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Node.js - Docs

If you're working with Node.js (versions 20+), the official posthog-node library is the simplest 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:

PostHog AI

npm

npm install posthog-node --save

Yarn

yarn add posthog-node

pnpm

pnpm add posthog-node

Bun

bun add posthog-node

In your app, set your project token before making any calls.

Node.js

PostHog AI

import { PostHog } from 'posthog-node'
const client = new PostHog(
    '<ph_project_token>',
    { host: 'https://us.i.posthog.com' }
)
await client.shutdown()

You can find your project token and instance address in the project settings page in PostHog.

Note: As a rule of thumb, we do not recommend hardcoding API keys or tokens. Setting it as an environment variable is preferred.

Identifying users

Identifying users is required. Backend events need a distinct_id that matches the ID your frontend uses when calling posthog.identify(). Without this, backend events are orphaned — they can't be linked to frontend event captures, session replays, LLM traces, or error tracking.

See our guide on identifying users for how to set this up.

Options

Note: When using PostHog in an AWS Lambda function or a similar serverless function environment, make sure you set flushAt to 1 and flushInterval to 0. Also, remember to always call await posthog.shutdown() at the end to flush and send all pending events.

Capturing events

You can send custom events using capture:

Node.js

PostHog AI

client.capture({
    distinctId: 'distinct_id_of_the_user',
    event: '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 include additional information with the event by including a properties object:

Node.js

PostHog AI

client.capture({
  distinctId: 'distinct_id_of_the_user',
  event: 'user signed up',
  properties: {
    login_type: 'email',
    is_free_trial: true,
  },
})

Capturing pageviews

If you're aiming for a backend-only implementation of PostHog and won't be capturing events from your frontend, you can send $pageview events from your backend like so:

Node.js

PostHog AI

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

Person profiles and properties

The Node SDK captures identified events by default. These create person profiles. To set person properties in these profiles, include them when capturing an event using $set and $set_once:

Node.js

PostHog AI

client.capture({
  distinctId: 'distinct_id_of_the_user',
  event: 'movie_played',
  properties: {
    $set: { name: 'Max Hedgehog'  },
    $set_once: { initial_url: '/blog' },
  },
})

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

To capture anonymous events without person profiles, set the event's $process_person_profile property to false:

Node.js

PostHog AI

client.capture({
  distinctId: 'distinct_id_of_the_user',
  event: 'movie_played',
  properties: {
    $process_person_profile: false,
  },
})

Alias

Sometimes, you want to assign multiple distinct IDs to a single user. This is helpful when your primary distinct ID is inaccessible. For example, if a distinct ID used on the frontend is not available in your backend.

In this case, you can use alias to assign another distinct ID to the same user.

Node.js

PostHog AI

client.alias({
  distinctId: 'distinct_id',
  alias: 'alias_id',
})

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

Super properties

Requires posthog-node version >= 5.25.0.

Super properties are properties that are automatically included with every event captured by the client. Use register to set them:

Node.js

PostHog AI

client.register({
  app_version: '1.2.0',
  environment: 'production',
})
// Both events include app_version and environment
client.capture({
  distinctId: 'distinct_id',
  event: 'page_viewed',
})
client.capture({
  distinctId: 'distinct_id',
  event: 'button_clicked',
})

If an event sets a property with the same key as a super property, the event's property takes precedence:

Node.js

PostHog AI

client.register({ environment: 'production' })
// This event is captured with environment='staging'
client.capture({
  distinctId: 'distinct_id',
  event: 'page_viewed',
  properties: { environment: 'staging' },
})

To remove a super property, use unregister:

Node.js

PostHog AI

client.unregister('environment')

Super properties are global — they apply to every event for the lifetime of the client instance. For properties that should only apply to a specific scope (e.g. a single request or transaction), use contexts instead.

Contexts

Requires posthog-node version >= 5.17.0.

The Node SDK uses nested contexts for managing state that's shared across events. Contexts are useful for adding properties to multiple events (including exceptions) during a single user's interaction with your product.

You can enter a context using withContext:

Node.js

PostHog AI

posthog.withContext(
  {
    distinctId: 'user-123',
    properties: { transactionId: 'abc123' }
  },
  () => {
    // This event is captured with the distinct ID and properties set above
    posthog.capture({ event: 'order_processed' })
  }
)

Contexts are persisted across function calls. If you enter one and then call a function and capture an event in the called function, it uses the context properties set in the parent context:

Node.js

PostHog AI

function someFunction() {
  // When called from `outerFunction`, this event is captured
  // with transactionId='abc123'
  posthog.capture({ event: 'order_processed' })
}
function outerFunction() {
  posthog.withContext(
    { properties: { transactionId: 'abc123' } },
    () => {
      someFunction()
    }
  )
}

By default, each context inherits from parent contexts. To disable nesting (where child contexts is fresh and has no properties), pass { fresh: true }:

Node.js

PostHog AI

posthog.withContext(
  {
    properties: {
      someKey: 'value-1',
      someOtherKey: 'another-value'
    }
  },
  () => {
    posthog.withContext(
      { properties: { someKey: 'value-2' } },
      () => {
        // Captured with someKey='value-2', someOtherKey='another-value'
        posthog.capture({ event: 'order_processed' })
      },
    )
    // Captured with someKey='value-1', someOtherKey='another-value'
    posthog.capture({ event: 'order_completed' })
  }
)

Note: Properties passed directly to capture calls override context state in the final event.

