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 a cookie.

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 changed with your user and you want to refetch their flag values, call 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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