LFW211 JSNAD CH-9
I have a question regarding the following. The above page says that
`import { once, EventEmitter } from 'events'
const uneventful = new EventEmitter()
await once(uneventful, 'ping')
console.log('pinged!')`
Execution will pause on the line starting await once, until the registered event fires. If it never fires, execution will never proceed past that point.
Am I supposed to see the process keep waiting on the await statement? (What I am experiencing is that I don't see the 'pinged' message but process exists may be because nothing exists to keep the Event loop active)
Best Answer
-
In Chapter 14. Process & Operating System you will probably find the answer to your questions.
https://trainingportal.linuxfoundation.org/learn/course/nodejs-application-development-lfw211/process-operating-system/process-operating-system?page=4Some API's have active handles. An active handle is a reference that keeps the process open. For instance, net.createServer creates a server with an active handle which will stop the process from exiting by itself so that it can wait for incoming requests. Timeouts and intervals also have active handles that keep the process from exiting
So, in the original question the process terminates immediately without printing 'pinged!' because 'ping' event is not emitted and there are no active handles (i.e. roughly speaking activities that could potentially emit given event)
The same applies to Question 1. The process remain live as long as there is active timer.
As for Question 2. It's a bit complicated. As I can see the resource is fetched successfully and 'ping' is emitted indeed. However, the subscription for 'ping' is made after the event is emitted.
If you rewrite the code following way, you will see 'pinged!' in output
import { once, EventEmitter } from 'events' const uneventful = new EventEmitter() setImmediate(async () => { await once(uneventful, 'ping') console.log('pinged!') }) try { const response = await fetch('https://jsonplaceholder.typicode.com/todos/1') const data = await response.json() console.log('before emit') uneventful.emit('ping') console.log('after emit') } catch (error) { console.error(error) }
2
Answers
-
Thanks @abraham.m.joseph for your questions.
I actually have no answer, but while investigating your questions, I have also faced some understanding
regarding the 'once' event.Question 1. Why the process is alive for 5 seconds. As there should not be any relation between the
setTimeout and await once(uneventful, 'ping')
import { once, EventEmitter } from 'events' const uneventful = new EventEmitter() setTimeout(() => { }, 5000) await once(uneventful, 'ping')
Question 2. Let's forget about the setTimeout, Now if I make a fake API call and fire an emit event after the response is successful, Why the "pinged!" is not printed in console.
import { once, EventEmitter } from 'events' const uneventful = new EventEmitter() try { const response = await fetch('https://jsonplaceholder.typicode.com/todos/1') const data = await response.json() uneventful.emit('ping') } catch (error) { } await once(uneventful, 'ping') console.log('pinged!')
1 -
Thanks a lot, @dmsheiko. got it.
0 -
Thanks a lot raselkarim7 for your thought provoking questions and @dmsheiko for the answers. I understand better now.
