When a user logs out in one tab, the other tabs should follow. When they update their cart, every open window should reflect it. The common solution is a localStorage trick: write a sentinel value, listen for the storage event, read it, parse it, check if it's "for you," and clean it up. It works — but it's a side-channel communication pattern built on a persistence API that was never meant for messaging. The Broadcast Channel API is the direct path.
The API
// Sender (any tab, worker, or iframe on the same origin)
const channel = new BroadcastChannel('app-sync');
channel.postMessage({ type: 'LOGOUT' });
