Integrate flags with Vercel Web Analytics

Vercel Web Analytics can look up the values of evaluated feature flags in the DOM. It can then enrich page views and client-side events with these feature flags.

1. Emit feature flags and connect them to Vercel Web Analytics

   To share your feature flags with Web Analytics you have to emit your feature flag values to the DOM as described in Supporting Feature Flags.

   This will automatically annotate all page views and client-side events with your feature flags.

2. Tracking feature flags in client-side events

   Client-side events in Web Analytics will now automatically respect your flags and attach those to custom events.

   To manually overwrite the tracked flags for a specific track event, call:

   component.ts

   import { track } from '@vercel/analytics';
    
   track('My Event', {}, { flags: ['summer-sale'] });

   If the flag values on the client are encrypted, the entire encrypted string becomes part of the event payload. This can lead to the event getting reported without any flags when the encrypted string exceeds size limits.

To track feature flags in server-side events:

1. First, report the feature flag value using reportValue to make the flag show up in Runtime Logs:

   app/api/test/route.ts

   import { reportValue } from '@vercel/flags';
    
   // force dynamic mode so the flag actually gets reported,
   // otherwise the route would be static
   export const dynamic = 'force-dynamic';
    
   export async function GET() {
     reportValue('summer-sale', false);
     return Response.json({ ok: true });
   }

2. Once reported, any calls to track can look up the feature flag while handling a specific request:

   app/api/test/route.ts

   import { track } from '@vercel/analytics/server';
   import { reportValue } from '@vercel/flags';
    
   // force dynamic mode so the flag actually gets reported,
   // otherwise the route would be static
   export const dynamic = 'force-dynamic';
    
   export async function GET() {
     reportValue('summer-sale', false);
     track('My Event', {}, { flags: ['summer-sale'] });
    
     return Response.json({ ok: true });
   }