Working with Events
1. Creating Event Emitters
Example
import { EventEmitter } from "node:events";
class Bus extends EventEmitter {}
const bus = new Bus();
2. Registering Event Listeners
| Method | Description |
|---|
on(event, fn) | Add listener |
addListener | Alias of on |
off(event, fn) | Remove listener |
removeAllListeners(event?) | Clear |
3. Using Once Listeners
Example
bus.once("ready", () => console.log("first time only"));
import { once } from "node:events";
const [data] = await once(bus, "data"); // promise-based
4. Emitting Events
| API | Returns |
|---|
emit(event, ...args) | true if any listeners |
5. Removing Listeners
| Pattern | Use |
|---|
bus.off("x", fn) | Remove a specific fn |
| Save reference to anon fn | So you can remove later |
6. Prepending Listeners (prependListener)
| Method | Effect |
|---|
prependListener | Insert at front of list |
prependOnceListener | Front + once |
7. Getting Listener Count (listenerCount)
| API | Returns |
|---|
bus.listenerCount("x") | Number |
bus.listeners("x") | Array of fns |
bus.eventNames() | All registered names |
8. Setting Max Listeners (setMaxListeners)
| API | Default |
|---|
setMaxListeners(n) | 10 |
EventEmitter.defaultMaxListeners | Global default |
Note: Exceeding triggers a "MaxListenersExceededWarning" (possible memory leak).
9. Handling Error Events
Warning: Unhandled "error" events crash the process.
Example
bus.on("error", (err) => logger.error(err));
bus.emit("error", new Error("boom"));
10. Using Async Iterators on Events
Example
import { on } from "node:events";
for await (const [msg] of on(bus, "message")) {
if (msg === "stop") break;
}
11. Creating Custom Event Emitters
Example: Typed emitter (TS)
type Events = { ready: []; data: [Buffer]; error: [Error] };
class Typed extends EventEmitter {
on<K extends keyof Events>(e: K, fn: (...a: Events[K]) => void) { return super.on(e, fn as any); }
emit<K extends keyof Events>(e: K, ...a: Events[K]) { return super.emit(e, ...a); }
}
| Class Variant | Detail |
|---|
EventEmitterAsyncResource | Integrates with async hooks |
captureRejections: true | Auto-emit "error" on async listener rejection |