Adding feature flag code

Boolean feature flags

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

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

Multivariate feature flags

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 your chosen persistence option (local storage by default).

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:

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 has changed with your user and you'd like to refetch their flag values, call:

posthog.reloadFeatureFlags()

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.

posthog.init('<ph_project_api_key>', { 
        api_host: '<ph_instance_address>',
        feature_flag_request_timeout_ms: 3000 // Time in milliseconds. Default is 3000 (3 seconds).
    }
)