The Node.js 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.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

const class EventEmitter<T extends EventMap<T> = DefaultEventMap>EventEmitter = 
var require: NodeJS.Require
(id: string) => anyrequire('node:events');

const const eventEmitter: EventEmitter<DefaultEventMap>eventEmitter = new new EventEmitter<DefaultEventMap>(options?: EventEmitterOptions): EventEmitter<DefaultEventMap>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', () => {
  var console: Consoleconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)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: anynumber => {
  var console: Consoleconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)log(`started ${number: anynumber}`);
});

eventEmitter.emit('start', 23);

Multiple arguments:

eventEmitter.on('start', (start: anystart, end: anyend) => {
  var console: Consoleconsole.Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)log(`started from ${start: anystart} to ${end: anyend}`);
});

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

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 more about these methods in the official documentation.