`emitter.once() / events.once()`

Registers a one-time listener, or awaits a single emission as a Promise.

Since Node 0.x; events.once Node 11.13 Spec ↗

Syntax

emitter.once(name, listener)  /  await once(emitter, name)

Parameters

Name	Type	Required	Description
`eventName`	`string \| symbol`	Yes	The event to listen for once.
`listener`	`function`	No	The callback (method form). Removed automatically after one call.

Returns

EventEmitter | Promise<any[]> — The emitter, or a Promise resolving with the emitted args.

Examples

import { EventEmitter } from 'node:events';

const ee = new EventEmitter();
ee.once('init', () => console.log('runs once'));
ee.emit('init');
ee.emit('init');

Output

runs once

import { once } from 'node:events';
import { createServer } from 'node:http';

const server = createServer().listen(3000);
await once(server, 'listening');
console.log('server is up');

Output

server is up

Notes

The standalone `events.once()` is ideal for awaiting startup events. It also rejects if the emitter emits `'error'` first, so wrap it in try/catch. The listener form auto-removes after the first emission.