console.log stops printing in while loop

[kenorb]

Version

v16.13.1

Platform

Linux Ubuntu-ROG 5.15.0-47-generic #51-Ubuntu SMP Thu Aug 11 07:51:15 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

No response

What steps will reproduce the bug?

I'm using fake-words module (npm install fake-words) with the following simple code:

#!/usr/bin/env node
const fake = require("fake-words");
while (true) {
  console.log(fake.sentence());
}

When I run ./genwords.js, everything works as expected.

However when I pipe into external program (on Ubuntu shell), the generation of words stops after a second.

$ ./genwords.js | cat
...
(output generation stops after a second)
$ ./genwords.js | tee
...
(stuck as well)
$ ./genwords.js | pv -l
...
4.64k 0:00:13 [0.00 /s]

How often does it reproduce? Is there a required condition?

Always, on Linux and macOS, but not on Windows.

What is the expected behavior?

console.log printing lines constantly.

What do you see instead?

Nothing. Printing stops after a second.

Additional information

I've posted a question here, but this seems more like a bug.

[Trott]

Duplicate of #6379?

[Trott]

The behavior of console.log() in code (such as the while loop in your example) that never relinquishes control to the event loop (especially when piped to another process) in Node.js is a longstanding...uh...quirk. It's a lot harder to fix than you might think. Here's the relevant issue in the tracker: #6379

Or more specifically on the whole handling-when-piped-to-another-process issue: #1741

You can work around the issue by restructuring the code to relinquish control to the event loop. Here is one possibility.

#!/usr/bin/env node
const fake = require("fake-words");

function getWords() {
  console.log(fake.sentence());
  setImmediate(getWords);
}

getWords()

[kenorb]

You can work around the issue by restructuring the code to relinquish control to the event loop.

Thank you, that works as expected.