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_instance_address> 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.

!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.async=!0,p.src=s.api_host+"/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="capture identify alias people.set people.set_once set_config register register_once unregister opt_out_capturing has_opted_out_capturing opt_in_capturing reset isFeatureEnabled onFeatureFlags getFeatureFlag getFeatureFlagPayload reloadFeatureFlags group updateEarlyAccessFeatureEnrollment getEarlyAccessFeatures getActiveMatchingSurveys getSurveys onSessionId".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: '<ph_instance_address>'})

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.

Option 2: Install via package manager

yarn add posthog-js

And then include it in your files:

import posthog from 'posthog-js'
posthog.init('<ph_project_api_key>', { api_host: '<ph_instance_address>' })

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

if (!window.location.host.includes('') && !window.location.host.includes('localhost')) {
posthog.init('<ph_project_api_key>', { api_host: '<ph_instance_address>' })

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


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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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