Fix reportAllChanges behavior for LCP when library is loaded late

src/onLCP.ts

@@ -58,24 +58,26 @@ export const onLCP = (onReport: LCPReportCallback, opts?: ReportOpts) => {
     let report: ReturnType<typeof bindReporter>;
 
     const handleEntries = (entries: LCPMetric['entries']) => {
-      const lastEntry = entries[entries.length - 1] as LargestContentfulPaint;
-      if (lastEntry) {
+      // If reportAllChanges is set then call this function for each entry
+      // As otherwise only want to emit the last one.
+      if (!opts!.reportAllChanges) {
+        entries = entries.slice(-1);
+      }
+
+      entries.forEach((entry) => {
         // Only report if the page wasn't hidden prior to LCP.
-        if (lastEntry.startTime < visibilityWatcher.firstHiddenTime) {
+        if (entry.startTime < visibilityWatcher.firstHiddenTime) {
           // The startTime attribute returns the value of the renderTime if it is
           // not 0, and the value of the loadTime otherwise. The activationStart
           // reference is used because LCP should be relative to page activation
           // rather than navigation start if the page was prerendered. But in cases
           // where `activationStart` occurs after the LCP, this time should be
           // clamped at 0.
-          metric.value = Math.max(
-            lastEntry.startTime - getActivationStart(),
-            0,
-          );
-          metric.entries = [lastEntry];
+          metric.value = Math.max(entry.startTime - getActivationStart(), 0);
+          metric.entries = [entry];
           report();
         }
-      }
+      });
     };
 
     const po = observe('largest-contentful-paint', handleEntries);

test/e2e/onLCP-test.js

@@ -121,14 +121,24 @@ describe('onLCP()', async function () {
     // Wait until all images are loaded and fully rendered.
     await imagesPainted();
 
-    // Load a new page to trigger the hidden state.
-    await navigateTo('about:blank');
+    await beaconCountIs(2);
+    const [lcp1, lcp2] = await getBeacons();
 
-    // Even though the test sets `reportAllChanges` to true, since the library
-    // is lazy loaded after all elements have been rendered, only a single
-    // change will be reported.
-    await beaconCountIs(1);
-    assertStandardReportsAreCorrect(await getBeacons());
+    assert(lcp1.value > 0);
+    assert(lcp1.id.match(/^v4-\d+-\d+$/));
+    assert.strictEqual(lcp1.name, 'LCP');
+    assert.strictEqual(lcp1.value, lcp1.delta);
+    assert.strictEqual(lcp1.rating, 'good');
+    assert.strictEqual(lcp1.entries.length, 1);
+    assert.strictEqual(lcp1.navigationType, 'navigate');
+
+    assert(lcp2.value > 500); // Greater than the image load delay.
+    assert(lcp2.id.match(/^v4-\d+-\d+$/));
+    assert.strictEqual(lcp2.name, 'LCP');
+    assert(lcp2.value > lcp2.delta);
+    assert.strictEqual(lcp2.rating, 'good');
+    assert.strictEqual(lcp2.entries.length, 1);
+    assert.strictEqual(lcp2.navigationType, 'navigate');
   });
 
   it('accounts for time prerendering the page', async function () {
@@ -332,7 +342,7 @@ describe('onLCP()', async function () {
 
     const [lcp1] = await getBeacons();
 
-    assert(lcp1.value > 0); // Greater than the image load delay.
+    assert(lcp1.value > 0);
     assert(lcp1.id.match(/^v4-\d+-\d+$/));
     assert.strictEqual(lcp1.name, 'LCP');
     assert.strictEqual(lcp1.value, lcp1.delta);
@@ -346,7 +356,7 @@ describe('onLCP()', async function () {
 
     const [lcp2] = await getBeacons();
 
-    assert(lcp2.value > 0); // Greater than the image load delay.
+    assert(lcp2.value > 0);
     assert(lcp2.id.match(/^v4-\d+-\d+$/));
     assert.strictEqual(lcp2.name, 'LCP');
     assert.strictEqual(lcp2.value, lcp2.delta);
@@ -377,7 +387,7 @@ describe('onLCP()', async function () {
 
     const [lcp1] = await getBeacons();
 
-    assert(lcp1.value > 0); // Greater than the image load delay.
+    assert(lcp1.value > 0);
     assert(lcp1.id.match(/^v4-\d+-\d+$/));
     assert.strictEqual(lcp1.name, 'LCP');
     assert.strictEqual(lcp1.value, lcp1.delta);
@@ -391,7 +401,7 @@ describe('onLCP()', async function () {
 
     const [lcp2] = await getBeacons();
 
-    assert(lcp2.value > 0); // Greater than the image load delay.
+    assert(lcp2.value > 0);
     assert(lcp2.id.match(/^v4-\d+-\d+$/));
     assert.strictEqual(lcp2.name, 'LCP');
     assert.strictEqual(lcp2.value, lcp2.delta);
@@ -415,7 +425,7 @@ describe('onLCP()', async function () {
 
     const [lcp] = await getBeacons();
 
-    assert(lcp.value > 0); // Greater than the image load delay.
+    assert(lcp.value > 0);
     assert(lcp.id.match(/^v4-\d+-\d+$/));
     assert.strictEqual(lcp.name, 'LCP');
     assert.strictEqual(lcp.value, lcp.delta);
@@ -654,7 +664,7 @@ describe('onLCP()', async function () {
 
       const [lcp2] = await getBeacons();
 
-      assert(lcp2.value > 0); // Greater than the image load delay.
+      assert(lcp2.value > 0);
       assert(lcp2.id.match(/^v4-\d+-\d+$/));
       assert.strictEqual(lcp2.name, 'LCP');
       assert.strictEqual(lcp2.value, lcp2.delta);