JavaScript Web

Note: This doc refers to our posthog-js library for use on the browser. For server-side JavaScript, see our Node SDK.

Installation

Option 1: Add JavaScript snippet to your HTML Recommended

This is the simplest way to get PostHog up and running on your website, and only takes a few minutes to set-up.

Add to your website & app

Paste this snippet within the <head> tags of your website - ideally just inside the closing </head> tag - on all pages that you wish to track.

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.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".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>'})
</script>

Be sure to replace <ph_project_api_key> and <ph_instance_address> with your project's values. (You can find the snippet pre-filled with this data in the PostHog app under Project / Settings. (Quick links if you use PostHog Cloud US or PostHog Cloud EU)

What this code does

After adding the snippet to your website, it will automatically start to:

Capture $pageview events when a user visits a page
Track when users click on links or buttons
Record videos of user sessions that you can play back (if you've enabled the Session recordings feature, which we highly recommend!)

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: '<ph_instance_address>' })

If you don't want to send a bunch of test data while you're developing, you could 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: '<ph_instance_address>' })
}

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

Track across marketing website & app

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.com and your app is on app.yourapp.com users will be followed when they go from one to the other.

Permitted domains

You can also configure "permitted domains" in your project settings. These are domains where you'll be able to record user sessions and use the PostHog toolbar.

Capturing events

You can send custom events using capture:

Web

posthog.capture('user signed up');

Tip: We recommend using a '[object][verb]' format for your event names, where '[object]' is the entity that the behavior relates to, and '[verb]' is the behavior itself. For example, project created, user signed up, or invite sent.

Setting event properties

Optionally, you can also include additional information in the event by setting the properties value:

Web

posthog.capture('user signed up', {
    login_type: "email",
    is_free_trial: true
})

Page views and autocapture

By default, PostHog automatically captures the following frontend events:

Pageviews, including the URL.
Autocaptured events, such as any click, change of input, or submission associated with a, button, form, input, select, textarea, and label tags.

If you prefer to disable these, set the appropriate values in your configuration options.

Single-page apps and pageviews

PostHog automatically sends pageview events whenever it gets loaded. If you have a single-page app, that means it only sends a pageview once (when your app loads).

To make sure any navigating a user does within your app gets captured, you can make a pageview call manually.

Web

posthog.capture('$pageview')

This automatically sends the current URL along with other relevant properties.

Setting user properties

To set user properties, include them when capturing an event:

Web

posthog.capture(
  'event_name', 
  { 
    $set: { name: 'Max Hedgehog'  },
    $set_once: { initial_url: '/blog' },
  }
)

Typically user properties are set when an event occurs like user updated email but there may be occasions where you simply want to set user properties as its own event.

JavaScript

posthog.setPersonProperties(
    { name: "Max Hedgehog" } // Thes properties are like the `$set` from above 
    { initial_url: "/blog" } // Thes properties are like the `$set` from above 
)

This creates a special $set event that is sent to PostHog.

For more details on the difference between $set and $set_once, see our user properties docs.

Identifying users

We strongly recommend reading our docs on identifying users to better understand how to correctly use this method.

Using identify, you can associate events with specific users. This enables you to gain full insights as to how they're using your product across different sessions, devices, and platforms.

Web

posthog.identify(
    'distinct_id', // Required. Replace 'distinct_id' with your user's unique identifier
    { email: 'max@hedgehogmail.com', name: 'Max Hedgehog' },  // $set, optional
    { first_visited_url: '/blog' } // $set_once, optional
);

Alias

Sometimes, you may want to assign multiple distinct IDs to a single user. This is helpful in scenarios where your primary distinct ID may be inaccessible. For example, if a distinct ID which is typically used on the frontend is not available in certain parts of your backend code. In this case, you can use alias to assign another distinct ID to the same user.

We strongly recommend reading our docs on alias to best understand how to correctly use this method.

Reset after logout

If a user logs out, you should call reset to unlink any future events made on that device with that user.

This is important if your users are sharing a computer, as otherwise all of those users are grouped together into a single user due to shared cookies between sessions. We strongly recommend you call reset on logout even if you don't expect users to share a computer.