Identification context

Contexts can be associated with a distinct ID:

Node.js

PostHog AI

posthog.withContext(
  { distinctId: 'user-123' },
  () => {
    // Associated with "user-123"
    posthog.capture({ event: 'order_processed' })
    // Overrides to "another-user"
    posthog.capture({
      distinctId: 'another-user',
      event: 'order_processed'
    })
  }
)

Session context

Node.js

PostHog AI

posthog.withContext(
  { sessionId: 'some-session' },
  () => {
    // Associated with session "some-session"
    posthog.capture({ event: 'image_uploaded' })
    // Overrides to "next-session"
    posthog.capture({
      event: 'image_uploaded',
      properties: { $sessionId: 'next-session' }
    })
  }
)

Custom context parameters

Node.js

PostHog AI

posthog.withContext(
  { flightNumber: 'TAC313' },
  () => {
    // Associated with flightNumber TAC313
    posthog.capture({ event: 'flight_cancelled' })
    // Overrides to PL7714
    posthog.capture({
      event: 'flight_cancelled',
      properties: { flightNumber: 'PL7714' }
    })
  }
)

Feature flags

PostHog's feature flags enable you to safely deploy and roll back new features as well as target specific users and groups with them.

There are 2 steps to implement feature flags in Node:

Step 1: Evaluate the feature flag value

Boolean feature flags

Node.js

PostHog AI

const isFeatureFlagEnabled = await client.isFeatureEnabled('flag-key', 'distinct_id_of_your_user')
if (isFeatureFlagEnabled) {
    // Your code if the flag is enabled
    // Optional: fetch the payload
    const matchedFlagPayload = await client.getFeatureFlagPayload('flag-key', 'distinct_id_of_your_user', isFeatureFlagEnabled)
}

Multivariate feature flags

Node.js

PostHog AI

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

Step 2: Include feature flag information when capturing events

If you want use your feature flag to breakdown or filter events in your insights, you'll need to include feature flag information in those events. This ensures that the feature flag value is attributed correctly to the event.

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Include the $feature/feature_flag_name property

In the event properties, include $feature/feature_flag_name: variant_key:

Node.js

PostHog AI

client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    properties: {
        '$feature/feature-flag-key': 'variant-key' // replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
    },
})

Method 2: Set sendFeatureFlags to true

The capture() method has an optional argument sendFeatureFlags, which is set to false by default. This parameter controls whether feature flag information is sent with the event.

Basic usage

Setting sendFeatureFlags to true will include feature flag information with the event:

Node.js

PostHog AI

client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    sendFeatureFlags: true,
})

Advanced usage (v5.5.0+)

As of version 5.5.0, sendFeatureFlags can also accept an options object for more granular control:

Node.js

PostHog AI

client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    sendFeatureFlags: {
        onlyEvaluateLocally: true,
        personProperties: { plan: 'premium' },
        groupProperties: { org: { tier: 'enterprise' } }
    }
})

Performance considerations

• With local evaluation: When local evaluation is configured, setting sendFeatureFlags: true will not make additional server requests. Instead, it uses the locally cached feature flags, and it provides an interface for including person and/or group properties needed to evaluate the flags in the context of the event, if required.

• Without local evaluation: PostHog will make an additional request to fetch feature flag information before capturing the event, which adds delay.

Breaking change in v5.5.0

Prior to version 5.5.0, feature flags were automatically sent with events when using local evaluation, even when sendFeatureFlags was not explicitly set. This behavior has been removed in v5.5.0 to be more predictable and explicit.

If you were relying on this automatic behavior, you must now explicitly set sendFeatureFlags: true to continue sending feature flags with your events.

Fetching all flags for a user

You can fetch all flag values for a single user by calling getAllFlags() or getAllFlagsAndPayloads().

This is useful when you need to fetch multiple flag values and don't want to make multiple requests.

