A pub/sub bus in 50 lines

LINK subscribes a cell to a topic. BIND publishes a message. Listeners fire on every subscriber. Open this page in two tabs — they share the bus (in the cloud) or run separately (in localStorage).

01Subscribe

Pick a topic and a callback cell. New messages on the topic will land in the cell.

02Publish

03Live feed

No messages yet. Subscribe and publish to see them flow.

04Subscriptions

Topic	Subscriber cell	Messages received
No subscriptions yet.

05The source

const sub = new Substrate();

function subscribe(topic, cell) {
  if (!sub.has(cell)) sub.bind(cell, []);
  sub.link(cell, topic, 'subscribes');
}

function publish(topic, message) {
  for (const [cellName, cell] of sub.cells) {
    for (const linkKey of sub.links.keys()) {
      if (linkKey.startsWith(`${cellName}|${topic}|`)) {
        const next = [...(cell.value || []), { topic, message, t: sub.t }];
        sub.bind(cellName, next);
      }
    }
  }
  sub.tick();
}

06Take it with you

Save the bus state. Open this page in two tabs. Subscribe in tab A. Publish in tab B. If both tabs are loaded from the same cloud save, the bus becomes a real shared substrate.