You can do that like so:

Web

posthog.reset()

If you also want to reset device_id, you can pass true as a parameter:

Web

posthog.reset(true)

Super Properties

Super Properties are properties associated with events that are set once and then sent with every capture call, be it a $pageview, an autocaptured button click, or anything else.

They are set using posthog.register, which takes a properties object as a parameter, and they persist across sessions.

For example, take a look at the following call:

Web

posthog.register({
    'icecream pref': 'vanilla',
    team_id: 22,
})

The call above ensures that every event sent by the user will include "icecream pref": "vanilla" and "team_id": 22. This way, if you filtered events by property using icecream_pref = vanilla, it would display all events captured on that user after the posthog.register call, since they all include the specified Super Property.

However, please note that this does not store properties against the User, only against their events. To store properties against the User object, you should use posthog.setPersonProperties. More information on this can be found on the Sending User Information section.

Furthermore, if you register the same property multiple times, the next event will use the new value of that property. If you want to register a property only once (e.g. for ad campaign properties) you can use register_once, like so:

Web

posthog.register_once({
    'campaign source': 'twitter',
})

Using register_once will ensure that if a property is already set, it will not be set again. For example, if the user already has property "icecream pref": "vanilla", calling posthog.register_once({"icecream pref": "chocolate"}) will not update the property.

Removing stored Super Properties

Setting Super Properties creates a cookie on the client with the respective properties and their values. In order to stop sending a Super Property with events and remove the cookie, you can use posthog.unregister, like so:

Web

posthog.unregister('icecream pref')

This will remove the Super Property and subsequent events will not include it.

Opt users out

PostHog JS offers a function to opt users out based on your cookie settings definition (e.g. preferences set via a cookie banner).

This is also the suggested way to prevent capturing any data from the admin on the page, as well as from team members of your organization. A simple way to do this is to access the page as the admin (or any other user on your team you wish to stop capturing data on), and call posthog.opt_out_capturing(); on the developer console. You can also add this logic in you app and call it directly after an admin/team member logs in.

If you still wish to capture these events but want to create a distinction between users and team in PostHog, you should look into Cohorts.

With PostHog, you can:

Opt a user out:

Web

posthog.opt_out_capturing()

See if a user has opted out:

Web

posthog.has_opted_out_capturing()

Opt a user back in:

Web

posthog.opt_in_capturing()

Feature Flags

PostHog's feature flags enable you to safely deploy and roll back new features.

Boolean feature flags

Web

if (posthog.isFeatureEnabled('flag-key') ) {
    // Do something differently for this user

    // Optional: fetch the payload
    const matchedFlagPayload = posthog.getFeatureFlagPayload('flag-key')
}

Multivariate feature flags

Web

if (posthog.getFeatureFlag('flag-key')  == 'variant-key') { // replace 'variant-key' with the key of your variant
    // Do something differently for this user
    
    // Optional: fetch the payload
    const matchedFlagPayload = posthog.getFeatureFlagPayload('flag-key')
}

Ensuring flags are loaded before usage

Every time a user loads a page, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in a cookie.

This means that for most pages, the feature flags are available immediately – except for the first time a user visits.

To handle this, you can use the onFeatureFlags callback to wait for the feature flag request to finish:

Web

posthog.onFeatureFlags(function () {
    // feature flags are guaranteed to be available at this point
    if (posthog.isFeatureEnabled('flag-key')) {
        // do something
    }
})

Reloading feature flags

Feature flag values are cached. If something changed with your user and you want to refetch their flag values, call posthog.reloadFeatureFlags().

Bootstrapping Flags

Since there is a delay between initializing PostHog and fetching feature flags, feature flags are not always available immediately. This is detrimental if you want to do something like redirecting to a different page based on a feature flag.

To have your feature flags available immediately, you can initialize PostHog with precomputed values until PostHog has had a chance to fetch them. This is called bootstrapping.

For details on how to implement bootstrapping, see our bootstrapping guide

Overriding server properties

Sometimes, you might want to evaluate feature flags using properties that haven't been ingested yet, or were set incorrectly earlier. You can do so by setting properties the flag depends on with these calls:

Web

posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'})

Note that these are set for the entire session. Successive calls are additive: all properties you set are combined together and sent for flag evaluation.

