refactor: move PostHog singleton to utils/posthog.ts

Extracts getPostHogClient() into a dedicated module so it can be reused for feature flags and other PostHog features beyond the mirror middleware.

Author: jakebromberg

apps/backend/middleware/legacy/mirror.middleware.ts

@@ -1,17 +1,6 @@
 import { NextFunction, Request, Response } from 'express';
 import { MirrorCommandQueue } from './commandqueue.mirror';
-
-import { PostHog } from 'posthog-node';
-
-let postHogClient: PostHog | null = null;
-function getPostHogClient(): PostHog {
-  if (!postHogClient) {
-    postHogClient = new PostHog(process.env.POSTHOG_API_KEY ?? '', {
-      host: 'https://us.i.posthog.com',
-    });
-  }
-  return postHogClient;
-}
+import { getPostHogClient } from '../../utils/posthog.js';
 
 export const createBackendMirrorMiddleware =
   <T>(createCommand: (req: Request, data: T) => Promise<string[]>) =>

apps/backend/utils/posthog.ts

@@ -0,0 +1,12 @@
+import { PostHog } from 'posthog-node';
+
+let postHogClient: PostHog | null = null;
+
+export function getPostHogClient(): PostHog {
+  if (!postHogClient) {
+    postHogClient = new PostHog(process.env.POSTHOG_API_KEY ?? '', {
+      host: 'https://us.i.posthog.com',
+    });
+  }
+  return postHogClient;
+}

tests/unit/middleware/legacy/mirror.posthog.test.ts

@@ -3,10 +3,10 @@ const mockPostHogInstance = {
   shutdown: jest.fn().mockResolvedValue(undefined),
 };
 
-const PostHogConstructor = jest.fn(() => mockPostHogInstance);
+const mockGetPostHogClient = jest.fn(() => mockPostHogInstance);
 
-jest.mock('posthog-node', () => ({
-  PostHog: PostHogConstructor,
+jest.mock('../../../../apps/backend/utils/posthog', () => ({
+  getPostHogClient: mockGetPostHogClient,
 }));
 
 jest.mock('../../../../apps/backend/middleware/legacy/commandqueue.mirror', () => ({
@@ -34,14 +34,14 @@ function createMockReqRes() {
   return { req, res };
 }
 
-describe('PostHog client instantiation', () => {
+describe('PostHog client usage', () => {
   beforeEach(() => {
-    PostHogConstructor.mockClear();
+    mockGetPostHogClient.mockClear();
     mockPostHogInstance.isFeatureEnabled.mockClear();
     mockPostHogInstance.shutdown.mockClear();
   });
 
-  it('creates PostHog at most once across multiple requests', async () => {
+  it('uses the shared PostHog singleton from utils/posthog', async () => {
     const createCommand = jest.fn().mockResolvedValue(['SQL1']);
     const middleware = createBackendMirrorMiddleware(createCommand);
 
@@ -63,6 +63,8 @@ describe('PostHog client instantiation', () => {
     // Allow async callbacks to settle
     await new Promise((r) => setTimeout(r, 50));
 
-    expect(PostHogConstructor).toHaveBeenCalledTimes(1);
+    // getPostHogClient is called per-request, but it returns the same singleton
+    expect(mockGetPostHogClient).toHaveBeenCalled();
+    expect(mockPostHogInstance.isFeatureEnabled).toHaveBeenCalledTimes(2);
   });
 });