Event Emitter

mediumcodingJavaScriptDesign PatternsClasses~25 min

Asked at Meta, Google, Stripe

Overview

Implement an EventEmitter class:

on(event, listener) — register listener
off(event, listener) — remove listener
emit(event, ...args) — call all listeners for event
once(event, listener) — listener fires only once

Examples

const emitter = new EventEmitter();
emitter.on('data', val => console.log(val));
emitter.emit('data', 42); // logs 42

Solution

Reveal solution

class EventEmitter {
  constructor() { this.events = new Map(); }
  on(event, listener) {
    if (!this.events.has(event)) this.events.set(event, []);
    this.events.get(event).push(listener);
    return this;
  }
  off(event, listener) {
    const listeners = this.events.get(event);
    if (listeners) this.events.set(event, listeners.filter(l => l !== listener && l._original !== listener));
    return this;
  }
  emit(event, ...args) {
    const listeners = this.events.get(event) || [];
    listeners.forEach(l => l(...args));
    return this;
  }
  once(event, listener) {
    const wrapper = (...args) => { this.off(event, wrapper); listener(...args); };
    wrapper._original = listener;
    return this.on(event, wrapper);
  }
}

event-emitter.js

Event Emitter

mediumcodingJavaScriptDesign Patterns

Overview

Implement an EventEmitter class:

on(event, listener) — register listener
off(event, listener) — remove listener
emit(event, ...args) — call all listeners for event
once(event, listener) — listener fires only once

Examples

const emitter = new EventEmitter();
emitter.on('data', val => console.log(val));
emitter.emit('data', 42); // logs 42

Solution

Reveal solution

class EventEmitter {
  constructor() { this.events = new Map(); }
  on(event, listener) {
    if (!this.events.has(event)) this.events.set(event, []);
    this.events.get(event).push(listener);
    return this;
  }
  off(event, listener) {
    const listeners = this.events.get(event);
    if (listeners) this.events.set(event, listeners.filter(l => l !== listener && l._original !== listener));
    return this;
  }
  emit(event, ...args) {
    const listeners = this.events.get(event) || [];
    listeners.forEach(l => l(...args));
    return this;
  }
  once(event, listener) {
    const wrapper = (...args) => { this.off(event, wrapper); listener(...args); };
    wrapper._original = listener;
    return this.on(event, wrapper);
  }
}

NameTopicDifficulty

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