Whenever you set these properties, we also trigger a reload of feature flags to ensure we have the latest values. You can disable this by passing in the optional parameter for reloading:

Web

posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'}, false)

At any point, you can reset these properties by calling resetPersonPropertiesForFlags:

Web

posthog.resetPersonPropertiesForFlags()

The same holds for group properties:

Web

// set properties for a group
posthog.setGroupPropertiesForFlags({'company': {'property1': 'value', property2: 'value2'}})

// reset properties for a given group:
posthog.resetGroupPropertiesForFlags('company')

// reset properties for all groups:
posthog.resetGroupPropertiesForFlags()

Note: You don't need to add the group names here, since these properties are automatically attached to the current group (set via posthog.group()). When you change the group, these properties are reset.

Automatic overrides

Whenever you call posthog.identify with person properties, we automatically add these properties to flag evaluation calls to help determine the correct flag values. The same is true for when you call posthog.group().

Default overridden properties

By default, we always override some properties based on the user IP address.

The list of properties that this overrides:

$geoip_city_name
$geoip_country_name
$geoip_country_code
$geoip_continent_name
$geoip_continent_code
$geoip_postal_code
$geoip_time_zone

This enables any geolocation-based flags to work without manually setting these properties.

Enriched analytics

You can send enriched analytics data for feature flags, which helps uncover replays where people interact with a flag, target people who've interacted with a feature, or build cohorts of people who've viewed a feature.

To enable this, you can either use our <PosthogFeature> React component (which implements this for you), or implement it on your own if you're not using react.

To implement it on your own, there are 3 things you need to do:

Whenever a feature is viewed, send the $feature_view event with the property feature_flag set to the name of the flag.

JavaScript

posthog.capture('$feature_view', { feature_flag: flag })

Whenever someone interacts with a feature, send the $feature_interaction event with the property feature_flag set to the name of the flag.
At the same time, set the person property $feature_interaction/<flag-key> to true. Here's a code example.

JavaScript

posthog.capture('$feature_interaction', { feature_flag: flag, $set: { [`$feature_interaction/${flag}`]: true } })

Here's a code example for the entire React component.

Experiments (A/B tests)

Since experiments use feature flags, the code for running an experiment is very similar to the feature flags code:

Web

// Ensure flags are loaded before usage.
// You'll only need to call this on the code the first time a user visits.
// See this doc for more details: https://posthog.com/docs/feature-flags/manual#ensuring-flags-are-loaded-before-usage
posthog.onFeatureFlags(function() {
    // feature flags should be available at this point
    if (posthog.getFeatureFlag('experiment-feature-flag-key')  == 'variant-name') {
        // do something
    }
})

// Otherwise, you can just do:
if (posthog.getFeatureFlag('experiment-feature-flag-key')  == 'variant-name') {
    // do something
}

// You can also test your code by overriding the feature flag:
// e.g., posthog.feature_flags.override({'experiment-feature-flag-key': 'test'})

It's also possible to run experiments without using feature flags.

Early access feature management

Early access features give you the option to release feature flags that can be controlled by your users. More information on this can be found here.

Web

posthog.getEarlyAccessFeatures((previewItemData) => {
    // do something with early access feature
})

Web

posthog.updateEarlyAccessFeatureEnrollment(flagKey, 'true')

Group analytics

Group analytics allows you to associate the events for that person's session with a group (e.g. teams, organizations, etc.). Read the Group Analytics guide for more information.

Note: This is a paid feature and is not available on the open-source or free cloud plan. Learn more here.

Associate the events for this session with a group

Web

posthog.group('company', 'company_id_in_your_db')

posthog.capture('upgraded plan') // this event is associated with company ID `company_id_in_your_db`

Associate the events for this session with a group AND update the properties of that group

Web

posthog.group('company', 'company_id_in_your_db', {
    name: 'Awesome Inc.',
    employees: 11,
})

The name is a special property which is used in the PostHog UI for the name of the Group. If you don't specify a name property, the group ID will be used instead.

Handling logging out

When the user logs out it's important to call posthog.reset() to avoid new events being registered under the previously active group.

Integrating groups with feature flags

If you have updated tracking, you can use group-based feature flags as normal.

