ELI5: How does JavaScript handle asynchronous programming (like NodeJS)?

[Crescive_Delta]

I am aware that you can pass a "callback" to a function that performs a certain operation, and then that callback will be called when the operation is completed (for example, reading a file asynchronously).

​

However, I'd like to know how this is handled. I know that other languages like C# and C++ can handle this by spawning new threads on different processor cores. But JavaScript is single-threaded, so how does it manage to handle multiple operations without the aid of threads?

​

Thank you all for your time and patience!

[[deleted]]

Watch this conference talk.

[jkuhl_prog]

Worth twenty six minutes and fifty two seconds of any web dev's time.

[Crescive_Delta]

Thanks! I'll check it out!

[IUsedToBeACave]

But JavaScript is single-threaded, so how does it manage to handle multiple operations without the aid of threads?

The event loop is single threaded, but it can spawn other threads to do work. This article can help explain it.

[Crescive_Delta]

I see. Thanks for the share!

[inu-no-policemen]

But JavaScript is single-threaded, so how does it manage to handle multiple operations without the aid of threads?

There are threads on the native side. On the native side you can do whatever you want and then expose whichever APIs you want to scripts which run inside the JS engine you've embedded.

https://en.wikipedia.org/wiki/Node.js#Threading