what is mean by event loop in node.js ? javascript event loop or libuv event loop?

Question

In Node.js we a lot talk about the event loop, so I want to know which event loop we are talking about, the Javascript event loop or the libuv event loop ? I guess libuv e

Answer 1

Currently Node uses the the event loop provided by libuv - namely its default event loop: uv_default_loop(). See: An Introduction to libuv by Nikhil Marathe:

A default loop is provided by libuv and can be accessed using uv_default_loop(). You should use this loop if you only want a single loop.

Note: node.js uses the default loop as its main loop. If you are writing bindings you should be aware of this.

There is a linuv architecture diagram on the Design overview page in the libuv API documentation:

In the past, libev's event loop was used in Node. See Understanding the node.js event loop by Mikito Takada:

Internally, node.js relies on libev to provide the event loop, which is supplemented by libeio which uses pooled threads to provide asynchronous I/O. To learn even more, have a look at the libev documentation.

Some good resources on the Node event loop:

• Understanding the node.js event loop by Mikito Takada
• Understanding node.js by Felix Geisendörfer
• Understanding the Node.js Event Loop by Trevor Norris
• Node.js itself is blocking, only its I/O is non-blocking by Jeremy Epstein
• An Introduction to libuv by Nikhil Marathe
• The libuv API documentation

Thanks to Saúl Ibarra Corretgé for the clarification in the comments.

Answer 2

There is not just 1 event loop but different implementations of the event loop depending on the context. For example Chrome browser uses the event loop of the V8 JS engine. NodeJS uses the V8 engine but not it's event loop - it uses the Libuv event loop instead.

I made a video with a detailed explanation recently here: https://www.youtube.com/watch?v=4xsvn6VUTwQ