Node.js

PostHog AI

await client.getAllFlags('distinct_id_of_your_user')
await client.getAllFlagsAndPayloads('distinct_id_of_your_user')

Sending $feature_flag_called events

Capturing $feature_flag_called events enable PostHog to know when a flag was accessed by a user and thus provide analytics and insights on the flag. By default, we send a these event when:

1. You call posthog.getFeatureFlag() or posthog.isFeatureEnabled(), AND

2. It's a new user, or the value of the flag has changed.

Note: Tracking whether it's a new user or if a flag value has changed happens in a local cache. This means that if you reinitialize the PostHog client, the cache resets as well – causing $feature_flag_called events to be sent again when calling getFeatureFlag or isFeatureEnabled. PostHog is built to handle this, and so duplicate $feature_flag_called events won't affect your analytics.

You can disable automatically capturing $feature_flag_called events. For example, when you don't need the analytics, or it's being called at such a high volume that sending events slows things down.

To disable it, set the sendFeatureFlagEvents argument in your function call, like so:

Node.js

PostHog AI

const isFeatureFlagEnabled = await client.isFeatureEnabled(
    'flag-key',
    'distinct_id_of_your_user',
    {
        'sendFeatureFlagEvents': false
    })

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

You can provide properties to evaluate the flag with by using the person properties, groups, and group properties arguments. PostHog will then use these values to evaluate the flag, instead of any properties currently stored on your PostHog server.

For example:

Node.js

PostHog AI

await client.getFeatureFlag(
    'flag-key',
    'distinct_id_of_the_user',
    {
        personProperties: {
            'property_name': 'value'
        },
        groups: {
            "your_group_type": "your_group_id",
            "another_group_type": "your_group_id",
        },
        groupProperties: {
            'your_group_type': {
                'group_property_name': 'value'
            },
            'another_group_type': {
                'group_property_name': 'value'
            }
        },
    }
)

Overriding GeoIP properties

By default, a user's GeoIP properties are set using the IP address they use to capture events on the frontend. You may want to override the these properties when evaluating feature flags. A common reason to do this is when you're not using PostHog on your frontend, so the user has no GeoIP properties.

You can override GeoIP properties by including them in the person_properties parameter when evaluating feature flags. This is useful when you're evaluating flags on your backend and want to use the client's location instead of your server's location.

The following GeoIP properties can be overridden:

• $geoip_country_code

• $geoip_country_name

• $geoip_city_name

• $geoip_city_confidence

• $geoip_continent_code

• $geoip_continent_name

• $geoip_latitude

• $geoip_longitude

• $geoip_postal_code

• $geoip_subdivision_1_code

• $geoip_subdivision_1_name

• $geoip_subdivision_2_code

• $geoip_subdivision_2_name

• $geoip_subdivision_3_code

• $geoip_subdivision_3_name

• $geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

Request timeout

You can configure the feature_flag_request_timeout_ms parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked in the case when PostHog's servers are too slow to respond. By default, this is set at 3 seconds.

JavaScript

PostHog AI

const client = new PostHog('<ph_project_token>', {
        api_host: 'https://us.i.posthog.com',
        feature_flag_request_timeout_ms: 3000 // Time in milliseconds. Default is 3000 (3 seconds).
    }
)

Error handling

When using the PostHog SDK, it's important to handle potential errors that may occur during feature flag operations. Here's an example of how to wrap PostHog SDK methods in an error handler:

JavaScript

PostHog AI

async function handleFeatureFlag(client, flagKey, distinctId) {
    try {
        const isEnabled = await client.isFeatureEnabled(flagKey, distinctId);
        console.log(`Feature flag '${flagKey}' for user '${distinctId}' is ${isEnabled ? 'enabled' : 'disabled'}`);
        return isEnabled;
    } catch (error) {
        console.error(`Error fetching feature flag '${flagKey}': ${error.message}`);
        // Optionally, you can return a default value or throw the error
        // return false; // Default to disabled
        throw error;
    }
}
// Usage example
try {
    const flagEnabled = await handleFeatureFlag(client, 'new-feature', 'user-123');
    if (flagEnabled) {
        // Implement new feature logic
    } else {
        // Implement old feature logic
    }
} catch (error) {
    // Handle the error at a higher level
    console.error('Feature flag check failed, using default behavior');
    // Implement fallback logic
}

Note: For remote config flags, see the remote config documentation. Remote config requires the Feature Flags secure API key passed as the personalApiKey option.

Local evaluation

Evaluating feature flags requires making a request to PostHog for each flag. However, you can improve performance by evaluating flags locally. Instead of making a request for each flag, PostHog will periodically request and store feature flag definitions locally, enabling you to evaluate flags without making additional requests.

It is best practice to use local evaluation flags when possible, since this enables you to resolve flags faster and with fewer API calls.

For details on how to implement local evaluation, see our local evaluation guide.

