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flavio ⚡️🔥
flavio ⚡️🔥

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The Node Event emitter

If you worked with JavaScript in the browser, you know how much of the interaction of the user is handled through events: mouse clicks, keyboard button presses, reacting to mouse movements, and so on.

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').EventEmitter()
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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', () => {
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When we run

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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)
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Multiple arguments:

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

eventEmitter.emit('start', 1, 100)
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The EventEmitter object also exposes several other methods to interact with events, like

  • once(): add a one-time listener
  • removeListener() / off(): remove an event listener from an event
  • removeAllListeners(): remove all listeners for an event

You can read all their details on the events module page at

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