BroadcastChannel✓Baseline · Widely available

// Every tab on the same origin that opens this channel gets the message
const channel = new BroadcastChannel("auth");

channel.postMessage({ type: "logout" });
channel.onmessage = (e) => {
  if (e.data.type === "logout") location.assign("/login");
};

if ("BroadcastChannel" in globalThis) {
  // BroadcastChannel is available
} else {
  // load a polyfill or use a fallback
}