Node.js feature flags installation

  1. Install

    Required

    Install the PostHog Node.js library using your package manager:

    npm install posthog-node

  2. Configure

    Required

    Initialize the PostHog client with your project API key:

    Node.js
    import { PostHog } from 'posthog-node'
    

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

  3. Send an event

    Recommended

    Once installed, you can manually send events to test your integration:

    Node.js
    client.capture({
        distinctId: 'distinct_id_of_the_user',
        event: 'event_name',
        properties: {
            property1: 'value',
            property2: 'value',
        },
    })

    By default, for backwards compatibility reasons, events are sent with person profile processing enabled. This means a person profile will be created for each user who triggers an event.

    If you want to disable person profile processing for certain events, send the event with the following property:

    Node.js
    "$process_person_profile": false

  4. Evaluate boolean feature flags

    Required

    Check if a feature flag is enabled:

    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)
    }

  5. Evaluate multivariate feature flags

    Optional

    For multivariate flags, check which variant the user has been assigned:

    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)
    }

  6. Include feature flag information in events

    Required

    If you want to 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.

    Set sendFeatureFlags to true in your capture call:

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

  7. Override server properties

    Optional

    Sometimes, you may want to evaluate feature flags using properties that haven't been ingested yet, or were set incorrectly earlier. You can provide properties to evaluate the flag with:

    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'
                },
            },
        }
    )

  8. Running experiments

    Optional

    Experiments run on top of our feature flags. Once you've implemented the flag in your code, you run an experiment by creating a new experiment in the PostHog dashboard.

  9. Next steps

    Recommended
    ResourceDescription
    Creating a feature flagHow to create a feature flag in PostHog
    Adding feature flag codeHow to check flags in your code for all platforms
    Framework-specific guidesSetup guides for React Native, Next.js, Flutter, and other frameworks
    How to do a phased rolloutGradually roll out features to minimize risk
    More tutorialsOther real-world examples and use cases

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