# The Node events module

> A reference to the Node.js events module and its EventEmitter class, covering methods like on(), emit(), once(), listenerCount(), and removeListener().

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2018-09-14 | Topics: [Node.js](https://flaviocopes.com/tags/node/) | Canonical: https://flaviocopes.com/node-module-events/

The `events` module provides us the EventEmitter class, which is key to working with events in Node.

[I published a full article on that](https://flaviocopes.com/node-event-emitter/), so here I will just describe the API without further examples on how to use it.

```js
const EventEmitter = require('events')
const door = new EventEmitter()
```

The event listener eats its own dog food and uses these events:

- `newListener` when a listener is added
- `removeListener` when a listener is removed

Here's a detailed description of the most useful methods:

<!-- TOC -->

- [emitter.addListener()](#emitteraddlistener)
- [emitter.emit()](#emitteremit)
- [emitter.eventNames()](#emittereventnames)
- [emitter.getMaxListeners()](#emittergetmaxlisteners)
- [emitter.listenerCount()](#emitterlistenercount)
- [emitter.listeners()](#emitterlisteners)
- [emitter.off()](#emitteroff)
- [emitter.on()](#emitteron)
- [emitter.once()](#emitteronce)
- [emitter.prependListener()](#emitterprependlistener)
- [emitter.prependOnceListener()](#emitterprependoncelistener)
- [emitter.removeAllListeners()](#emitterremovealllisteners)
- [emitter.removeListener()](#emitterremovelistener)
- [emitter.setMaxListeners()](#emittersetmaxlisteners)

<!-- /TOC -->

## emitter.addListener()

Alias for `emitter.on()`.

## emitter.emit()

Emits an event. It synchronously calls every event listener in the order they were registered.

## emitter.eventNames()

Return an array of strings that represent the events registered on the current EventListener:

```js
door.eventNames()
```

## emitter.getMaxListeners()

Get the maximum amount of listeners one can add to an EventListener object, which defaults to 10 but can be increased or lowered by using `setMaxListeners()`

```js
door.getMaxListeners()
```

## emitter.listenerCount()

Get the count of listeners of the event passed as parameter:

```js
door.listenerCount('open')
```

## emitter.listeners()

Gets an array of listeners of the event passed as parameter:

```js
door.listeners('open')
```

## emitter.off()

Alias for `emitter.removeListener()` added in Node 10

## emitter.on()

Adds a callback function that's called when an event is emitted.

Usage:

```js
door.on('open', () => {
  console.log('Door was opened')
})
```

## emitter.once()

Adds a callback function that's called when an event is emitted for the first time after registering this. This callback is only going to be called once, never again.

```js
const EventEmitter = require('events')
const ee = new EventEmitter()

ee.once('my-event', () => {
  //call callback function once
})
```

## emitter.prependListener()

When you add a listener using `on` or `addListener`, it's added last in the queue of listeners, and called last. Using `prependListener` it's added, and called, before other listeners.

## emitter.prependOnceListener()

When you add a listener using `once`, it's added last in the queue of listeners, and called last. Using `prependOnceListener` it's added, and called, before other listeners.

## emitter.removeAllListeners()

Removes all listeners of an event emitter object listening to a specific event:

```js
door.removeAllListeners('open')
```

## emitter.removeListener()

Remove a specific listener. You can do this by saving the callback function to a variable, when added, so you can reference it later:

```js
const doSomething = () => {}
door.on('open', doSomething)
door.removeListener('open', doSomething)
```

## emitter.setMaxListeners()

Sets the maximum amount of listeners one can add to an EventListener object, which defaults to 10 but can be increased or lowered.

```js
door.setMaxListeners(50)
```