diff --git a/index.html b/index.html
index a84a99e..deb494f 100644
--- a/index.html
+++ b/index.html
@@ -1725,6 +1725,25 @@ <h3 style="margin-bottom:0">Life Events</h3>
   return cutoff.toISOString().slice(0, 10);
 }
 
+// SUMMARY-STATS-START (extracted verbatim by tests/js/summary_stats.test.mjs)
+function computeSummaryStats(allDays) {
+  const avgStat = vals => { const c = vals.filter(v => v != null); return c.length ? +(c.reduce((a,b)=>a+b,0)/c.length).toFixed(1) : null; };
+  // Seed-free min/max: a null seed would coerce to 0 in Math.min/Math.max.
+  const extreme = (vals, pick) => { const c = vals.filter(v => v != null); return c.length ? pick(...c) : null; };
+  const bestStepsDay = allDays.reduce((best, d) => (!best || (d.steps||0) > (best.steps||0)) ? d : best, null);
+  return {
+    avg_steps:       avgStat(allDays.map(d => d.steps)),
+    avg_sleep_hours: avgStat(allDays.map(d => d.sleep_hours)),
+    avg_resting_hr:  avgStat(allDays.map(d => d.resting_hr)),
+    avg_hrv:         avgStat(allDays.map(d => d.hrv)),
+    best_sleep:      extreme(allDays.map(d => d.sleep_hours), Math.max),
+    worst_sleep:     extreme(allDays.map(d => d.sleep_hours), Math.min),
+    best_steps_day:  bestStepsDay ? bestStepsDay.date : null,
+    total_days:      allDays.length,
+  };
+}
+// SUMMARY-STATS-END
+
 async function parseHealthXML(file, onProgress) {
   onProgress(0, 'Reading file...');
   const text = await file.text();
@@ -1812,20 +1831,8 @@ <h3 style="margin-bottom:0">Life Events</h3>
   onProgress(90, 'Computing summary...');
   await new Promise(r => setTimeout(r, 0));
 
-  // Summary stats
   const allDays = Object.values(filtered);
-  const avgStat = vals => { const c = vals.filter(v => v != null); return c.length ? +(c.reduce((a,b)=>a+b,0)/c.length).toFixed(1) : null; };
-  const bestStepsDay = allDays.reduce((best, d) => (!best || (d.steps||0) > (best.steps||0)) ? d : best, null);
-  const summary = {
-    avg_steps:       avgStat(allDays.map(d => d.steps)),
-    avg_sleep_hours: avgStat(allDays.map(d => d.sleep_hours)),
-    avg_resting_hr:  avgStat(allDays.map(d => d.resting_hr)),
-    avg_hrv:         avgStat(allDays.map(d => d.hrv)),
-    best_sleep:      allDays.map(d=>d.sleep_hours).filter(v=>v!=null).reduce((a,b)=>Math.max(a,b), null),
-    worst_sleep:     allDays.map(d=>d.sleep_hours).filter(v=>v!=null).reduce((a,b)=>Math.min(a,b), null),
-    best_steps_day:  bestStepsDay ? bestStepsDay.date : null,
-    total_days:      allDays.length,
-  };
+  const summary = computeSummaryStats(allDays);
 
   return {
     daily: filtered,
diff --git a/tests/js/summary_stats.test.mjs b/tests/js/summary_stats.test.mjs
new file mode 100644
index 0000000..3f5ad45
--- /dev/null
+++ b/tests/js/summary_stats.test.mjs
@@ -0,0 +1,42 @@
+// Run: node --test tests/js/summary_stats.test.mjs
+// Extracts computeSummaryStats from the single-file frontend so the real code is tested.
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+
+const html = readFileSync(fileURLToPath(new URL('../../index.html', import.meta.url)), 'utf8');
+const m = html.match(/\/\/ SUMMARY-STATS-START[^\n]*\n([\s\S]*?)\/\/ SUMMARY-STATS-END/);
+assert.ok(m, 'summary-stats markers not found in index.html');
+const computeSummaryStats = new Function(`${m[1]}; return computeSummaryStats;`)();
+
+const days = sleeps => sleeps.map((s, i) => ({ date: `2025-01-0${i + 1}`, steps: 1000 * (i + 1), sleep_hours: s }));
+
+test('best/worst sleep over positive values', () => {
+  const s = computeSummaryStats(days([7, 6.5, 8]));
+  assert.equal(s.worst_sleep, 6.5);
+  assert.equal(s.best_sleep, 8);
+});
+
+test('nulls are ignored', () => {
+  const s = computeSummaryStats(days([null, 7, null, 5.5]));
+  assert.equal(s.worst_sleep, 5.5);
+  assert.equal(s.best_sleep, 7);
+});
+
+test('no sleep data stays null, not 0', () => {
+  for (const input of [[], days([null, null])]) {
+    const s = computeSummaryStats(input);
+    assert.equal(s.worst_sleep, null);
+    assert.equal(s.best_sleep, null);
+    assert.equal(s.avg_sleep_hours, null);
+  }
+});
+
+test('other stats unchanged', () => {
+  const s = computeSummaryStats(days([7, 6.5, 8]));
+  assert.equal(s.best_steps_day, '2025-01-03');
+  assert.equal(s.total_days, 3);
+  assert.equal(s.avg_steps, 2000);
+  assert.equal(computeSummaryStats([]).best_steps_day, null);
+});