Node.js library

If you're working with Node.js, the official posthog-node library is the simple way to integrate your software with PostHog. This library uses an internal queue to make calls fast and non-blocking. It also batches requests and flushes asynchronously, making it perfect to use in any part of your web app or other server-side application that needs performance. And in addition to event capture, feature flags are supported as well.

Installation

Run either npm or yarn in terminal to add it to your project:

npm install posthog-node --save
# or
yarn add posthog-node

In your app, set your API key before making any calls.

import { PostHog } from 'posthog-node'

const client = new PostHog(
    '<ph_project_api_key>',
    { host: '<ph_instance_address>' } // You can omit this line if using PostHog Cloud
)

// On program exit, call shutdown to stop pending pollers and flush any remaining events
client.shutdown()

You can find your key in the 'Project Settings' page in PostHog.

Note: As a rule of thumb, we do not recommend hardcoding API keys. Setting it as an environment variable would be best.

Options

Note: When using PostHog in an AWS Lambda function or a similar serverless function environment, make sure you set flushAt and flushInterval to 0.

Making calls

Capture

Capture allows you to capture anything a user does within your system, which you can later use in PostHog to find patterns in usage, work out which features to improve or where people are giving up.

A capture call requires:

• distinct id which uniquely identifies your user
• event name to identify the event

• We recommend naming events with "[noun][verb]", such as movie played or movie updated, in order to easily identify what your events mean later on (we know this from experience).

Optionally you can submit:

• properties, which is an object with any information you'd like to add

For example:

client.capture({
    distinctId: 'distinct id',
    event: 'movie played',
    properties: {
        movieId: '123',
        category: 'romcom',
    },
})

Setting user properties via an event

To set properties on your users via an event, you can leverage the event properties $set and $set_once.

$set

Example

client.capture({
    distinctId: 'distinct id',
    event: 'movie played',
    properties: {
        $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.

$set_once

Example

client.capture({
    distinctId: 'distinct id',
    event: 'movie played',
    properties: {
        $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.

Identify

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

Identify lets you add metadata to your users so you can easily identify who they are in PostHog, as well as do things like segment users by these properties.

An identify call requires:

• distinctId – a distinct ID belonging to the user
• properties – a user properties object

For example:

client.identify({
    distinctId: 'user:123',
    properties: {
        email: 'john@doe.com',
        proUser: false,
    },
})

The most obvious place to make this call is whenever a user signs up, or when they update their information.

Alias

To connect whatever a user does before they sign up or log in with what they do after you need to make an alias call. This will allow you to answer questions like "Which marketing channels leads to users churning after a month?" or "What do users do on our website before signing up?"

In a purely back-end implementation, this means whenever an anonymous user does something, you'll want to send a session ID with the capture call. Then, when that users signs up, you want to do an alias call with the session ID and the newly created user ID.

The same concept applies for when a user logs in.

If you're using PostHog in the front-end and back-end, doing the identify call in the frontend will be enough.

An alias call requires:

• distinctId – the current distinct ID of the user
• alias – the other distinct ID of the user, which will be aliased to the current one

For example:

client.alias({
    distinctId: 'user:123',
    alias: 'user:12345',
})

Sending page views

If you're aiming for a full back-end implementation of PostHog, you can send pageviews from your backend, like so:

client.capture({
    distinctId: 'distinct id',
    event: '$pageview',
    properties: {
        $current_url: 'https://example.com',
    },
})

Feature flags

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

When using them with one of libraries, you should check if a feature flag is enabled and use the result to toggle functionality on and off in you application.

How to check if a flag is enabled

Note: Whenever we face an error computing the flag, the library returns undefined, instead of true, false, or a string variant value.

// isFeatureEnabled(key: string, distinctId: string, options: {}): Promise<boolean | undefined>
const isMyFlagEnabledForUser = await client.isFeatureEnabled('flag-key', 'user distinct id')

if (isMyFlagEnabledForUser) {
    // Do something differently for this user
}

Get a flag value

If you're using multivariate feature flags, you can also get the value of the flag, as well as whether or not it is enabled.

Note: Whenever we face an error computing the flag, the library returns None, instead of true or false or a string variant value.

