Event Loop (Node.js)

TL;DR

The Event Loop is what allows Node.js to perform non-blocking I/O operations despite JavaScript being single-threaded. Unlike the browser event loop, the Node.js event loop runs in specific phases, each with its own queue of callbacks.

Mental Model

(Note: Microtasks, like Promises and process.nextTick, are executed between every single phase!)

How It Works (The Phases)

1. Timers: Executes callbacks scheduled by setTimeout() and setInterval().
2. Pending Callbacks: Executes I/O callbacks deferred to the next loop iteration (e.g., TCP errors).
3. Poll: Retrieves new I/O events; executes I/O related callbacks (almost all, with the exception of close callbacks, timers, and setImmediate()). Node will block here if the other queues are empty.
4. Check: Executes setImmediate() callbacks.
5. Close Callbacks: Executes close events (e.g., socket.on('close', ...)).

Example

const fs = require('fs');

fs.readFile(__filename, () => {
  console.log('1. I/O Callback (Poll Phase)');

  setTimeout(() => console.log('2. setTimeout (Timers Phase)'), 0);
  setImmediate(() => console.log('3. setImmediate (Check Phase)'));
});

// Output order: 1 -> 3 -> 2
// Why? Once Poll finishes the readFile callback, it moves to the Check phase next!

Common Interview Questions

What is the difference between the Node.js Event Loop and the Browser Event Loop?

The browser event loop is simpler: it has one Macrotask queue and one Microtask queue. Node.js uses libuv, which splits the macrotask queue into distinct phases (Timers, Poll, Check) that execute in a specific round-robin order.

At what phase does Node.js spend most of its time?

The Poll phase. If there are no timers waiting, Node.js waits in the Poll phase for incoming I/O events (like HTTP requests) to arrive.