A/B testing installation

A/B testing enables you to test the impact of product changes and understand how they affect your users' behaviour. For example:

How changes to your onboarding flow affect your signup rate.
If different designs of your app's dashboard increase user engagement and retention.
The impact a free trial period versus money-back guarantee to determine which results in more customers.

A/B tests are also referred to as "experiments", and this is how we refer to them in the PostHog app.

To start using A/B tests, install PostHog with the library you want to run tests in (if you haven't already):

Option 1: Add the JavaScript snippet to your HTML Recommended

This is the simplest way to get PostHog up and running. It only takes a few minutes.

Copy the snippet below and replace <ph_project_api_key> and <ph_client_api_host> with your project's values, then add it within the <head> tags at the base of your product - ideally just before the closing </head> tag. This ensures PostHog loads on any page users visit.

You can find the snippet pre-filled with this data in your project settings.

HTML

<script>
    !function(t,e){var o,n,p,r;e.__SV||(window.posthog=e,e._i=[],e.init=function(i,s,a){function g(t,e){var o=e.split(".");2==o.length&&(t=t[o[0]],e=o[1]),t[e]=function(){t.push([e].concat(Array.prototype.slice.call(arguments,0)))}}(p=t.createElement("script")).type="text/javascript",p.crossOrigin="anonymous",p.async=!0,p.src=s.api_host.replace(".i.posthog.com","-assets.i.posthog.com")+"/static/array.js",(r=t.getElementsByTagName("script")[0]).parentNode.insertBefore(p,r);var u=e;for(void 0!==a?u=e[a]=[]:a="posthog",u.people=u.people||[],u.toString=function(t){var e="posthog";return"posthog"!==a&&(e+="."+a),t||(e+=" (stub)"),e},u.people.toString=function(){return u.toString(1)+".people (stub)"},o="init capture register register_once register_for_session unregister unregister_for_session getFeatureFlag getFeatureFlagPayload isFeatureEnabled reloadFeatureFlags updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures on onFeatureFlags onSessionId getSurveys getActiveMatchingSurveys renderSurvey canRenderSurvey getNextSurveyStep identify setPersonProperties group resetGroups setPersonPropertiesForFlags resetPersonPropertiesForFlags setGroupPropertiesForFlags resetGroupPropertiesForFlags reset get_distinct_id getGroups get_session_id get_session_replay_url alias set_config startSessionRecording stopSessionRecording sessionRecordingStarted captureException loadToolbar get_property getSessionProperty createPersonProfile opt_in_capturing opt_out_capturing has_opted_in_capturing has_opted_out_capturing clear_opt_in_out_capturing debug".split(" "),n=0;n<o.length;n++)g(u,o[n]);e._i.push([i,s,a])},e.__SV=1)}(document,window.posthog||[]);
    posthog.init('<ph_project_api_key>',{api_host:'https://us.i.posthog.com', defaults:'2025-05-24'})
</script>

Once the snippet is added, PostHog automatically captures $pageview and other events like button clicks. You can then enable other products, such as session replays, within your project settings.

Set up a reverse proxy (recommended)

We recommend setting up a reverse proxy, so that events are less likely to be intercepted by tracking blockers.

We have our own managed reverse proxy service included in the platform add-ons, which routes through our infrastructure and makes setting up your proxy easy.

If you don't want to use our managed service then there are several other options for creating a reverse proxy, including using Cloudflare, AWS Cloudfront, and Vercel.

Grouping products in one project (recommended)

If you have multiple customer-facing products (e.g. a marketing website + mobile app + web app), it's best to install PostHog on them all and group them in one project.

This makes it possible to track users across their entire journey (e.g. from visiting your marketing website to signing up for your product), or how they use your product across multiple platforms.

Include ES5 support (optional)

If you need ES5 support for example to track Internet Explorer 11 replace /static/array.js in the snippet with /static/array.full.es5.js

Working with AI code editors?

If you’re working with AI code editors (like Lovable, Bolt.new, Replit, and others), it’s easy to install PostHog. All you have to do is ask (seriously). Specifically: in your tool, type a prompt like this: “I’d like to use PostHog to track events here; here is my install snippet.” Paste in your snippet. The tool will do the rest.

As of this time, some web-based AI builders (such as artifacts by Claude) don’t support access to external JS libraries. You’ll be able to ask it to use a placeholder, and then update it manually when you deploy to production.

Option 2: Install via package manager

yarn add posthog-js

And then include it with your project API key and host (which you can find in your project settings):

Web

import posthog from 'posthog-js'

posthog.init('<ph_project_api_key>', {
  api_host: 'https://us.i.posthog.com',
  defaults: '2025-05-24'
})

See our framework specific docs for Next.js, React, Vue, Angular, Astro, Remix, and Svelte for more installation details.

Bundle all required extensions (advanced)

By default, the JavaScript Web library only loads the core functionality. It lazy-loads extensions such as surveys or the session replay 'recorder' when needed.

This can cause issues if:

You have a Content Security Policy (CSP) that blocks inline scripts.
You want to optimize your bundle at build time to ensure all dependencies are ready immediately.
Your app is running in environments like the Chrome Extension store or Electron that reject or block remote code loading.

To solve these issues, we have multiple import options available below.

Note: With any of the no-external options, the toolbar will be unavailable as this is only possible as a runtime dependency loaded directly from us.posthog.com.

Web

// No external code loading possible (this disables all extensions such as Replay, Surveys, Exceptions etc.)
import posthog from 'posthog-js/dist/module.no-external'

// No external code loading possible but all external dependencies pre-bundled
import posthog from 'posthog-js/dist/module.full.no-external'

// All external dependencies pre-bundled and with the ability to load external scripts (primarily useful is you use Site Apps)
import posthog from 'posthog-js/dist/module.full'

// Finally you can also import specific extra dependencies 
import "posthog-js/dist/recorder"
import "posthog-js/dist/surveys"
import "posthog-js/dist/exception-autocapture"
import "posthog-js/dist/tracing-headers"
import "posthog-js/dist/web-vitals"
import posthog from 'posthog-js/dist/module.no-external'

// All other posthog commands are the same as usual
posthog.init('<ph_project_api_key>', { api_host: 'https://us.i.posthog.com', defaults: '2025-05-24' })

Note: You should ensure if using this option that you always import posthog-js from the same module, otherwise multiple bundles could get included. At this time posthog-js/react does not work with any module import other than the default.

Don't want to send test data while developing?

If you don't want to send test data while you're developing, you can do the following:

Web

if (!window.location.host.includes('127.0.0.1') && !window.location.host.includes('localhost')) {
    posthog.init('<ph_project_api_key>', { api_host: 'https://us.i.posthog.com', defaults: '2025-05-24' })
}

What is the `defaults` option?

The defaults is a date, such as 2025-05-24, for a configuration snapshot used as defaults to initialize PostHog. This default is overridden when you explicitly set a value for any of the options.

Option 1: Add the JavaScript snippet to your HTML Recommended

Option 2: Install via package manager

Questions? Ask Max AI.

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