Once listeners in Node.js - Mini Project: Build & Apply

Using Once Listeners in Node.js EventEmitter

📖 Scenario: You are building a simple Node.js program that reacts to a special event only once. This is useful when you want to run some code the first time something happens, but not on later times.

🎯 Goal: Create an EventEmitter, add a once listener for a custom event called greet, and emit the event twice. The listener should run only the first time.

📋 What You'll Learn

Create an EventEmitter instance called emitter

Add a once listener for the event named greet that logs 'Hello once!'

Emit the greet event twice

Ensure the listener runs only once

💡 Why This Matters

🌍 Real World

Once listeners are useful in real applications where you want to respond to an event only the first time it happens, such as initializing a resource or handling a one-time setup.

💼 Career

Understanding event-driven programming and Node.js EventEmitter is essential for backend developers working with asynchronous code and real-time applications.

1

Create EventEmitter instance

Import the EventEmitter class from the events module and create an instance called emitter.

Node.js

const { EventEmitter } = require('events');
// Your code here

Need a hint?

Use require('events') to import EventEmitter and then create emitter with new EventEmitter().

2

Add a once listener for 'greet'

Add a once listener to emitter for the event 'greet' that logs 'Hello once!' when triggered.

Node.js

const { EventEmitter } = require('events');
const emitter = new EventEmitter();
// Add a once listener for 'greet' event
// Your code here

Need a hint?

Use emitter.once('greet', callback) where the callback logs the message.

3

Emit the 'greet' event twice

Emit the 'greet' event two times using emitter.emit('greet').

Node.js

const { EventEmitter } = require('events');
const emitter = new EventEmitter();
emitter.once('greet', () => {
  console.log('Hello once!');
});
// Emit 'greet' event twice
// Your code here

Need a hint?

Call emitter.emit('greet') two times to trigger the event.

4

Complete the program

Ensure the program is complete with the import, emitter creation, once listener, and two emits as shown.

Node.js

const { EventEmitter } = require('events');
const emitter = new EventEmitter();
emitter.once('greet', () => {
  console.log('Hello once!');
});
emitter.emit('greet');
emitter.emit('greet');
// Program complete

Need a hint?

Check that all parts are present: import, emitter, once listener, and two emits.