart?: number;
-    // Early Hints support
+    // https://w3c.github.io/resource-timing/#dom-performanceresourcetiming-firstinterimresponsestart
     firstInterimResponseStart?: number;
     finalResponseHeadersStart?: number;
   }