Feature Flags

Feature flags control access to specific features at runtime. @repo/feature-flags exists to:

• Roll out features gradually to users
• A/B test new functionality
• Enable/disable features based on user segments
• Control access to beta or experimental features

How it works

The package is built on the Flags SDK, which is largely an architectural pattern rather than a service.

createFlag is the entry point. A flag created through it carries authentication and PostHog resolution.

Adding a new flag

To add a new flag, simply create a feature flag in PostHog and then modify the feature flags index file. Let's add a new flag called showAnalyticsFeature that will be enabled for all users:

import { createFlag } from './lib/create-flag';

export const showBetaFeature = createFlag('showBetaFeature');
export const showAnalyticsFeature = createFlag('showAnalyticsFeature');

Make sure the key you're using matches the key in PostHog exactly.

Usage in your application

To use the flag in your application, simply import it from the @repo/feature-flags package and use it like a normal boolean value.

import { showAnalyticsFeature } from '@repo/feature-flags';
import { notFound } from 'next/navigation';

export default function Page() {
  const isEnabled = showAnalyticsFeature();

  if (!isEnabled) {
    notFound();
  }

  return <div>Analytics feature is enabled</div>;
}

Because feature flags are tied to the user, they only work if the user is signed in. Otherwise, the flag will always return false.

Overriding flags in development

Override a flag in development from the PostHog project that resolves it. The in-browser toolbar that used to provide overrides was removed with the move to Cloudflare; it was a hosting-platform feature, not part of the Flags SDK.