Web

if (posthog.isFeatureEnabled('new-groups-feature')) {
    // do something
}

To check flag status for a different group, first switch the active group by calling posthog.group().

Persistence

In order for PostHog to work optimally, we require storing a small amount of information about the user on the user's browser. This ensures that if the user navigates away, and comes back to your site at a later time, we will still identify them properly. We store the following information in the user's browser:

User's ID
Session ID & Device ID
Active & enabled feature flags
Any super properties you have defined.
Some PostHog configuration options (e.g. whether session recording is enabled)

By default we store all this information in a cookie, which means that PostHog will still be able to identify your users even across subdomains. By default, this cookie is set to expire after 365 days.

If you would like to change how PostHog stores this information, you can do so with the persistence parameter.

persistence: "cookie" (default). Everything is stored in a cookie.
persistence: "localStorage+cookie". User's distinct ID is stored in a cookie and everything else is stored in the browser's localStorage.
persistence: "localStorage". Everything is stored in localStorage.
persistence: "memory". Stores in page memory, which means data is only persisted for the duration of the page view.

As a note, due to the size limitation of cookies you may run into 431 Request Header Fields Too Large errors (e.g. if you have a lot of feature flags). In that case, use localStorage+cookie.

Note: Please be aware that localStorage can't be used across subdomains. If you have multiple sites on the same domain, you may want to consider the cookie option or make sure to set all super properties across each subdomain.

If you don't want PostHog to store anything on the user's browser (e.g. if you want to rely on your own identification mechanism only, or want completely anonymous users), you can set disable_persistence: true in PostHog's config. Warning: Remember to call posthog.identify every time your app loads or every page refresh will be treated as a different user.

Config

When calling posthog.init, there are various configuration options you can set in addition to loaded and api_host.

To configure these options, pass them as an object to the posthog.init call, like so:

Web

posthog.init('<ph_project_api_key>', {
    api_host: '<ph_instance_address>',
    loaded: function (posthog) {
        posthog.identify('[user unique id]')
    },
    autocapture: false,
    // ... more options
})

There are multiple different configuration options, most of which you do not have to ever worry about. For brevity, only the most relevant ones are used here. However you can view all the configuration options in posthog-core.js.

Some of the most relevant options are:

Attribute	Description
`api_host` Type: String Default: `https://app.posthog.com`	URL of your PostHog instance.
`ui_host` Type: String Default: undefined	If using a reverse proxy for `api_host` then this should be the actual PostHog app URL (e.g. app.posthog.com)
`autocapture` Type: Boolean Default: `true`	Determines if PostHog should autocapture events. This setting does not affect capturing pageview events (see `capture_pageview`).
`bootstrap` Type: Object Default: `{}`	An object containing the `distinctID`, `isIdentifiedID`, and `featureFlags` keys, where `distinctID` is a string, and `featureFlags` is an object of key-value pairs
`capture_pageview` Type: Boolean Default: `true`	Determines if PostHog should automatically capture pageview events.
`capture_pageleave` Type: Boolean Default: `true`	Determines if PostHog should automatically capture pageleave events.
`cross_subdomain_cookie` Type: Boolean Default: `true`	Determines if cookie should be set on the top level domain (example.com). If PostHog-js is loaded on a subdomain (test.example.com), and `cross_subdomain_cookie` is set to false, it'll set the cookie on the subdomain only (test.example.com).
`disable_persistence` Type: Boolean Default: `false`	Disable persisting user data across pages. This will disable cookies, session storage and local storage.
`disable_session_recording` Type: Boolean Default: `false`	Determines if users should be opted out of session recording.
`enable_recording_console_log` Type: Boolean Default: `false`	Determines if console logs should be recorded as part of the session recording. More information.
`loaded` Type: Function Default: `function () {}`	A function to be called once the PostHog scripts have loaded successfully.
`mask_all_text` Type: Boolean Default: `false`	Prevent PostHog autocapture from capturing any text from your elements.
`mask_all_element_attributes` Type: Boolean Default: `false`	Prevent PostHog autocapture from capturing any attributes from your elements.
`opt_out_capturing_by_default` Type: Boolean Default: `false`	Determines if users should be opted out of PostHog tracking by default, requiring additional logic to opt them into capturing by calling `posthog.opt_in_capturing`.
`persistence` Type: `localStorage` or `cookie` or `memory` or `localStorage+cookie` Default: `cookie`	Determines how PostHog stores information about the user. See persistence for details.
`property_blacklist` Type: Array Default: `[]`	A list of properties that should never be sent with `capture` calls.
`sanitize_properties` Type: Function Default: `null`	A function that allows you to sanitize or modify the properties that get sent. Example: `sanitize_properties: function(properties, event) { if(properties['$ip']) { properties['$ip'] = null } return properties }`
`session_recording` Type: Object Default: See here.	Configuration options for recordings. More details found here
`xhr_headers` Type: Object Default: `{}`	Any additional headers you wish to pass with the XHR requests to the PostHog API.

