Skip to main content

Use EventEmitter from node:events

Edit on Github

EventEmitter is the classic Node.js building block for things that emit named events, and countless npm packages hand you one. Deno supports it through the node:events module. This example registers listeners, emits events, and shows the special behavior of the error event.

import { EventEmitter, once } from "node:events";

const emitter = new EventEmitter();
on registers a listener that runs for every matching emit. The extra emit arguments are passed to the listener.
function onMessage(text: string) {
  console.log(`received: ${text}`);
}
emitter.on("message", onMessage);
emitter.emit("message", "hello"); // received: hello
emitter.emit("message", "again"); // received: again
once (the method) registers a listener that runs a single time and is then removed automatically.
emitter.once("connected", () => console.log("connected fired"));
emitter.emit("connected"); // connected fired
emitter.emit("connected"); // (nothing, the listener is gone)
off removes a previously registered listener. You need the same function reference you passed to on.
emitter.off("message", onMessage);
emitter.emit("message", "ignored"); // (nothing)
emit returns whether any listener was called, which is an easy way to see the effect.
console.log(emitter.emit("message", "ignored")); // false
The error event is special: emitting it with no error listener registered throws the error instead of silently doing nothing. Always attach an error listener to emitters that can fail.
try {
  emitter.emit("error", new Error("disk full"));
} catch (err) {
  console.log(`threw: ${(err as Error).message}`); // threw: disk full
}
emitter.on("error", (err: Error) => console.log(`handled: ${err.message}`));
emitter.emit("error", new Error("disk full")); // handled: disk full
The once function (imported from node:events, not the method) returns a promise that resolves with the event arguments, which lets you await an event in async code.
setTimeout(() => emitter.emit("ready", "database"), 10);
const [what] = await once(emitter, "ready");
console.log(`ready: ${what}`); // ready: database

Run this example locally using the Deno CLI:

deno run https://docs.deno.com/examples/scripts/event_emitter.ts

Additional resources

Did you find what you needed?

Privacy policy