Web feature flags installation

Install the PostHog JavaScript library using your package manager:

Import and initialize the PostHog library with your project API key and host:

Once installed, PostHog will automatically start capturing events. You can also manually send events to test your integration:

Click around and view a couple pages to generate some events. PostHog automatically captures pageviews, clicks, and other interactions for you.

If you'd like, you can also manually capture custom events:

Check if a feature flag is enabled:

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

Feature flags can include payloads with additional data. Fetch the payload like this:

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:

Feature flag values are cached. If something has changed with your user and you'd like to refetch their flag values:

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.