Match the example: Your implementation should follow the example project's patterns as closely as possible.
Framework guidelines
A missing PostHog configuration must never break the app — read keys optionally (never a required setting), guard init and capture behind their presence, and keep build and boot working with no PostHog environment set — but never silently: in development or debug builds fail loudly, using the language's idiomatic error, with the message " variable required by PostHog is missing or un-configured, this causes events to be silently missed. This error stops appearing once is configured" (substituting the actual variable name); production stays a no-op
For Next.js 15.3+, initialize PostHog in instrumentation-client.ts for the simplest setup
For feature flags, use useFeatureFlagEnabled() or useFeatureFlagPayload() hooks - they handle loading states and external sync automatically
Add analytics capture in event handlers where user actions occur, NOT in useEffect reacting to state changes
Do NOT use useEffect for data transformation - calculate derived values during render instead
Do NOT use useEffect to respond to user events - put that logic in the event handler itself
Do NOT use useEffect to chain state updates - calculate all related updates together in the event handler
Do NOT use useEffect to notify parent components - call the parent callback alongside setState in the event handler
To reset component state when a prop changes, pass the prop as the component's key instead of using useEffect
useEffect is ONLY for synchronizing with external systems (non-React widgets, browser APIs, network subscriptions)
Remember that source code is available in the node_modules directory
Check package.json for type checking or build scripts to validate changes
When identity comes from framework-bridged state (Inertia or SSR shared props, a serialized session), confirm the backend actually shares that field — add the share server-side if missing — before identifying from it
When a reverse proxy is configured, both /static/* AND /array/* must route to the assets origin (us-assets.i.posthog.com or eu-assets.i.posthog.com).
posthog-js is the JavaScript SDK package name
posthog.init() MUST be called before any other PostHog methods (capture, identify, etc.)
posthog-js is browser-only — do NOT import it in Node.js or server-side contexts (use posthog-node instead)
Autocapture is ON by default with posthog-js (tracks clicks, form submissions, pageviews). Keep autocapture enabled unless the user explicitly asks to turn it off.
NEVER send PII in posthog.capture() event properties — no emails, full names, phone numbers, physical addresses, IP addresses, or user-generated content
PII belongs in posthog.identify() person properties (email, name, role), NOT in capture() event properties
Identifying users
Identify users during login and signup events. Refer to the example code and documentation for the correct identify pattern for this framework. If both frontend and backend code exist, pass the client-side session and distinct ID using X-POSTHOG-DISTINCT-ID and X-POSTHOG-SESSION-ID headers to maintain correlation.
Error tracking
Add PostHog error tracking to relevant files, particularly around critical user flows and API boundaries.
Call posthog.identify(userId, { email, name, role }) on login AND on page refresh if the user is already logged in
Call posthog.reset() on logout — the transition out of an identified session, never an initially anonymous page load (that discards the anonymous id and its history) — and before identify() when switching directly between accounts
For SPAs without a framework router, capture pageviews with posthog.capture($pageview) or use the capture_pageview history_change option in init for History API routing
When verifying with an automated browser (Playwright, Puppeteer, Selenium), posthog-js's bot filter silently drops every capture while flags and asset loads still succeed. Override navigator.webdriver, the user agent, AND navigator.userAgentData before concluding events do not send. Diagnose with ?__posthog_debug=true ("likely bot" in the console).
posthog-node is the Node.js server-side SDK package name; posthog-js is browser-only, so use posthog-node on the server instead
Include enableExceptionAutocapture: true in the PostHog constructor options
Add posthog.capture() calls in route handlers for meaningful user actions – every route that creates, updates, or deletes data should track an event with contextual properties
Add posthog.captureException(err, distinctId) in the application's error handler (e.g., Express error middleware, Fastify setErrorHandler, Koa app.on('error'))
The SDK batches events and flushes asynchronously. await flush() or await shutdown() before letting that process exit. If unsure, set flushAt 1 and flushInterval 0.
posthog.capture() enqueues synchronously and returns; the batched HTTP send happens afterwards. Treat every per-request handler as short-lived even when the framework feels like a server: Next.js / Nuxt / SvelteKit / Remix route handlers, serverless and edge functions, and Lambda are torn down per invocation before the send runs. Create the client with flushAt 1 and flushInterval 0, then await the send before returning. Always use await posthog.flush() for a shared/singleton client, await posthog.shutdown() for a per-request client. Never skip the awaited flush or risk the enqueued event being silently dropped.
Reverse proxy is NOT needed for server-side Node.js – only client-side JavaScript needs a proxy to avoid ad blockers
