Is NodeJS really single-threaded?

No! Not really!

NodeJS has one main thread called event-loop and it also has a worker pool to handle blocking IO and CPU-intensive tasks. The default number of threads in the worker pool is 4, and you can change it.

Node.js uses a small number of threads to handle many clients. In Node.js there are two types of threads: one Event Loop (aka the main loop, main thread, event thread, etc.), and a pool of k Workers in a Worker Pool (aka the threadpool).
– Don’t Block the Event Loop (or the Worker Pool)

You can test it by yourself. Firstly, run the below code snippet, it gonna log out the PID – process id of the app.

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const http = require('http');
const hostname = '127.0.0.1';const port = 3000;
const server = http.createServer(
    (req, res) => {
        res.statusCode = 200;
        res.setHeader('Content-Type', 'text/plain');
        res.end('Hello World');
    }
);
server.listen(port, hostname, () => { console.log(`PID: ${process.pid}`);});

Next, execute the below command in the Linux terminal, it gonna print out the number of threads of a PID.

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ps huH p <PID> | wc -lv

For MacOS:

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ps -M PID | grep -v USER | wc -l

On my computer, the number of threads is 7. The number of threads varies according to different machines, but my point is that NodeJS use more than one thread.

References

https://kariera.future-processing.pl/blog/on-problems-with-threads-in-node-js/ https://stackoverflow.com/questions/61550822/why-node-js-spins-7-threads-per-process

References#

References