Question for my Frontend Interview at Facebook

facebook.js

/*
 * Create an event emitter that goes like this
 * emitter = new Emitter();
 *
 * Allows you to subscribe to some event
 * sub1 = emitter.subscribe('function_name', callback1);
 * (you can have multiple callbacks to the same event)
 * sub2 = emitter.subscribe('function_name', callback2);
 *
 * You can emit the event you want with this api
 * (you can receive 'n' number of arguments)
 * sub1.emit('function_name', foo, bar);
 *
 * And allows you to release the subscription like this
 * (but you should be able to still emit from sub2)
 * sub1.release();
 */
class Emitter {
  constructor(events = {}) {
    this.events = events
  }

  subscribe(name, cb) {
    (this.events[name] || (this.events[name] = [])).push(cb)

    return {
      release: () => this.events[name] &&
        this.events[name].splice(this.events[name].indexOf(cb) >>> 0, 1)
    }
  }

  emit(name, ...args) {
    return (this.events[name] || []).map(fn => fn(...args))
  }
}

export default Emitter

test.js

const expect = require('expect')

const Emitter = require('./facebook')

// Step 1

let test = 0

const emitter = new Emitter()

emitter.emit('sum', 1)

expect(test).toBe(0) // should trigger nothing

const sub1 = emitter.subscribe('sum', num => (test = test + num))

emitter.emit('sum', 1)

expect(test).toBe(1) // should trigger 1 callback

const sub2 = emitter.subscribe('sum', num => (test = test * num))

emitter.emit('sum', 2)

expect(test).toBe(6) // should trigger 2 callbacks

// Step 2

sub1.release()

emitter.emit('sum', 3)

expect(test).toBe(18) // should trigger 1 callback

// Step 3

const myEvent1 = emitter.subscribe('myEvent', () => 1)
const myEvent2 = emitter.subscribe('myEvent', () => 2)
const myEvent3 = emitter.subscribe('myEvent', () => true)
const result = emitter.emit('myEvent')

expect(result).toEqual([1, 2, true])

console.info('You passed the test!!! 👏🏼 👏🏼 👏🏼')

thanks for this solution. could you please explain a little about why use indexOf(cb) >>> 0 ? I am a little confused about it

to prevent a negative number

Hi Kutyel,
why did you add "return" in release and emit?
is class is the scope of the "this" of the release function? or is it because the subscribe?

thanks