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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.js 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: 
import EventEmitter from 'node: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

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Emitting and listening to events

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.

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Passing arguments to event handlers

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);

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Additional EventEmitter methods

The EventEmitter object also exposes several other methods to interact with events:
  • 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 more about these methods in the official documentation.

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