What are the main phases of the Node.js event loop?

Think of a security guard doing rounds in a hostel, visiting the same checkpoints in the same order, again and again. Each checkpoint has its own to-do list. At each stop he finishes the tasks that are ready and moves on.

The loop, run by a C library called libuv, visits these in order:

1. Timers: runs callbacks of setTimeout and setInterval whose time has come.
2. Pending callbacks: some deferred system-level callbacks, such as certain network errors.
3. Idle, prepare: internal use only.
4. Poll: gets new I/O events (file reads finished, sockets with data) and runs their callbacks. If nothing is ready, it may wait here.
5. Check: runs setImmediate callbacks.
6. Close callbacks: things like socket.on("close").

Between each callback, Node empties the process.nextTick queue and then the promise microtask queue.

const fs = require("fs");

fs.readFile(__filename, () => {
  setTimeout(() => console.log("timeout"), 0);
  setImmediate(() => console.log("immediate"));
});
// prints inside an I/O callback, always: immediate, then timeout

Inside an I/O callback the loop is in the poll phase, so the next stop is check, so setImmediate always wins.

• Believing the event loop is multi-threaded; callbacks run on one thread.
• Memorising phases without knowing where setTimeout and setImmediate sit.