What are the Node.js event loop phases?

Direct answer

libuv's loop runs timers → pending callbacks → idle/prepare → poll → check → close callbacks — with process.nextTick and Promise microtasks draining between every phase transition.

Each phase has a FIFO queue of callbacks. Node runs all callbacks in the current phase (or until a limit), then drains microtasks, then moves on — order matters for setTimeout vs setImmediate (Q7).

Phase	Runs
timers	setTimeout / setInterval callbacks whose threshold expired
pending callbacks	Deferred I/O callbacks (e.g. some TCP errors)
idle, prepare	Internal libuv housekeeping — ignore in apps
poll	Fetch new I/O events; execute I/O callbacks; may block waiting
check	setImmediate callbacks
close callbacks	e.g. socket.on('close') handlers

Between phases — process.nextTick queue drains completely, then Promise microtasks, then next phase.
Poll blocking — if no timers and no setImmediate, poll waits for I/O — efficient for servers.
Node 11+ — timers phase uses libuv timer refactor; behavior stable for interviews but mention version if asked.

phases-demo.js

import fs from "node:fs";

setTimeout(() => console.log("timers phase"), 0);

setImmediate(() => console.log("check phase"));

fs.readFile(__filename, () => {
  console.log("poll phase — I/O callback");
  setTimeout(() => console.log("timer inside I/O cb"), 0);
  setImmediate(() => console.log("immediate inside I/O cb"));
});