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Lifecycle events

Besides the events you define, every Tabus instance automatically emits two internal events to notify its peers about its lifecycle.

Event Payload When it’s emitted
tab:join { tabId: string } Shortly after the instance is created, notifying peers that a new tab joined the channel.
tab:leave { tabId: string } When destroy() is called, as long as tab:join had already been emitted previously.

They’re listened to just like any other event:

bus.on("tab:join", ({ tabId }) => console.log("new tab connected:", tabId));
bus.on("tab:leave", ({ tabId }) => console.log("tab closed:", tabId));

Why tab:join is deferred

Section titled “Why tab:join is deferred”

tab:join isn’t emitted synchronously in the constructor: it’s deferred with setTimeout(0). This gives whoever creates the instance a chance to register their handlers before the notice reaches peers:

const bus = new Tabus("my-app");


// This handler is already registered by the time tab:join is broadcast,
// because the actual emission happens on the next tick.
bus.on("tab:join", ({ tabId }) => console.log("new tab:", tabId));

If tab:join were emitted immediately (synchronously), there would be a race window: another tab could react and respond before this instance got around to subscribing to anything.

The destroy() guard

Section titled “The destroy() guard”

Because tab:join is asynchronous, it’s possible for someone to call destroy() in the same tick the instance was created, before the timer fires. Tabus accounts for this with an internal flag:

  • If destroy() is called before the tab:join timer fires, neither tab:join nor tab:leave are sent. Peers never got to know this tab existed, so there’s no point telling them it’s gone either.
  • If destroy() is called afterward, tab:leave is emitted normally.

Next step

Section titled “Next step”