// getFeatureFlag(key: string, distinctId: string, options: {}): Promise<string | boolean | undefined>
const flagValue = await client.getFeatureFlag('flag-key', 'user distinct id')

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.

For example, if the beta-feature depends on the is_authorized property, and you know the value of the property, you can tell PostHog to use this property, like so:

// getFeatureFlag(
//    key: string,
//    distinctId: string,
//    options?: {
//      groups?: Record<string, string>
//      personProperties?: Record<string, string>
//      groupProperties?: Record<string, Record<string, string>>
//      onlyEvaluateLocally?: boolean
//      sendFeatureFlagEvents?: boolean
//    }
//  ): Promise<string | boolean | undefined>
const flagValue = await client.getFeatureFlag('flag-key', 'user distinct id', {personProperties: {is_authorized: true}})

The same holds for groups. If you have a group named organisation, you can add properties like so:

const flagValue = await client.getFeatureFlag('flag-key', 'user distinct id', {groups:{'organisation': 'google'}, groupProperties:{'organisation': {'is_authorized': True}})

Getting all flag values

You can also get all known flag values as well. This is useful when you want to seed a frontend client with initial known flags. Like all methods above, this also takes optional person and group properties, if known.

await client.getAllFlags('distinct id', {groups: {}, personProperties: {'is_authorized': True}, groupProperties: {}})
# returns dict of flag key and value pairs.

Local Evaluation

Note: To enable local evaluation of feature flags you must also set a personal_api_key when configuring the integration, as described in the Installation section.

Note: This feature requires version 2.0 of the library, which in turn requires a minimum PostHog version of 1.38

All feature flag evaluation requires an API request to your PostHog servers to get a response. However, where latency matters, you can evaluate flags locally. This is much faster, and requires two things to work:

1. The library must be initialised with a personal API key
2. You must know all person or group properties the flag depends on.

Then, the flag can be evaluated locally. The method signature looks exactly like above.

await client.getFeatureFlag('beta-feature', 'distinct id', {personProperties: {'is_authorized': True}})
# returns string or None

Note: New feature flag definitions are polled every 30 seconds by default, which means there will be up to a 30 second delay between you changing the flag definition, and it reflecting on your servers. You can change this default on the client by setting featureFlagsPollingInterval during client initialisation.

Restricting evaluation to local only

Sometimes, performance might matter to you so much that you never want an HTTP request roundtrip delay when computing flags. In this case, you can set the only_evaluate_locally parameter to true, which tries to compute flags only with the properties it has. If it fails to compute a flag, it returns None, instead of going to PostHog's servers to get the value.

Reloading feature flags

When initializing PostHog, you can configure the interval at which feature flags are polled (fetched from the server). However, if you need to force a reload, you can use reloadFeatureFlags:

await client.reloadFeatureFlags()

// Do something with feature flags here

Group analytics

PostHog 1.31.0 introduced support for group analytics, which allows you to associate users and events with larger groups (teams, organizations, etc.). This feature requires a posthog-node version of 1.2.0 or above.

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

• Capture an event and associate it with a group

client.capture({
    event: 'some event',
    distinctId: '[distinct id]',
    groups: { company: 'id:5' },
})

• Update properties on a group

client.groupIdentify({
    groupType: 'company',
    groupKey: 'id:5',
    properties: {
        company_name: 'Awesome Inc',
        employees: 11,
    },
})

Shutdown

You should call shutdown on your program's exit to exit cleanly:

// Stop pending pollers and flush any remaining events
client.shutdown()

Upgrading from V1 to V2

V2.x.x of the Node.js library is completely rewritten in Typescript and is based on a new JS core shared with other JavaScript based libraries with the goal of ensuring new features and fixes reach the different libraries at the same pace.

With the release of V2, the API was kept mostly the same but with some small changes and deprecations:

1. The minimum PostHog version requirement is 1.38
2. The callback parameter passed as an optional last argument to most of the methods is no longer supported
3. The method signature for isFeatureEnabled and getFeatureFlag is slightly modified. See the above documentation for each method for more details.
4. For specific changes, see the CHANGELOG