Tidy obs.speedcurve.js

Normalise SpeedCurve data writes, reuse cached navigation data,
and guard against a missing HEAD_End performance entry.

Author: csswizardry

js/obs.speedcurve.js

@@ -5,10 +5,17 @@
  */
 
 (() => {
+  const lux = window.LUX;
+
   // Bail out if SpeedCurve is not available.
-  if (!window.LUX || typeof window.LUX.addData !== 'function') return;
+  if (!lux || typeof lux.addData !== 'function') return;
 
   const obs = window.obs || Object.create(null);
+  const navigation = performance.getEntriesByType('navigation')[0];
+
+  const addData = (key, value) => {
+    lux.addData(key, value);
+  };
 
   // Keys we intend to send. Keep in sync with obs.js
   const keys = [
@@ -31,55 +38,45 @@
 
   for (const key of keys) {
     if (Object.prototype.hasOwnProperty.call(obs, key)) {
-      window.LUX.addData(key, obs[key]);
+      addData(key, obs[key]);
     }
   }
 
-  // Was the response from HTTP cache or the network?
-  const navigation = performance.getEntriesByType('navigation')[0];
-
   if (!navigation) return;
 
+  // Was the response from HTTP cache or the network?
   const { transferSize } = navigation;
 
   if (transferSize === 0) {
-    LUX.addData('fromCache', true);
+    addData('fromCache', true);
   } else if (transferSize > 0) {
-    LUX.addData('fromCache', false);
+    addData('fromCache', false);
   }
 
   // Was the response from the back–forward cache?
   window.addEventListener('pageshow', (event) => {
-    if (event.persisted) {
-      LUX.addData('frombfCache', true);
-    } else {
-      LUX.addData('frombfCache', false);
-    }
+    addData('frombfCache', event.persisted);
   });
 
-  if (document.prerendering) {
-  } else if (performance.getEntriesByType("navigation")[0]?.activationStart > 0) {
-    LUX.addData('fromPrerender', true);
-  } else {
-    LUX.addData('fromPrerender', false);
+  if (!document.prerendering) {
+    addData('fromPrerender', navigation.activationStart > 0);
   }
 
   // Time to Last Byte (TTLB)
   if (navigation.responseEnd && navigation.startTime >= 0) {
     const ttlb = Math.round(navigation.responseEnd - navigation.startTime);
     if (Number.isFinite(ttlb) && ttlb >= 0) {
-      LUX.addData('ttlb', ttlb);
+      addData('ttlb', ttlb);
     }
   }
 
   // First Potential Paint (FPP)
-  if (navigation.startTime >= 0) {
-    const headEnd = performance.getEntriesByName('HEAD_End')[0];
+  const headEnd = performance.getEntriesByName('HEAD_End')[0];
+
+  if (headEnd && navigation.startTime >= 0) {
     const fpp = Math.round(headEnd.startTime - navigation.startTime);
     if (Number.isFinite(fpp) && fpp >= 0) {
-      LUX.addData('fpp', fpp);
+      addData('fpp', fpp);
     }
   }
-
-
 })();