Is console.log atomic?

Question

The print statement in Python is not thread-safe. Is it safe to use console.log in Node.js concurrently?

If so, then is it also interleave-safe? That is, if multiple (even hundreds) of callbacks write to the console, can I be sure that the output won't be clobbered or interleaved?

Looking at the source code, it seems that Node.js queues concurrent attempts to write to a stream ( here ). On the other hand, console.log 's substitution flags come from printf(3) . If console.log wraps around printf , then that can interleave output on POSIX machines ( as shown here ).

Please show me where the async ._write(chunk, encoding, cb) is implemented inside Node.js in your response to this question.

EDIT: If it is fine to write to a stream concurrently, then why does this npm package exist?

Answer 1

Everything in node.js is basically "atomic". That's because node.js is single threaded - no code can ever be interrupted.

Answer 2

The events loop of nodejs is single thread, but all the async calls of nodejs are multi-threaded, it use libuv under the hood, libuv is library that use multi threads.

link: https://medium.com/the-node-js-collection/what-you-should-know-to-really-understand-the-node-js-event-loop-and-its-metrics-c4907b19da4c

Answer 3

Based on what I see on my Node.js console it is NOT "interleave-safe".

I can see my console-output is sometimes "clobbered or interleaved". Not always. When I run my program it is maybe every 5th time that I see interleaved output from multiple log-statements.

This may of course depend on your Node.js version and the OS you are running it on. For the record my Node.js version is v12.13.0 and OS is Windows 10.0.19042.