fix(analytics): use nullish checks in resolveDistinctId

CodeRabbit/Copilot feedback: truthy checks (`if (req.user?.id)`) would
coerce `0` or empty-string identifiers to `'anonymous'`. Switch to
`!= null` so any defined id/sessionID is preserved and stringified.

Added 2 tests (`id: 0`, empty sessionID) — suite now 13 tests.

Author: PierreBrisorgueil

lib/services/analytics.js

@@ -108,8 +108,10 @@ const isFeatureEnabled = async (flag, distinctId, options) => {
  */
 const resolveDistinctId = (req) => {
   if (!req) return 'anonymous';
-  if (req.user?.id) return String(req.user.id);
-  if (req.sessionID) return String(req.sessionID);
+  // Use nullish checks so legitimate identifiers like `0` or empty strings
+  // are preserved — any defined id is a valid PostHog distinctId.
+  if (req.user?.id != null) return String(req.user.id);
+  if (req.sessionID != null) return String(req.sessionID);
   return 'anonymous';
 };
 

lib/services/tests/analytics.forRequest.unit.tests.js

@@ -117,6 +117,30 @@ describe('analytics request-aware feature-flag helpers:', () => {
       );
     });
 
+    test('accepts user.id === 0 (nullish-check, not truthy)', async () => {
+      const req = { user: { id: 0 }, sessionID: 'sess-abc' };
+
+      await AnalyticsService.getFeatureFlagForRequest('checkout-v2', req);
+
+      expect(mockPostHogInstance.getFeatureFlag).toHaveBeenCalledWith(
+        'checkout-v2',
+        '0',
+        undefined,
+      );
+    });
+
+    test('accepts empty string sessionID when req.user is absent', async () => {
+      const req = { sessionID: '' };
+
+      await AnalyticsService.getFeatureFlagForRequest('checkout-v2', req);
+
+      expect(mockPostHogInstance.getFeatureFlag).toHaveBeenCalledWith(
+        'checkout-v2',
+        '',
+        undefined,
+      );
+    });
+
     test('returns undefined when PostHog is not configured', async () => {
       await AnalyticsService.shutdown();
       jest.resetModules();