JavaScript library

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Note: You can just use our snippet to start capturing events with our JS.

This page of the Docs refers to our posthog-js library.

Why does this exist?

The reason this exists is that whilst the default snippet captures every click on certain elements (like a, button, input etc.) and page views, it's often worth sending more context whenever a user does something. This might also be useful if you have a one page app.

Installation

You can either load the snippet as a script in your 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".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>

Place the snippet in the <head> tags of your website, ideally just above the closing </head> tag. You will need to do this for all pages that you wish to track.

Or you can include it using npm, by doing either:

yarn add posthog-js

or:

npm install --save 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 a bunch of test data while you're developing, you could do the following:

if (!window.location.href.includes('127.0.0.1')) {
posthog.init("<ph_project_api_key>", {api_host: '<ph_instance_address>'})
}

Usage

Autocapture

When you call posthog.init the PostHog JS library begins automatically capturing user 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

PostHog puts a great amount of effort into making sure it doesn't capture any sensitive data from your website. If there are other elements you don't want to be captured, you can add the ph-no-capture class name.

<button class='ph-no-capture'>Sensitive information here</button>

Important notes on autocapture

While autocapture allows you to track the majority of general events on your website right out of the gate, it is important to note that, for security reasons, PostHog is very conservative regarding input tags. In order to prevent passwords or other sensitive data from being collected, very little data is collected from inputs with autocapture.

Specifically, PostHog autocapture will grab only the name, id, and class attributes from input tags.

As such, you should be aware of this when you start, in order to understand why you may be getting less data than expected.

If you need to collect more data from inputs, you should look into Custom events and Actions.

Another important thing to note is that disabling autocapture will not disable session recording. You can disable session recording too using disable_session_recording or by turning it off in your project's settings.

Website vs 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.

Capture

This allows you to send more context than the default event info that PostHog captures whenever a user does something. In that case, you can send an event with any metadata you may wish to add.

posthog.capture('[event-name]', {property1: 'value', property2: 'another value'});

Setting user properties via an event

To set properties on a user, you can use the posthog.people.set and posthog.people.set_once methods.

However, you can also leverage the event properties $set and $set_once to do this via an event.

$set

Example

posthog.capture('some event', { $set: { userProperty: 'value' } });

Usage

When capturing an event, you can pass a property called $set as an event property, and specify its value to be an object with properties to be set on the user that will be associated with the user who triggered the event.

This works the same way as posthog.people.set.

$set_once

Example

posthog.capture('some event', { $set_once: { userProperty: 'value' } });

Usage

$set_once works just like $set, except that it will only set the property if the user doesn't already have that property set.

This works the same way as posthog.people.set_once.

Identifying users

We highly recommend reading our section on Identifying users to better understand how to correctly use this method.

To make sure you understand which user is performing actions within your app, you can identify users at any point. From the moment you make this call, all events will be identified with that distinct id.

The ID can by anything, but is usually the unique ID that you identify users by in the database. Normally, you would put this below posthog.init if you have the information there.

If a user was previously anonymous (because they hadn't signed up or logged in yet), we'll automatically alias their anonymous ID with their new unique ID. That means all their events from before and after they signed up will be shown under the same user.

posthog.identify(
'[user unique id]', // distinct_id, required
{ userProperty: 'value1' }, // $set, optional
{ anotherUserProperty: 'value2' } // $set_once, optional
);

You can also pass two more arguments to posthog.identify. Both take objects consisting of as many properties as you want to be set on that user's profile. The difference is that the second argument will use the $set mechanism, whereas the third argument will use $set_once.

You can read more about the difference between this in the setting properties section.

Warning! You can't call identify straight after an `.init` (as `.init` sends a pageview event, probably with the user's anonymous ID).

