JS-exercise-event-emitter

Exercício de event emitter em Javascript usando TDD

Esse é um exercício de uma entrevista técnica de emprego para vaga de Engenheiro de Software. Então você pode utilizá-la para treinar seus conhecimentos antes de uma entrevista javascript.

Para fazer o exercício você terá que instalar o Yarn em sua máquina e estar levemente familiarizado com o desenvolvimento de software orientado a testes (TDD).

Baixe a pasta “exercise-event-emitter-master” e tente solucioná-lo antes de ver a resposta. Procure se focar mais em solucionar o problema do que em eficiência.

Lembre-se: Sempre há mais de uma maneira de se chegar na resposta correta. E o mais importante se divirta!

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

---

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)

---

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