Node.js

PostHog AI

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

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:

Node.js

PostHog AI

await client.reloadFeatureFlags()
// Do something with feature flags here

Distributed environments

In multi-worker or edge environments, you can implement custom caching for flag definitions using Redis, Cloudflare KV, or other storage backends. This enables sharing definitions across workers and coordinating fetches. See our guide for local evaluation in distributed environments for details.

Experiments (A/B tests)

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

Node.js

PostHog AI

const variant = await client.getFeatureFlag('experiment-feature-flag-key', 'user_distinct_id')
if (variant === 'variant-name') {
  // do something
}

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

Group analytics

Group analytics enable you to associate an event with a group (e.g. teams, organizations, etc.). Read the group analytics guide for more information.

To create a group or update its properties, use groupIdentify:

Node.js

PostHog AI

client.groupIdentify({
  groupType: 'company',
  groupKey: 'company_id_in_your_db',
  properties: {
    name: 'Awesome Inc',
    employees: 11,
  },
  // optional distinct ID to associate event with an existing person
  distinctId: 'xyz'
})

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 is used instead.

If the optional distinctId parameter is not provided in the group identify call, it defaults to ${groupType}_${groupKey} (e.g., $company_company_id_in_your_db in the example above). This default behavior results in each group appearing as a separate person in PostHog. To avoid this, it's often more practical to use a consistent distinctId, such as group_identifier.

Once a group is created, you can use the capture method and pass in the groups parameter to capture an event with group analytics.

Node.js

PostHog AI

client.capture({
  event: 'some_event',
  distinctId: 'user_distinct_id',
  groups: { company: 'company_id_in_your_db' },
})

GeoIP properties

Before posthog-node v3.0, we added GeoIP properties to all incoming events by default. We also used these properties for feature flag evaluation, based on the IP address of the request. This isn't ideal since they are created based on your server IP address, rather than the user's, leading to incorrect location resolution.

As of posthog-node v3.0, the default now is to disregard the server IP, not add the GeoIP properties, and not use the values for feature flag evaluations.

You can go back to previous behavior by setting disableGeoip to false in your initialization:

Node.js

PostHog AI

const posthog = new PostHog('<ph_project_token>', {
  host: 'https://us.i.posthog.com',
  disableGeoip: false
})

The list of properties that this overrides:

1. $geoip_city_name

2. $geoip_country_name

3. $geoip_country_code

4. $geoip_continent_name

5. $geoip_continent_code

6. $geoip_postal_code

7. $geoip_time_zone

You can also explicitly chose to enable or disable GeoIP for a single capture request like so:

Node.js

PostHog AI

client.capture({
  distinctId: distinctId,
  event: 'your_event',
  disableGeoip: `true`,
})

Shutdown

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

Node.js

PostHog AI

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

Debug mode

If you're not seeing the expected events being captured, the feature flags being evaluated, or the surveys being shown, you can enable debug mode to see what's happening.

You can enable debug mode by calling the debug() method in your code. This will enable verbose logs about the inner workings of the SDK.

Node.js

PostHog AI

client.debug()

Handling errors thrown by the SDK

If you are experiencing issues with the SDK it could be a number of things from an incorrectly configured API key, to some other network related issues.

The SDK does not throw errors for things happening in the background to ensure it doesn't affect your process. You can however hook into the errors to get more information:

Node.js

PostHog AI

client.on("error", (err) => {
  // Whatever handling you want
  console.error("PostHog had an error!", err)
})

Short-lived processes like serverless environments

The Node SDK is designed to queue and batch requests in the background to optimize API calls and network time. As serverless environments like AWS Lambda or Vercel Functions are short-lived, we provide a few options to ensure all events are captured.

First, we recommend using the captureImmediate method instead of capture to ensure the event is captured before the function shuts down. It guarantees the HTTP request finishes before your function continues (or shuts down).

Second, we recommend setting flushAt to 1 and flushInterval to 0 to ensure the events are sent immediately. These set the queue to flush immediately, both in terms of events and time.

Third, we provide a method shutdown() which can be awaited to ensure all queued events are sent to the API. For example:

Node.js

PostHog AI

export const handler() {
  client.capture({
    distinctId: 'distinct_id_of_the_user',
    event: 'thing_happened'
  })
  client.capture({
    distinctId: 'distinct_id_of_the_user',
    event: 'other_thing_happened'
  })
  // So far 2 events are queued but not sent
  // Calling shutdown, flushed the queue but batched into 1 API call for maximum efficiency
  await client.shutdown()
}

This is also useful for shutting down a standard Node.js app.

LLM analytics

You can capture LLM usage and performance data by combining the posthog-node and @posthog/ai libraries. These work with LLM providers like OpenAI and Vercel's AI SDKs. Learn more in our LLM analytics docs.

Error tracking

You can capture errors using the posthog-node library. This enables you to see stack traces, source code, and watch associated session recordings to improve your application stability. Learn more in our error tracking docs.

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

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