javascript 43 lines · 7 steps

Building an EventEmitter in JavaScript

A pub/sub class that registers handlers per event, supports one-shot listeners, and fires them on emit.

Explained by highlit
Intermediate pub-sub closures map set method-chaining
1class EventEmitter {
2 constructor() {
3 this.listeners = new Map();
4 }
5 
6 on(event, handler) {
7 if (!this.listeners.has(event)) {
8 this.listeners.set(event, new Set());
9 }
10 this.listeners.get(event).add(handler);
11 return this;
12 }
13 
14 once(event, handler) {
15 const wrapper = (...args) => {
16 this.off(event, wrapper);
17 handler(...args);
18 };
19 wrapper.original = handler;
20 return this.on(event, wrapper);
21 }
22 
23 off(event, handler) {
24 const handlers = this.listeners.get(event);
25 if (!handlers) return this;
26 for (const h of handlers) {
27 if (h === handler || h.original === handler) {
28 handlers.delete(h);
29 }
30 }
31 if (handlers.size === 0) this.listeners.delete(event);
32 return this;
33 }
34 
35 emit(event, ...args) {
36 const handlers = this.listeners.get(event);
37 if (!handlers) return false;
38 for (const handler of [...handlers]) {
39 handler(...args);
40 }
41 return true;
42 }
43}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1A Map of event names to Sets gives fast lookup and automatic dedup of handlers.
  2. 2Wrapping a handler in a self-removing closure turns any listener into a one-shot without new machinery.
  3. 3Tagging the wrapper with a reference to the original handler lets off() cancel once-listeners by their public identity.

Related explainers

javascript 
import { useCallback, useState } from "react";
 
const MIN = 0;
const MAX = 1000;

Building a dual-thumb price slider in React

controlled-components state-clamping usecallback
Intermediate 8 steps
javascript 
class LyricsSync {
  constructor(audio, container, lines) {
    this.audio = audio;
    this.container = container;

Building a synced lyrics highlighter

binary-search dom-manipulation event-handling
Intermediate 9 steps
javascript 
const express = require('express');
 
const app = express();
 

Enforcing HTTPS with Express middleware

middleware https security
Intermediate 6 steps
javascript 
const express = require('express');
const router = express.Router();
 
router.get('/articles/:slug', async (req, res, next) => {

Conditional GET caching in Express

http-caching conditional-get routing
Intermediate 8 steps
rust 
use std::time::Duration;
 
#[derive(Debug, Clone, Copy)]
pub struct LatencyStats {

Computing latency percentiles in Rust

percentiles interpolation closures
Intermediate 6 steps
javascript 
function parseHexColor(hex) {
  const cleaned = hex.trim().replace(/^#/, '');
 
  const expand = (short) =>

Parsing hex colors into RGBA channels

parsing bitwise regex
Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

Building an EventEmitter in JavaScript — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code