0
Categories
- All Categories
- 154 LFX Mentorship
- 154 LFX Mentorship: Linux Kernel
- 665 Linux Foundation IT Professional Programs
- 319 Cloud Engineer IT Professional Program
- 141 Advanced Cloud Engineer IT Professional Program
- 54 DevOps Engineer IT Professional Program
- 120 Cloud Native Developer IT Professional Program
- 91 Express Training Courses
- 91 Express Courses - Discussion Forum
- 5.4K Training Courses
- 19 LFC110 Class Forum - Discontinued
- 12 LFC131 Class Forum
- 32 LFD102 Class Forum
- 191 LFD103 Class Forum
- 9 LFD110 Class Forum
- 24 LFD121 Class Forum
- 14 LFD133 Class Forum
- 14 LFD137 Class Forum
- 62 LFD201 Class Forum
- 2 LFD210 Class Forum
- 1 LFD210-CN Class Forum
- 1 LFD213 Class Forum - Discontinued
- 127 LFD232 Class Forum - Discontinued
- LFD233 Class Forum
- LFD237 Class Forum
- 22 LFD254 Class Forum
- 644 LFD259 Class Forum
- 108 LFD272 Class Forum
- 1 LFD272-JP クラス フォーラム
- 7 LFD273 Class Forum
- 2 LFS101 Class Forum
- LFS112 Class Forum
- LFS116 Class Forum
- LFS118 Class Forum
- LFS142 Class Forum
- LFS144 Class Forum
- 2 LFS145 Class Forum
- LFS146 Class Forum
- LFS151 Class Forum
- LFS157 Class Forum
- LFS158 Class Forum
- LFS162 Class Forum
- LFS166 Class Forum
- LFS167 Class Forum
- LFS170 Class Forum
- LFS171 Class Forum
- LFS178 Class Forum
- LFS180 Class Forum
- LFS182 Class Forum
- LFS183 Class Forum
- 28 LFS200 Class Forum
- 735 LFS201 Class Forum - Discontinued
- 1 LFS201-JP クラス フォーラム
- 13 LFS203 Class Forum
- 98 LFS207 Class Forum
- 299 LFS211 Class Forum
- 54 LFS216 Class Forum
- 47 LFS241 Class Forum
- 41 LFS242 Class Forum
- 36 LFS243 Class Forum
- 12 LFS244 Class Forum
- LFS245 Class Forum
- 42 LFS250 Class Forum
- 1 LFS250-JP クラス フォーラム
- LFS251 Class Forum
- 141 LFS253 Class Forum
- LFS254 Class Forum
- LFS255 Class Forum
- 4 LFS256 Class Forum
- LFS257 Class Forum
- 1.2K LFS258 Class Forum
- 9 LFS258-JP クラス フォーラム
- 110 LFS260 Class Forum
- 147 LFS261 Class Forum
- 39 LFS262 Class Forum
- 82 LFS263 Class Forum - Discontinued
- 15 LFS264 Class Forum - Discontinued
- 11 LFS266 Class Forum - Discontinued
- 20 LFS267 Class Forum
- 18 LFS268 Class Forum
- 26 LFS269 Class Forum
- 198 LFS272 Class Forum
- 1 LFS272-JP クラス フォーラム
- LFS274 Class Forum
- 3 LFS281 Class Forum
- LFW111 Class Forum
- 254 LFW211 Class Forum
- 173 LFW212 Class Forum
- 9 SKF100 Class Forum
- SKF200 Class Forum
- 781 Hardware
- 198 Drivers
- 68 I/O Devices
- 37 Monitors
- 95 Multimedia
- 174 Networking
- 87 Printers & Scanners
- 83 Storage
- 742 Linux Distributions
- 80 Debian
- 66 Fedora
- 15 Linux Mint
- 13 Mageia
- 23 openSUSE
- 143 Red Hat Enterprise
- 31 Slackware
- 13 SUSE Enterprise
- 347 Ubuntu
- 459 Linux System Administration
- 39 Cloud Computing
- 69 Command Line/Scripting
- Github systems admin projects
- 89 Linux Security
- 77 Network Management
- 101 System Management
- 46 Web Management
- 51 Mobile Computing
- 17 Android
- 24 Development
- 1.2K New to Linux
- 1K Getting Started with Linux
- 357 Off Topic
- 109 Introductions
- 169 Small Talk
- 18 Study Material
- 504 Programming and Development
- 283 Kernel Development
- 203 Software Development
- 1.1K Software
- 210 Applications
- 180 Command Line
- 3 Compiling/Installing
- 398 Games
- 308 Installation
- 51 All In Program
- 51 All In Forum
Upcoming Training
-
August 20, 2018
Kubernetes Administration (LFS458)
-
August 20, 2018
Linux System Administration (LFS301)
-
August 27, 2018
Open Source Virtualization (LFS462)
-
August 27, 2018
Linux Kernel Debugging and Security (LFD440)