Adding feature flag code

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Once you've created your feature flag in PostHog, the next step is to add your 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:


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:

posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'})

Note: These are set for the entire session. Successive calls are additive: all properties you set are combined together and sent for flag evaluation.

Whenever you set these properties, we also trigger a reload of feature flags to ensure we have the latest values. You can disable this by passing in the optional parameter for reloading:

posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'}, false)

At any point, you can reset these properties by calling resetPersonPropertiesForFlags:


The same holds for group properties:

// set properties for a group
posthog.setGroupPropertiesForFlags({'company': {'property1': 'value', property2: 'value2'}})
// reset properties for a given group:
// reset properties for all groups:

Note: You don't need to add the group names here, since these properties are automatically attached to the current group (set via When you change the group, these properties are reset.

Automatic overrides

Whenever you call posthog.identify with person properties, we automatically add these properties to flag evaluation calls to help determine the correct flag values. The same is true for when you call

Default overridden properties

By default, we always override some properties based on the user IP address.

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

This enables any geolocation-based flags to work without manually setting these properties.

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: '',
feature_flag_request_timeout_ms: 3000 // Time in milliseconds. Default is 3000 (3 seconds).


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Testing your feature flag

Once you've written your code, it's a good idea to test that each variant behaves as you'd expect. There are 3 ways you can do this: Method 1: Assign a user a specific flag value For boolean flags, you can roll out the flag to a specific user. For multivariate flags, you can assign a user to a specific variant by adding an optional override to your release conditions . To do this: Go to your feature flag. Ensure the feature flag is enabled by checking the "Enable feature flag" box. Add a new…

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