Labs ch-9.1

davidwz

I have implemented the once method of the eventEmitter to call the event just once and then the listener is removed. However the test is not passing, because the event emitter is undefined in the assertion assert.equal(this, ee). Any hints how to solve it ?

k0dard

Hello davidwz,

I don't think that you should touch the assertion...

Could you post your code please ?

krave

Perhaps the error is caused by not assigning the return value of new EventEmitter() to the constant ee. You have to keep ee reference to the instance of the event emitter until the assertion finishes.

davidwz

Thats the code:
'use strict'
const assert = require('assert')
const { EventEmitter } = require('events')

const ee = new EventEmitter()
ee.once('tick', ()=>console.log('one time called'))
let count = 0

setInterval(() => {

ee.emit('tick');

}, 100)

function listener () {

count++
setTimeout(() => {
assert.equal(count, 1)
assert.equal(this, ee)
console.log('passed!')
}, 250)
}

listener()

And thats the error message: AssertionError [ERR_ASSERTION]: undefined == EventEmitter

krave

@davidwz said:
Thats the code:
'use strict'
const assert = require('assert')
const { EventEmitter } = require('events')

const ee = new EventEmitter()
ee.once('tick', ()=>console.log('one time called'))
let count = 0

setInterval(() => {

ee.emit('tick');

}, 100)

function listener () {

count++
setTimeout(() => {
assert.equal(count, 1)
assert.equal(this, ee)
console.log('passed!')
}, 250)
}

listener()

And thats the error message: AssertionError [ERR_ASSERTION]: undefined == EventEmitter

listener should be passed as event handler to event function such as ee.once() or ee.on. Invoking listener function directly as you did in the code - listener() will cause this in the listener function being undefined under strict mode.

So let's check this assertion:
asser.equal(this, ee)

this would be undefined and ee would be the Event Emmitter instance you created before.
As a result, as mentioned in the error message: undefined == EventEmitter which is not true.

krave

Here is my code

'use strict'
const assert = require('assert')
const { EventEmitter } = require('events')

const ee = new EventEmitter()

ee.once('tick', listener)

let count = 0
setInterval(() => {
  ee.emit('tick')
}, 100)

function listener() {
  count++
  setTimeout(() => {
    assert.equal(count, 1)
    assert.equal(this, ee)
    console.log('passed!')
  }, 250)
}

davidwz

Thanks a lot !

davidmarkclements

@krave is correct

emers0n

@davidmarkclements my code is the same as krave, and I receive 'passed!' in the console. However, the process continues to run and I am required to ctrl+c to end it, is this normal?

xdxmxc

yes that's normal @emers0n - the setInterval is an active handle, so it keeps the event-loop open. If you wanted the process to end do const interval = setInterval(...) and then clearInterval(interval) after the 'passed!' log.