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Inspecting listener counts ​

Problem ​

During debugging or in a test, you want to assert that exactly the right number of listeners are registered on the bus — detecting leaks from missing cleanup or verifying that setup ran correctly.

Solution ​

Useful for debugging, conditional logic, or test assertions:

ts
const bus = createBus<AppEvents>();

const unsub1 = bus.on('user:login', handler1);
const unsub2 = bus.on('user:login', handler2);
bus.on('user:logout', handler3);

bus.listenerCount('user:login'); // 2
bus.listenerCount('user:logout'); // 1
bus.listenerCount(); // 3 — total across all events

unsub1();
bus.listenerCount('user:login'); // 1

bus.dispose();
bus.listenerCount(); // 0

Pitfalls ​

  • listenerCount(event) only counts listeners for that exact event name. Listeners registered under a different casing or alias are not included.
  • The count is live. If you cache it, it becomes stale the moment a listener is added or removed.
  • eventNames() only returns names with at least one active listener. A name with no listeners does not appear, even if it was previously used.