Feature flag installation

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Feature flags enable safer deployment and roll back of new features and code. This helps roll out new features to your users in a managed way. If something goes wrong, flags enable roll backs without having to re-deploy your application.

Feature flags also help you control access to certain parts of your product, such as only showing paid features to users with an active subscription or early access features to users enrolled in a beta.

To start using feature flags, you first need to install PostHog (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'})
</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 Teams plan, 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.

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

Option 2: Install via package manager

yarn add posthog-js

And then include it in your files:

Web
import posthog from 'posthog-js'
posthog.init('<ph_project_api_key>', { api_host: 'https://us.i.posthog.com' })

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' })
}

If you're using React or Next.js, checkout our React SDK or Next.js integration.

Advanced option - bundle all required extensions

By default, the PostHog JS library will only load the core functionality, lazy-loading 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 or if you want to optimize your bundle at build time to ensure all dependencies are ready immediately. In addition environments like the Chrome Extension store will reject code that loads remote code. To solve this issue we have multiple import options available.

Please 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' })

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.

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Creating feature flags

To create a new feature flag, navigate to the feature flags tab in your PostHog app, where you should see a 'Create feature flag' button that we'll click. This presents you with a form where you can complete the details of your new flag: Here's a breakdown of each field in the form: Key This is the name of the key you use in your code to evaluate the feature flag. Description and tags Metadata to describe and share information about your feature flag in the PostHog app. Enable feature flag…

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