Custom events in JavaScript

Many of the code we write involves reacting to events. I/O events like mouse clicks or keyboard events. Network events, when you listen to an HTTP call.

Those are what I call built-in events.

In JavaScript we can create custom events, and the way it works changes in the browser and in Node.js.

In the frontend we use the Event object which is provided by the browser:

const anEvent = new Event('start');

You can trigger the event using

document.dispatchEvent(anEvent)

and when this happens, the event listener is triggered:

document.addEventListener('start', event => {   
  console.log('started!')
})

You can send custom data using the CustomEvent built-in object instead of Event, which accepts an object as the second parameter:

const anotherEvent = new CustomEvent('start', {
  detail: {
    color: 'white'
  }
})

Using CustomEvent, in the event listener you can ask the data to the event object using event.detail (you can't use another property):

document.addEventListener('start', event => {   
  console.log('started!')
  console.log(event.detail)
})

On the backend side, Node offers us the option to build a similar system using the events module.

This module, in particular, offers the EventEmitter class, which we'll use to handle our events.

You initialize that using

const EventEmitter = require('events')
const eventEmitter = new EventEmitter()

This object exposes, among many others, the on and emit methods.

• emit is used to trigger an event
• on is used to add a callback function that's going to be executed when the event is triggered

For example, let's create a start event, and as a matter of providing a sample, we react to that by just logging to the console:

eventEmitter.on('start', () => {
  console.log('started')
})

When we run

eventEmitter.emit('start')

the event handler function is triggered, and we get the console log.

You can pass arguments to the event handler by passing them as additional arguments to emit():

eventEmitter.on('start', (number) => {
  console.log(`started ${number}`)
})

eventEmitter.emit('start', 23)

Multiple arguments:

eventEmitter.on('start', (start, end) => {
  console.log(`started from ${start} to ${end}`)
})

eventEmitter.emit('start', 1, 100)