Events and EventEmitter

EventEmitter patterns for libraries, requests, and process signals. Results appear in the same fence: same-line // comments when short, multiline // blocks below the sample when not.

Emit and on

import { EventEmitter } from "node:events";
const bus = new EventEmitter();
const seen: string[] = [];
bus.on("msg", (text: string) => seen.push(text));
bus.emit("msg", "hello");
seen  // ["hello"]

once Promise

events.once returns a promise for the next event - great with async/await.

import { once, EventEmitter } from "node:events";
const ee = new EventEmitter();
setTimeout(() => ee.emit("ready", 42), 0);
const [value] = await once(ee, "ready");
value  // 42

error Event Convention

Emitters that emit error without a listener crash the process - always attach handling.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
const errs: string[] = [];
ee.on("error", (err: Error) => errs.push(err.message));
ee.emit("error", new Error("boom"));
errs  // ["boom"]

removeListener Cleanup

Remove listeners on shutdown to avoid leaks - prefer named functions over anonymous.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
function onData(n: number) { return n; }
ee.on("data", onData);
ee.listenerCount("data")  // 1
ee.off("data", onData);
ee.listenerCount("data")  // 0

EventEmitter Async

Keep handlers fast or queue work - avoid blocking the event loop inside listeners.

import { EventEmitter } from "node:events";
const bus = new EventEmitter();
const jobs: number[] = [];
bus.on("job", (n: number) => queueMicrotask(() => jobs.push(n)));
bus.emit("job", 1);
await Promise.resolve();
jobs  // [1]

prependListener Order

prependListener runs before listeners registered with on.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
const order: string[] = [];
ee.on("x", () => order.push("second"));
ee.prependListener("x", () => order.push("first"));
ee.emit("x");
order  // ["first", "second"]

listenerCount Inspect

Debug how many listeners are attached to an event name.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
ee.on("data", () => {});
ee.on("data", () => {});
ee.listenerCount("data")  // 2
ee.eventNames()           // ["data"]

setMaxListeners

Raise the max listeners warning threshold only when the fan-out is intentional.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
ee.setMaxListeners(50);
ee.getMaxListeners()  // 50

rawListeners Snapshot

Inspect wrappers including once wrappers during debugging.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
ee.once("ready", () => {});
ee.rawListeners("ready").length  // 1

AbortSignal event

Tie cancellation to emitters with AbortSignal event listeners.

const ac = new AbortController();
let canceled = false;
ac.signal.addEventListener("abort", () => { canceled = true; }, { once: true });
ac.abort();
canceled  // true
ac.signal.aborted  // true

EventTarget Web Style

Some Node APIs use Web EventTarget - same mental model, different method names.

const t = new EventTarget();
let n = 0;
t.addEventListener("ping", () => { n += 1; });
t.dispatchEvent(new Event("ping"));
n  // 1

captureRejections

Enable rejection capture so async listener failures become error events when configured.

import { EventEmitter } from "node:events";
EventEmitter.captureRejections = true;
EventEmitter.captureRejections  // true

Subclass Emitter

Domain objects often extend EventEmitter for lifecycle hooks.

import { EventEmitter } from "node:events";
class Job extends EventEmitter {
  start() { this.emit("start", "go"); }
}
const job = new Job();
const seen: string[] = [];
job.on("start", (m: string) => seen.push(m));
job.start();
seen  // ["go"]

process nextTick Order

process.nextTick queues before other microtasks - use sparingly for fairness.

const order: string[] = [];
process.nextTick(() => order.push("tick"));
Promise.resolve().then(() => order.push("promise"));
await new Promise((r) => setImmediate(r));
order
// ["tick", "promise"]

Signal Events

Handle graceful shutdown via process signal events.

// process.on("SIGINT", () => {
//   shutdown().finally(() => process.exit(0));
// });
// emits when user hits Ctrl+C (when supported)

Streams and Pipelines - streams are EventEmitters underneath
Process and Workers - process signals and IPC events
Errors and Debugging - error event handling

Node.js API Documentation - Complete reference for core modules and APIs.
Node.js Learn Guides - Official learning path.
Node.js Best Practices (GitHub) - Curated production best practices.
Node.js Design Patterns, 4th Edition - Production-grade application patterns.
Node.js in Action, 2nd Edition - Classic practical Node.js.

Stack versions: Node.js 24.18.0 (LTS line 24) · TypeScript 5.6+ · npm 10+

Events and EventEmitter

EventEmitter patterns for libraries, requests, and process signals. Results appear in the same fence: same-line // comments when short, multiline // blocks below the sample when not.

