What do the uv__run_idle and uv__run_prepare operations do in node.js?

[dSalieri]

I've read a lot of articles online and also read the node.js documentation and also read an overview of the libuv.

uv__run_idle (loop)
uv__run_prepare (loop)

What says libuv:

5. Idle handle callbacks are called. Despite the unfortunate name, idle handles are run on every loop iteration, if they are active.
6. Prepare handle callbacks are called. Prepare handles get their callbacks called right before the loop will block for I/O.

Can idle callbacks and prepare callbacks be created via javascript on node.js platform? If not, then why are they and what do they affect?

What node.js doc and lot of articles say:

idle, prepare: only used internally.

And what does it mean? That is, on the node.js platform we cannot reach these pointed stages in any way?

Then there are two questions:

1. What do these operations actually do? (their definition is similar to that of uv__run_check(loop) and this stage is somewhere tied to setImmediate processing but I don’t know where).
2. If you remove uv__run_idle(loop) and uv__run_prepare(loop) from the node.js codebase, will something break or what will this affect?

I still haven't seen answers to these questions, which is very strange because the (idle, prepare) phase in the node.js documentation is quoted from every corner, but no one knows what this phase does, which is sad.

P.S. I looked at the source code in libuv, but it didn’t give me anything, because you have to know the structure of node.js and its components.

[mateuspaulino]

@dSalieri Hey there! Sadly, no, you can't create idle and prepare callbacks directly via JavaScript in Node.js.

So, here's the deal:

Node.js documentation and many articles say:

Idle, prepare: only used internally.

What does this mean?

This means that as a developer using Node.js, you won't have direct access to these stages (idle and prepare) in your code. They're part of the internal mechanics of Node.js, specifically managed by libuv (the library Node.js uses for its asynchronous I/O).
Questions:

1 - What do these operations actually do?
Idle handles: These are called on every loop iteration if they are active. They let the event loop perform certain actions when there are no other tasks to handle.
Prepare handles: These are called right before the event loop will block for I/O operations. They help the loop get ready for any incoming I/O.

Their role is somewhat similar to uv__run_check(loop), and they're part of the phases in the event loop where things like setImmediate might be processed. However, the exact connection to setImmediate isn't clearly documented.

2 - If you remove uv__run_idle(loop) and uv__run_prepare(loop) from the Node.js codebase, will something break or what will this affect?

If you yank these functions out of the Node.js codebase, it could mess with the internal workings of the event loop. These handles make sure the event loop runs efficiently, managing idle times and getting ready for I/O operations. Without them, the event loop might not perform as well, and you could see issues with how asynchronous tasks are handled.

In a nutshell, while you can't create or manipulate idle and prepare callbacks directly in your JavaScript code, these internal handles are super important for the smooth operation of the Node.js event loop. They keep things running efficiently by handling idle times and preparing for I/O, making sure your Node.js applications perform well and stay reliable.