Next.js

PostHog makes it easy to get data about traffic and usage of your Next.js app. Integrating PostHog into your site enables analytics about user behavior, custom events capture, session recordings, feature flags, and more.

This guide walks you through integrating PostHog into your Next.js app using the React and the Node.js SDKs.

You can see a working example of this integration in our Next.js demo app.

Next.js has both client and server-side rendering, as well as pages and app routers. We'll cover all of these options in this guide.

Prerequisites

To follow this guide along, you need:

A PostHog instance (either Cloud or self-hosted)
A Next.js application

Client-side setup

Install posthog-js using your package manager:

Terminal

yarn add posthog-js
# or
npm install --save posthog-js
# or
pnpm add posthog-js

Add your environment variables to your .env.local file and to your hosting provider (e.g. Vercel, Netlify, AWS). You can find your project API key in your project settings.

.env.local

NEXT_PUBLIC_POSTHOG_KEY=<ph_project_api_key>
NEXT_PUBLIC_POSTHOG_HOST=https://us.i.posthog.com

These values need to start with NEXT_PUBLIC_ to be accessible on the client-side.

Pages router

If your Next.js app uses the pages router, you can integrate PostHog at the root of your app (pages/_app.js).

JavaScript

// pages/_app.js
import { useEffect } from 'react'
import { useRouter } from 'next/router'

import posthog from 'posthog-js'
import { PostHogProvider } from 'posthog-js/react'

// Check that PostHog is client-side (used to handle Next.js SSR)
if (typeof window !== 'undefined') {
  posthog.init(process.env.NEXT_PUBLIC_POSTHOG_KEY, {
    api_host: process.env.NEXT_PUBLIC_POSTHOG_HOST || 'https://us.i.posthog.com',
    person_profiles: 'identified_only',
    // Enable debug mode in development
    loaded: (posthog) => {
      if (process.env.NODE_ENV === 'development') posthog.debug()
    }
  })
}

export default function App({ Component, pageProps }) {
  const router = useRouter()

  useEffect(() => {
    // Track page views
    const handleRouteChange = () => posthog?.capture('$pageview')
    router.events.on('routeChangeComplete', handleRouteChange)

    return () => {
      router.events.off('routeChangeComplete', handleRouteChange)
    }
  }, [])

  return (
    <PostHogProvider client={posthog}>
      <Component {...pageProps} />
    </PostHogProvider>
  )
}

App router

If your Next.js app to uses the app router, you can integrate PostHog by creating a providers file in your app folder. This is because the posthog-js library needs to be initialized on the client-side using the Next.js 'use client' directive.

// app/providers.js
'use client'
import posthog from 'posthog-js'
import { PostHogProvider } from 'posthog-js/react'

if (typeof window !== 'undefined') {
  posthog.init(process.env.NEXT_PUBLIC_POSTHOG_KEY, {
    api_host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
    person_profiles: 'identified_only',
    capture_pageview: false // Disable automatic pageview capture, as we capture manually
  })
}

export function PHProvider({ children }) {
  return <PostHogProvider client={posthog}>{children}</PostHogProvider>
}

PostHog's $pageview autocapture relies on page load events. Since Next.js acts as a single-page app, this event doesn't trigger on navigation and we need to capture $pageview events manually.

To do this, we set up a PostHogPageView component to listen to URL path changes:

// app/PostHogPageView.jsx
'use client'

import { usePathname, useSearchParams } from "next/navigation";
import { useEffect } from "react";
import { usePostHog } from 'posthog-js/react';

export default function PostHogPageView() {
  const pathname = usePathname();
  const searchParams = useSearchParams();
  const posthog = usePostHog();
  useEffect(() => {
    // Track pageviews
    if (pathname && posthog) {
      let url = window.origin + pathname
      if (searchParams.toString()) {
        url = url + `?${searchParams.toString()}`
      }
      posthog.capture(
        '$pageview',
        {
          '$current_url': url,
        }
      )
    }
  }, [pathname, searchParams, posthog])
  
  return null
}

Then, import the PHProvider component into your app/layout file and wrap your app with it. We also dynamically import the PostHogPageView component and include it as a child of PHProvider.

Why is PostHogPageView dynamically imported? It contains the useSearchParams hook, which deopts the entire app into client-side rendering if it is not dynamically imported.

// app/layout.js

import './globals.css'
import { PHProvider } from './providers'
import dynamic from 'next/dynamic'

const PostHogPageView = dynamic(() => import('./PostHogPageView'), {
  ssr: false,
})

export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <PHProvider>
        <body>
          <PostHogPageView /> 
          {children}
        </body>
      </PHProvider>
    </html>
  )
}

PostHog is now set up and ready to go. Files and components accessing PostHog on the client-side need the 'use client' directive.

