diff --git a/src/attribution/onCLS.ts b/src/attribution/onCLS.ts
index 7197a4c0..95b75187 100644
--- a/src/attribution/onCLS.ts
+++ b/src/attribution/onCLS.ts
@@ -25,7 +25,7 @@ import {
 } from '../types.js';
 
 const getLargestLayoutShiftEntry = (entries: LayoutShift[]) => {
-  return entries.reduce((a, b) => (a?.value > b.value ? a : b));
+  return entries.reduce((a, b) => (a.value > b.value ? a : b));
 };
 
 const getLargestLayoutShiftSource = (sources: LayoutShiftAttribution[]) => {
@@ -53,7 +53,12 @@ const attributeCLS = (metric: CLSMetric): CLSMetricWithAttribution => {
     }
   }
 
-  return {...metric, attribution};
+  // Use `Object.assign()` to ensure the original metric object is returned.
+  const metricWithAttribution: CLSMetricWithAttribution = Object.assign(
+    metric,
+    {attribution},
+  );
+  return metricWithAttribution;
 };
 
 /**
diff --git a/src/attribution/onFCP.ts b/src/attribution/onFCP.ts
index aec0524f..5cc1143d 100644
--- a/src/attribution/onFCP.ts
+++ b/src/attribution/onFCP.ts
@@ -51,7 +51,12 @@ const attributeFCP = (metric: FCPMetric): FCPMetricWithAttribution => {
     }
   }
 
-  return {...metric, attribution};
+  // Use `Object.assign()` to ensure the original metric object is returned.
+  const metricWithAttribution: FCPMetricWithAttribution = Object.assign(
+    metric,
+    {attribution},
+  );
+  return metricWithAttribution;
 };
 
 /**
diff --git a/src/attribution/onINP.ts b/src/attribution/onINP.ts
index cc4876a2..41b1c903 100644
--- a/src/attribution/onINP.ts
+++ b/src/attribution/onINP.ts
@@ -219,7 +219,7 @@ const getIntersectingLoAFs = (
   start: DOMHighResTimeStamp,
   end: DOMHighResTimeStamp,
 ) => {
-  const intersectingLoAFs = [];
+  const intersectingLoAFs: PerformanceLongAnimationFrameTiming[] = [];
 
   for (const loaf of pendingLoAFs) {
     // If the LoAF ends before the given start time, ignore it.
@@ -281,7 +281,12 @@ const attributeINP = (metric: INPMetric): INPMetricWithAttribution => {
     loadState: getLoadState(firstEntry.startTime),
   };
 
-  return {...metric, attribution};
+  // Use `Object.assign()` to ensure the original metric object is returned.
+  const metricWithAttribution: INPMetricWithAttribution = Object.assign(
+    metric,
+    {attribution},
+  );
+  return metricWithAttribution;
 };
 
 /**
diff --git a/src/attribution/onLCP.ts b/src/attribution/onLCP.ts
index 932099e6..3f91d68e 100644
--- a/src/attribution/onLCP.ts
+++ b/src/attribution/onLCP.ts
@@ -84,7 +84,12 @@ const attributeLCP = (metric: LCPMetric): LCPMetricWithAttribution => {
     }
   }
 
-  return {...metric, attribution};
+  // Use `Object.assign()` to ensure the original metric object is returned.
+  const metricWithAttribution: LCPMetricWithAttribution = Object.assign(
+    metric,
+    {attribution},
+  );
+  return metricWithAttribution;
 };
 
 /**
diff --git a/src/attribution/onTTFB.ts b/src/attribution/onTTFB.ts
index cb19b5e2..5e96b408 100644
--- a/src/attribution/onTTFB.ts
+++ b/src/attribution/onTTFB.ts
@@ -73,7 +73,12 @@ const attributeTTFB = (metric: TTFBMetric): TTFBMetricWithAttribution => {
     };
   }
 
-  return {...metric, attribution};
+  // Use `Object.assign()` to ensure the original metric object is returned.
+  const metricWithAttribution: TTFBMetricWithAttribution = Object.assign(
+    metric,
+    {attribution},
+  );
+  return metricWithAttribution;
 };
 
 /**
diff --git a/src/lib/getNavigationEntry.ts b/src/lib/getNavigationEntry.ts
index 42bdf3f2..5fa79bdd 100644
--- a/src/lib/getNavigationEntry.ts
+++ b/src/lib/getNavigationEntry.ts
@@ -22,12 +22,15 @@ export const getNavigationEntry = (): PerformanceNavigationTiming | void => {
   // In some cases no value is reported by the browser (for
   // privacy/security reasons), and in other cases (bugs) the value is
   // negative or is larger than the current page time. Ignore these cases:
-  // https://github.com/GoogleChrome/web-vitals/issues/137
-  // https://github.com/GoogleChrome/web-vitals/issues/162
-  // https://github.com/GoogleChrome/web-vitals/issues/275
+  // - https://github.com/GoogleChrome/web-vitals/issues/137
+  // - https://github.com/GoogleChrome/web-vitals/issues/162
+  // - https://github.com/GoogleChrome/web-vitals/issues/275
   if (
-    navigationEntry?.responseStart > 0 &&
-    navigationEntry?.responseStart < globalThis.performance?.now()
+    navigationEntry &&
+    navigationEntry.responseStart > 0 &&
+    // Note: if `navigationEnrtry` exists, then we don't need to
+    // feature-detect `performance.now()`.
+    navigationEntry.responseStart < performance.now()
   ) {
     return navigationEntry;
   }
diff --git a/src/lib/observe.ts b/src/lib/observe.ts
index 06d4aece..56fca7b4 100644
--- a/src/lib/observe.ts
+++ b/src/lib/observe.ts
@@ -48,7 +48,7 @@ export const observe = <K extends keyof PerformanceEntryMap>(
           callback(list.getEntries() as PerformanceEntryMap[K]);
         });
       });
-      po.observe({type, buffered: true, ...opts} as PerformanceObserverInit);
+      po.observe({type, buffered: true, ...opts});
       return po;
     }
   } catch {
diff --git a/src/onINP.ts b/src/onINP.ts
index 0f9312cc..63205d25 100644
--- a/src/onINP.ts
+++ b/src/onINP.ts
@@ -66,8 +66,12 @@ export const onINP = (
   opts: ReportOpts = {},
 ) => {
   // Return if the browser doesn't support all APIs needed to measure INP.
-  const eventTimingProto = globalThis.PerformanceEventTiming?.prototype;
-  if (!(eventTimingProto && 'interactionId' in eventTimingProto)) {
+  if (
+    !(
+      globalThis.PerformanceEventTiming &&
+      'interactionId' in PerformanceEventTiming.prototype
+    )
+  ) {
     return;
   }
 
@@ -92,7 +96,7 @@ export const onINP = (
 
         const inp = estimateP98LongestInteraction();
 
-        if (inp?.latency !== metric.value) {
+        if (inp && inp.latency !== metric.value) {
           metric.value = inp.latency;
           metric.entries = inp.entries;
           report();