Advanced configuration

In this section we describe some additional details on advanced configuration available.

Attribute	Description
`advanced_disable_decide` Type: Boolean Default: `false`	Will completely disable the `/decide` endpoint request (and features that rely on it). More details below.
`advanced_disable_feature_flags` Type: Boolean Default: `false`	Will keep `/decide` running, but without any feature flag requests
`advanced_disable_feature_flags_on_first_load` Type: Boolean Default: `false`	Stops from firing feature flag requests on first page load. Only requests feature flags when user identity or properties are updated, or you manually request for flags to be loaded.
`secure_cookie` Type: Boolean Default: `false`	If this is `true`, PostHog cookies will be marked as secure, meaning they will only be transmitted over HTTPS.
`custom_campaign_params` Type: Array Default: `[]`	List of query params to be automatically captured (see UTM Segmentation )

These are features for advanced users and may lead to unintended side effects if not reviewed carefully. If you are unsure about something, just reach out.

Disable `/decide` endpoint

This feature was introduced in posthog-js 1.10.0. Previously, disabling autocapture would inherently disable the /decide endpoint altogether. This meant that disabling autocapture would inadvertenly turn off session recording, feature flags, compression and the toolbar too.

One of the very first things the PostHog library does when init() is called is make a request to the /decide endpoint on PostHog's backend. This endpoint contains information on how to run the PostHog library so events are properly received in the backend. This endpoint is required to run most features of the library (detailed below). However, if you're not using any of the described features, you may wish to turn off the call completely to avoid an extra request and reduce resource usage on both the client and the server.

The /decide endpoint can be disabled by setting advanced_disable_decide = true in PostHog config.

Resources dependent on /decide

These are features/resources that will be fully disabled when the /decide endpoint is disabled.

Autocapture. The /decide endpoint contains information on whether autocapture should be enabled or not (apart from local configuration).
Session recording. The endpoint contains information on where to send relevant session recording events.
Compression. The endpoint contains information on what compression methods are supported on the backend (e.g. LZ64, gzip) for event payloads.
Feature flags. The endpoint contains the feature flags enabled for the current person.
Toolbar. The endpoint contains authentication information and other toolbar capabilities information required to run it.

Any custom event capturing (posthog.capture), $identify, $set, $set_once and basically any other calls not detailed above will work as expected when /decide is disabled.

Debugging

In your dev console you can run posthog.debug(). This will enable debugging, easily allowing you to see all data that is being sent to PostHog.

Development

For instructions on how to run posthog-js locally and setup your development environment, please checkout the README on the posthog-js repository.

Which features are available in this library?

Installation

Option 1: Add JavaScript snippet to your HTML Recommended

Add to your website & app

What this code does

Option 2: Install via package manager

Track across marketing website & app

Permitted domains

Capturing events

Setting event properties

Page views and autocapture

Single-page apps and pageviews

Setting user properties

Identifying users

Alias

Reset after logout

Super Properties

Removing stored Super Properties

Opt users out

Feature Flags

Boolean feature flags

Multivariate feature flags

Ensuring flags are loaded before usage

Reloading feature flags

Bootstrapping Flags

Overriding server properties

Enriched analytics

Experiments (A/B tests)

Early access feature management

Group analytics

Handling logging out

Integrating groups with feature flags

Persistence

Config

Advanced configuration

Disable /decide endpoint

Debugging

Development

Questions?

Was this page useful?

Android

Disable `/decide` endpoint