Event Emitter

The goal of this exercise is to create an event emitter which can listen, unlisten and emit events.

The main module exports an object with a create method which act as a factory and should return a new event emitter every time it is called. The implementation of the emitter itself is up to you.

• Running tests
• Example
• Documentation
  • Creating a new Emitter
  • Methods
    • on
    • off
    • emit
    • once
    • race

Running tests

By executing the package.json test script, a jest process will start and re-run the test suite after every file change you made.

---

Example

import Emitter from './emitter.js'

const emitter = Emitter.create()

const handleMyEvent = () => {
  console.log('Hello!')
}

// Registers the callback `handleMyEvent` to be called when we call emitter.emit passing `myEvent` as parameter
emitter.on('myEvent', handleMyEvent)

// This will call the `handleMyEvent` function
emitter.emit('myEvent')

// Will print "Hello!"

// Unregisters the callback `handleMyEvent`
emitter.off('myEvent', handleMyEvent)

// No function will be called since `myEvent` does not have any callbacks assigned to it anymore
emitter.emit('myEvent')

Documentation

Creating a new Emitter

const EventEmitter = require('...')

// Return a new event emitter
const emitter = EventEmitter.create()

Methods

on

on(event: string, callback: function): function

Registers a callback that runs when event is emitted.

Example:

emitter.on('click', () => {
  console.log('Click!')
})
// register a new listener for the 'click' event

It returns a method which unregisters the listener:

Example:

const cancel = emitter.on('click', () => {
  console.log('Click!')
})

cancel()
// unregister the click listener

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off

off(event: string, callback: function)

Unregisters a callback from the event callback list.

Example:

const callback = () => {
  console.log('Click!')
}

// register the listener
emitter.on('click', callback)

// unregister the listener
emitter.off('click', callback)

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emit

emit(event: string, data: object)

Execute all callbacks registered for event passing data as a parameter.

Example:

emitter.on('click', () => console.log('Click 1'))
emitter.on('click', () => console.log('Click 2'))
emitter.on('click', () => console.log('Click 3'))

emitter.emit('click')
// Prints:
// Click 1
// Click 2
// Click 3

Every listener callback receives a data object which contains the type of the event and any other property passed on .emit():

Example:

emitter.on('loaded', e => console.log(e))

emitter.emit('loaded', { foo: 'potato' })
// Prints:
// { type: 'loaded', foo: 'potato' }

---

once

once(event: string, callback: function): function

Registers a callback tha runs only once when event is emitted.

Example:

emitter.once('click', () => console.log('Single click!'))

emitter.emit('click')
emitter.emit('click')

// Prints 'Single click!' only once

---

race

race(Array<[event: string, callback: function]>): function

Receives a list of [event, callback] and when any of the passed events is emitted, it unregisters all of them.

Example:

emitter.race([
  ['success', () => console.log('Success!')],
  ['failure', () => console.log('Failure :(')],
])

emitter.emit('success') // Prints 'Success!', `success` and `failure` are unregistered.
emitter.emit('failure') // nothing happens