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

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


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

Multivariate feature flags

Web
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:

Web
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:

Web
posthog.reloadFeatureFlags()

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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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