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Inspecting listener counts ​

Problem ​

Implement inspecting listener counts in a production-friendly way with @vielzeug/eventit while keeping setup and cleanup explicit.

Runnable Example ​

The snippet below is copy-paste runnable in a TypeScript project with @vielzeug/eventit installed.

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

Expected Output ​

  • The example runs without type errors in a standard TypeScript setup.
  • The main flow produces the behavior described in the recipe title.

Common Pitfalls ​

  • Forgetting cleanup/dispose calls can leak listeners or stale state.
  • Skipping explicit typing can hide integration issues until runtime.
  • Not handling error branches makes examples harder to adapt safely.