🟩

Events Module

Table of contents

What is Event ?
Events Module
- Note
Events Argument
- Note
Extend From Events Class

What is Event ?

A signal that something has happened.

Events Module

EventEmitter is one of the core classes in Node that allows us to raise (emit) and handle events. Several built-in classes in Node derive from EventEmitter.

const EventEmitter = require('events');
const emitter = new EventEmitter();
// register a listener
emitter.on('messageLogged', function () {
    console.log("Listener called");
})
// raise an event
emitter.emit('messageLogged');

Note

Emit function is synchronous then we should register listener before call emit function.

Events Argument

Sometimes we want to pass data about the event to the listener, we can do it with the below way.

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('messageLogged', function (arg) { // e,eventArg
    console.log("Listener called", arg);
})
emitter.emit('messageLogged', { id: 1, url: "http://" });

Note

In ES6 we have an arrow function we can use like the below sample

const EventEmitter = require('events');
const emitter = new EventEmitter();
emitter.on('messageLogged', (arg) => { // Arrow Function
    console.log("Listener called", arg);
})
emitter.emit('messageLogged', { id: 1, url: "http://" });

Extend From Events Class

In the real world, we don't use the Events class directly instead we make classes that extend from Events class. To create a class with the ability to raise events, we should extend EventEmitter:

class Logger extends EventEmitter {
}

See the belwo sample:

logger.js

const EventEmitter = require('events');
class Logger extends EventEmitter {
    log(message) {
        console.log(message)
        this.emit('messageLogged', { id: 1, url: "http://" });
    }
}
module.exports = Logger;

app.js

const Logger = require('./logger');
const logger = new Logger();
logger.on('messageLogged', (arg) => { // e,eventArg
    console.log("Listener called", arg);
})
logger.log("helllo")

Edit this page