To address the issue described above, you can call .init passing a loaded callback function, inside which you can then call identify, like so:

posthog.init('[your api key]', {
api_host: 'https://posthog.[your-domain].com',
loaded: function(posthog) { posthog.identify('[user unique id]'); }
});

Multiple IDs

If you use multiple distinct IDs for the same user (e.g. a logical ID and a UUID), you may want to set both for the same user. This way, if PostHog receives events for either ID, it will properly match them to the same person. This is also helpful if you want to associate anonymous IDs with an identified person (we actually do this automatically when you call .identify). To associate multiple IDs to the same person, you do an alias call, as shown below.

posthog.alias('[new ID]', '[original ID]')

The original ID will be kept as the default, but any events sent to either will be received under the same person.

Reset after logout

If a user is logged out, you most likely want to call reset to unset any distinct_ids.

This is especially important if your users are sharing a computer, as otherwise all of those users will be grouped together into a single user due to shared cookies between sessions.

We strongly recommend you do this on logout even if you don't expect users to share a computer. This can help make sure all your users are properly tracked in the odd case a user logs in with a different account.

You can do that like so:

posthog.reset();

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

posthog.reset(true);

Sending user information

An ID alone might not be enough to work out which user is who within PostHog. That's why it's useful to send over more metadata of the user. At minimum, we recommend sending the email property, which is also what we use to display in PostHog.

You can make this call on every page view to make sure this information is up-to-date. Alternatively, you can also do this whenever a user first appears (after signup) or when they change their information.

posthog.people.set({email: 'john@gmail.com'});

One-page apps and page views

This JS snippet automatically sends pageview events whenever it gets loaded. If you have a one-page app, that means it'll only send 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.

posthog.capture('$pageview');

This will automatically send the current URL.

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:

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.people.set. 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:

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:

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:

posthog.opt_out_capturing();

See if a user has opted out:

posthog.has_opted_out_capturing();

Opt a user back in:

posthog.opt_in_capturing();

Feature Flags

PostHog v1.10.0 introduced Feature Flags, which allow you to safely deploy and roll back new features.

Here's how you can use them:

  • Do something when the feature flags load:

    The argument callback(flags: string[]) will be called when the feature flags are loaded.

    In case the flags are already loaded, it'll be called immediately. Additionally, it will also be called when the flags are re-loaded e.g. after calling identify or reloadFeatureFlags.

posthog.onFeatureFlags(callback)
  • Check if a feature is enabled:
posthog.isFeatureEnabled('keyword')
  • Trigger a reload of the feature flags:
posthog.reloadFeatureFlags()
  • By default, this function will send a $feature_flag_called event to your instance every time it's called so you're able to do analytics. You can disable this by passing the send_event property:
posthog.isFeatureEnabled('keyword', { send_event: false })

Complete signup pseudocode

As an example, here is how to put some of the above concepts together:

function signup(email) {
// Your own internal logic for creating an account and getting a user_id
let userId = createAccount(email)
// Identify user with internal ID
posthog.identify(userId)
// Set email or any other data
posthog.people.set({ email: email })
}

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:

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:

AttributeDescription
api_host

Type: String
Default: https://app.posthog.com
URL of your PostHog instance.
autocapture

Type: Boolean
Default: true
Determines if PostHog should autocapture events.
capture_pageview

Type: Boolean
Default: true
Determines if PostHog should automatically capture pageview events.
disable_cookie

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.
loaded

Type: Function
Default: function () {}
A function to be called once the PostHog scripts have loaded successfully.
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.
property_blacklist

Type: Array
Default: []
A list of properties that should never be sent with capture calls.
xhr_headers

Type: Object
Default: {}
Any additional headers you wish to pass with the XHR requests to the PostHog API.
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.
session_recording

Type: Object
Default: See here.
Advanced feature - use with caution. Configuration options for rrweb.
advanced_decide_disabled

Type: Boolean
Default: false
Advanced! Will completely disable the /decide endpoint request (and features that rely on it). More details below.

Advanced configuration

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

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_decide_disabled = 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.