Setting the file. One moment.
Chapter 47 · Instrument Integration
Subchapter 47.46
references/js.mdMarkdown12 KBView on GitHub
Note: This doc refers to our posthog-js library for use on the browser. For server-side JavaScript, see our .
HTML
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<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_token>',{api_host:'https://us.i.posthog.com', defaults:'2026-01-30'})
</script>Keeping the SDK version up to date
Be careful to avoid things which can cause the SDK version to be cached and fail to update. See: Ways SDK versions fall behind (opens in a new tab)
Using TypeScript with the script tag?
If you’re using TypeScript and want type safety for window.posthog, install the @posthog/types package:
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npm install @posthog/typesThen create a type declaration file:
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// posthog.d.ts
import type { PostHog } from '@posthog/types'
declare global {
interface Window {
posthog?: PostHog
}
}
export {}See the TypeScript types documentation (opens in a new tab) for more details.
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npm install --save posthog-jsyarn add posthog-jspnpm add posthog-jsbun add posthog-jsAnd then include it with your project token and host (which you can find in your project settings (opens in a new tab)):
Web
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import posthog from 'posthog-js'
posthog.init('<ph_project_token>', {
api_host: 'https://us.i.posthog.com',
defaults: '2026-01-30'
})See our framework specific docs for Next.js (opens in a new tab), React (opens in a new tab), Vue (opens in a new tab), Angular (opens in a new tab), Astro (opens in a new tab), Remix (opens in a new tab), and Svelte (opens in a new tab) for more installation details.
Update early, update often
We ship weirdly fast, especially for our JavaScript web SDK. If you choose the npm package instead of the HTML snippet, be sure to update it frequently:
To actually update the package, you need to update the version constraint in your package.json file and then reinstall, or run update instead of install:
PostHog AI
npm update posthog-jspnpm update posthog-jsyarn upgrade posthog-jsBundle 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:
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.
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// 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/posthog-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_token>', { api_host: 'https://us.i.posthog.com', defaults: '2026-01-30' })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/react does not work with any module import other than the default.
Tree shaking with the slim bundle (advanced)
If you only need a subset of PostHog features, you can use the slim bundle to reduce your bundle size. It gives you the core functionality (event capture, identify, group analytics) and lets you explicitly opt in to additional features via extension bundles. This is currently experimental, but offers the biggest reduction in bundle size.
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import posthog from 'posthog-js/dist/module.slim'
import {
SessionReplayExtensions,
AnalyticsExtensions,
} from 'posthog-js/dist/extension-bundles'
posthog.init('<ph_project_token>', {
api_host: 'https://us.i.posthog.com',
defaults: '2026-01-30',
__extensionClasses: {
...SessionReplayExtensions,
...AnalyticsExtensions,
}
})Note: Always import posthog-js from the same module path (posthog-js/dist/module.slim) throughout your app, otherwise multiple bundles could get included.
| Bundle | What’s included |
|---|---|
| FeatureFlagsExtensions | Feature Flags (opens in a new tab) |
| SessionReplayExtensions | Session Replay (opens in a new tab) |
| AnalyticsExtensions | Autocapture (opens in a new tab), pageview tracking, heatmaps (opens in a new tab), dead click detection, web vitals (opens in a new tab) |
| ErrorTrackingExtensions | Error Tracking (opens in a new tab) |
| SurveysExtensions | Surveys (opens in a new tab) |
| ExperimentsExtensions | Experiments (opens in a new tab) |
| SiteAppsExtensions | Site apps (opens in a new tab) |
| TracingExtensions | Distributed tracing header injection |
| ToolbarExtensions | Toolbar (opens in a new tab) |
| LogsExtensions | Log capture (opens in a new tab) |
| ConversationsExtensions | Conversations |
| AllExtensions | Everything (equivalent to the default posthog-js bundle) |
Note: Each extension bundle includes its own dependencies. You don’t need to worry about adding them separately.
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:
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if (!window.location.host.includes('127.0.0.1') && !window.location.host.includes('localhost')) {
posthog.init('<ph_project_token>', { api_host: 'https://us.i.posthog.com', defaults: '2026-01-30' })
}What is the `defaults` option?
The defaults is a date, such as 2026-01-30, 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.
Identifying users is required. Call
posthog.identify('your-user-id')after login to link events to a known user. This is what connects frontend event captures, session replays (opens in a new tab), LLM traces (opens in a new tab), and error tracking (opens in a new tab) to the same person — and lets backend events link back too.See our guide on identifying users (opens in a new tab) for how to set this up.
Once you’ve installed PostHog, see our features doc (opens in a new tab) for more information about what you can do with it. You can also install the PostHog VS Code extension (opens in a new tab) to see live analytics, flag status, and session replay links inline in your code.
We recommend putting PostHog both on your homepage and your application if applicable. That means you’ll be able to follow a user from the moment they come onto your website, all the way through signup and actually using your product.
PostHog automatically sets a cross-domain cookie, so if your website is
yourapp.comand your app is onapp.yourapp.comusers will be followed when they go from one to the other. See our tutorial on cross-website tracking (opens in a new tab) if you need to track users across different domains.
You can configure “replay triggers” in your project settings (opens in a new tab). You can configure triggers to enable or pause session recording when the user visit a page that matches the URL(s) you configure.
You are also able to setup “event triggers”. Session recording will be started immediately before PostHog queues any of these events to be sent to the backend.
You can completely opt-out users from data capture. To do this, there are two options:
opt_out_capturing_by_default to true in your PostHog config (opens in a new tab).Web
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posthog.init('<ph_project_token>', {
opt_out_capturing_by_default: true,
});posthog.opt_out_capturing().Similarly, you can opt users in:
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posthog.opt_in_capturing()To check if a user is opted out:
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posthog.has_opted_out_capturing()While not a first-class citizen, PostHog allows you to run more than one instance of PostHog at the same time if you, for example, want to track different events in different posthog instances/projects.
posthog.init accepts a third parameter that can be used to create named instances.
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posthog.init('<ph_project_token>', {}, 'project1')
posthog.init('<ph_project_token>', {}, 'project2')You can then call these different instances by accessing it on the global posthog object
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posthog.project1.capture('some_event')
posthog.project2.capture('other_event')Note: You’ll probably want to disable autocapture (and some other events) to avoid them from being sent to both instances. Check all of our config options (opens in a new tab) to better understand that.
For instructions on how to run posthog-js locally and setup your development environment, please checkout the README on the posthog-js (opens in a new tab) repository.
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