Pageleave events (optional)

To capture pageleave events, we need to set capture_pageleave: true in the initialization because setting capture_pageview: false disables it.

// app/providers.js
'use client'
import posthog from 'posthog-js'
import { PostHogProvider } from 'posthog-js/react'

if (typeof window !== 'undefined') {
  posthog.init(process.env.NEXT_PUBLIC_POSTHOG_KEY, {
    api_host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
    person_profiles: 'identified_only',
    capture_pageview: false,
    capture_pageleave: true // Enable pageleave capture
  })
}

export function PHProvider({ children }) {
  return <PostHogProvider client={posthog}>{children}</PostHogProvider>
}

Accessing PostHog using the provider

PostHog can then be accessed throughout your Next.js app by using the usePostHog hook. See the React SDK docs for examples of how to use:

You can also read the full posthog-js documentation for all the usable functions.

Server-side analytics

Server-side rendering enables you to render pages on the server instead of the client. This can be useful for SEO, performance, and user experience.

To integrate PostHog into your Next.js app on the server-side, you can use the Node SDK.

First, install the posthog-node library:

Terminal

yarn add posthog-node
# or
npm install --save posthog-node

Pages router

For the pages router, we can use the getServerSideProps function to access PostHog on the server-side, send events, evaluate feature flags, and more.

This looks like this:

JavaScript

// pages/posts/[id].js
import { useContext, useEffect, useState } from 'react'
import { getServerSession } from "next-auth/next"
import { PostHog } from 'posthog-node'

export default function Post({ post, flags }) {
  const [ctaState, setCtaState] = useState()

  useEffect(() => {
    if (flags) {
      setCtaState(flags['blog-cta'])
    }
  })

  return (
    <div>
      <h1>{post.title}</h1>
      <p>By: {post.author}</p>
      <p>{post.content}</p>
      {ctaState &&
        <p><a href="/">Go to PostHog</a></p>
      }
      <button onClick={likePost}>Like</button>
    </div>
  )
}

export async function getServerSideProps(ctx) {

  const session = await getServerSession(ctx.req, ctx.res)
  let flags = null

  if (session) {
    const client = new PostHog(
      process.env.NEXT_PUBLIC_POSTHOG_KEY,
      {
        host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
      }
    )

    flags = await client.getAllFlags(session.user.email);
    client.capture({
      distinctId: session.user.email,
      event: 'loaded blog article',
      properties: {
        $current_url: ctx.req.url,
      },
    });

    await client.shutdown()
  }

  const { posts } = await import('../../blog.json')
  const post = posts.find((post) => post.id.toString() === ctx.params.id)
  return {
    props: {
      post,
      flags
    },
  }
}

Note: Make sure to always call await client.shutdown() after sending events from the server-side. PostHog queues events into larger batches, and this call forces all batched events to be flushed immediately.

App router

For the app router, we can initialize the posthog-node SDK once with a PostHogClient function, and import it into files.

This enables us to send events and fetch data from PostHog on the server – without making client-side requests.

JavaScript

// app/posthog.js
import { PostHog } from 'posthog-node'

export default function PostHogClient() {
  const posthogClient = new PostHog(process.env.NEXT_PUBLIC_POSTHOG_KEY, {
    host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
    flushAt: 1,
    flushInterval: 0
  })
  return posthogClient
}

Note: Because our server-side posthog-node initializations are short-lived, we set flushAt to 1 and flushInterval to 0. flushAt sets how many capture calls we should flush the queue (in one batch). flushInterval sets how many milliseconds we should wait before flushing the queue. Setting them to the lowest number ensures events are sent immediately and not batched. We also need to call await posthog.shutdown() once done.

JavaScript

import Link from 'next/link'
import PostHogClient from '../posthog'

export default async function About() {

  const posthog = PostHogClient()
  const flags = await posthog.getAllFlags(
    'user_distinct_id' // replace with a user's distinct ID
  );
  await posthog.shutdown()

  return (
    <main>
      <h1>About</h1>
      <Link href="/">Go home</Link>
      { flags['main-cta'] &&
        <Link href="http://posthog.com/">Go to PostHog</Link>
      }
    </main>
  )
}

Configuring a reverse proxy to PostHog

To improve the reliability of client-side tracking and make requests less likely to be intercepted by tracking blockers, you can setup a reverse proxy in Next.js. Read more about deploying a reverse proxy using Next.js rewrites, Next.js middleware, and Vercel rewrites.

Which features are available in this library?

Prerequisites

Client-side setup

Pages router

App router

Pageleave events (optional)

Accessing PostHog using the provider

Server-side analytics

Pages router

App router

Configuring a reverse proxy to PostHog

Further reading

Questions?

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Nuxt.js

Ian Vanagas

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