Emit and on

import { EventEmitter } from "node:events";
const bus = new EventEmitter();
const seen: string[] = [];
bus.on("msg", (text: string) => seen.push(text));
bus.emit("msg", "hello");
seen  // ["hello"]

once Promise

events.once returns a promise for the next event - great with async/await.

import { once, EventEmitter } from "node:events";
const ee = new EventEmitter();
setTimeout(() => ee.emit("ready", 42), 0);
const [value] = await once(ee, "ready");
value  // 42

error Event Convention

Emitters that emit error without a listener crash the process - always attach handling.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
const errs: string[] = [];
ee.on("error", (err: Error) => errs.push(err.message));
ee.emit("error", new Error("boom"));
errs  // ["boom"]

removeListener Cleanup

Remove listeners on shutdown to avoid leaks - prefer named functions over anonymous.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
function onData(n: number) { return n; }
ee.on("data", onData);
ee.listenerCount("data")  // 1
ee.off("data", onData);
ee.listenerCount("data")  // 0

EventEmitter Async

Keep handlers fast or queue work - avoid blocking the event loop inside listeners.

import { EventEmitter } from "node:events";
const bus = new EventEmitter();
const jobs: number[] = [];
bus.on("job", (n: number) => queueMicrotask(() => jobs.push(n)));
bus.emit("job", 1);
await Promise.resolve();
jobs  // [1]

prependListener Order

prependListener runs before listeners registered with on.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
const order: string[] = [];
ee.on("x", () => order.push("second"));
ee.prependListener("x", () => order.push("first"));
ee.emit("x");
order  // ["first", "second"]

listenerCount Inspect

Debug how many listeners are attached to an event name.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
ee.on("data", () => {});
ee.on("data", () => {});
ee.listenerCount("data")  // 2
ee.eventNames()           // ["data"]

setMaxListeners

Raise the max listeners warning threshold only when the fan-out is intentional.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
ee.setMaxListeners(50);
ee.getMaxListeners()  // 50

rawListeners Snapshot

Inspect wrappers including once wrappers during debugging.

import { EventEmitter } from "node:events";
const ee = new EventEmitter();
ee.once("ready", () => {});
ee.rawListeners("ready").length  // 1

AbortSignal event

Tie cancellation to emitters with AbortSignal event listeners.

const ac = new AbortController();
let canceled = false;
ac.signal.addEventListener("abort", () => { canceled = true; }, { once: true });
ac.abort();
canceled  // true
ac.signal.aborted  // true

EventTarget Web Style

Some Node APIs use Web EventTarget - same mental model, different method names.

const t = new EventTarget();
let n = 0;
t.addEventListener("ping", () => { n += 1; });
t.dispatchEvent(new Event("ping"));
n  // 1

captureRejections

Enable rejection capture so async listener failures become error events when configured.

import { EventEmitter } from "node:events";
EventEmitter.captureRejections = true;
EventEmitter.captureRejections  // true

Subclass Emitter

Domain objects often extend EventEmitter for lifecycle hooks.

import { EventEmitter } from "node:events";
class Job extends EventEmitter {
  start() { this.emit("start", "go"); }
}
const job = new Job();
const seen: string[] = [];
job.on("start", (m: string) => seen.push(m));
job.start();
seen  // ["go"]

process nextTick Order

process.nextTick queues before other microtasks - use sparingly for fairness.

const order: string[] = [];
process.nextTick(() => order.push("tick"));
Promise.resolve().then(() => order.push("promise"));
await new Promise((r) => setImmediate(r));
order
// ["tick", "promise"]

Signal Events

Handle graceful shutdown via process signal events.

// process.on("SIGINT", () => {
//   shutdown().finally(() => process.exit(0));
// });
// emits when user hits Ctrl+C (when supported)

Streams and Pipelines - streams are EventEmitters underneath
Process and Workers - process signals and IPC events
Errors and Debugging - error event handling

Node.js API Documentation - Complete reference for core modules and APIs.
Node.js Learn Guides - Official learning path.
Node.js Best Practices (GitHub) - Curated production best practices.
Node.js Design Patterns, 4th Edition - Production-grade application patterns.
Node.js in Action, 2nd Edition - Classic practical Node.js.

Stack versions: Node.js 24.18.0 (LTS line 24) · TypeScript 5.6+ · npm 10+

Events and EventEmitter

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Events and EventEmitter