To be fair

Gottcha! 😱

If you were anything like me, you would’ve come here thinking:

But that’s only partially true! Bare with me.

Node.js’s Main Thread is indeed single threaded, simply because it’s plain JavaScript. There’s no other way.
This very thread is the one that’s executing all user written code (and the place where the Event Loop is doing its magic!)

Yet there are also a lot of moving parts of Node.js that aren’t necessarily single threaded.

Going from here we’re about to act on OS level and therefore we can of course leverage multi threading!
libuv is managing something that is called a “thread pool”.
Each of those threads in this pool can get assigned (by the event loop) a specific task from your Event Queue and work on it concurrently.

Alrighty, this one takes a bit less than a second to finish.
So, in a truly single threaded world this means if I call the “doExpensiveHasing” function 4 times it would take a total time of about 4 seconds (each one about one second), right?

Let’s see than.

Jup, you just saw multiple threads kicking in inside of a Node.js application.
To prove that it would be different if it actually executed on one thread only, we can change our environment variable:

As you can see we JavaScript developers have to be careful when claiming Node.js would be entirely single threaded because de facto it isn’t.
And this is actually a good thing.
Imagine having the power of asynchronous code but only executing it in a single thread.

Knowing this seems to be part of really understanding Node.js and hence writing performant code and leveraging its real powers.

Thank you for reading!
Please let me know if there is something unclear or wrong. 🤓

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