What are the key points to understand about Node.js Event Loop?

Node.js event loop has 6 phases (not just micro/macro queues). Timers phase: executes setTimeout/setInterval callbacks. Poll phase: retrieves new I/O events, executes I/O callbacks. Check phase: executes setImmediate callbacks. process.nextTick runs BETWEEN phases (highest priority). Blocking the event loop freezes your entire server

What are common mistakes with Node.js Event Loop?

Confusing browser event loop with Node.js event loop. Using recursive process.nextTick (causes starvation). Blocking the event loop with CPU-intensive code. Assuming setTimeout(fn, 0) runs immediately

What are the interview tips for Node.js Event Loop?

Know all 6 phases of the libuv event loop. Explain the difference between setImmediate and setTimeout. Understand why process.nextTick exists and when to use it. Know how to handle CPU-intensive tasks (Worker Threads)

♻️

Node.js Event Loop

advanced

Node.js uses libuv to implement its event loop, which has 6 distinct phases. Understanding these phases helps you predict execution order and avoid blocking the server.

🎮Interactive Visualization

Codesync

1setTimeout(() => {
2  console.log('timeout');
3}, 0);
4 
5setImmediate(() => {
6  console.log('immediate');
7});
8 
9console.log('sync');

process.nextTick

(empty)

Promise Queue

(empty)

Timers

Pending

Poll

Check

Step 1/6Script starts. Node.js event loop initializes.

Key Insight: In main module, setTimeout vs setImmediate order varies! Inside I/O callbacks, setImmediate always runs first.

Key Points

Node.js event loop has 6 phases (not just micro/macro queues)
Timers phase: executes setTimeout/setInterval callbacks
Poll phase: retrieves new I/O events, executes I/O callbacks
Check phase: executes setImmediate callbacks
process.nextTick runs BETWEEN phases (highest priority)
Blocking the event loop freezes your entire server

💻Code Examples

Event Loop Phases

// Phase 1: Timers (setTimeout)
setTimeout(() => console.log("timer"), 0);

// Phase 4: Poll (I/O callbacks)
fs.readFile("file.txt", () => {
  console.log("file read");
});

// Phase 5: Check (setImmediate)
setImmediate(() => console.log("immediate"));

// Between phases: process.nextTick
process.nextTick(() => console.log("nextTick"));

// Output order depends on I/O!

Different callbacks run in different phases

setTimeout vs setImmediate

// In main module: order varies!
setTimeout(() => console.log("timeout"), 0);
setImmediate(() => console.log("immediate"));

// Inside I/O callback: immediate first!
fs.readFile("file.txt", () => {
  setTimeout(() => console.log("timeout"), 0);
  setImmediate(() => console.log("immediate"));
});
// Output: immediate, timeout
// (Check phase runs before Timers)

setImmediate runs before setTimeout in I/O callbacks

process.nextTick

// nextTick runs BETWEEN phases
// (even before Promises!)

Promise.resolve().then(() => {
  console.log("promise");
});

process.nextTick(() => {
  console.log("nextTick");
});

// Output: nextTick, promise
// nextTick has highest priority!

process.nextTick runs before Promise callbacks

nextTick Starvation

// DANGER: Recursive nextTick
// blocks the event loop forever!

function recurse() {
  process.nextTick(recurse);
}
recurse();

// I/O callbacks will NEVER run
// Server becomes unresponsive!

// Fix: use setImmediate instead
function safeRecurse() {
  setImmediate(safeRecurse);
}

nextTick can starve I/O if abused

Blocking the Event Loop

// BAD: Blocks entire server!
app.get("/slow", (req, res) => {
  // 10 billion iterations
  for (let i = 0; i < 1e10; i++) {}
  res.send("done");
});

// GOOD: Use Worker Threads
const { Worker } = require("worker_threads");

app.get("/fast", (req, res) => {
  const worker = new Worker("./heavy.js");
  worker.on("message", () => res.send("done"));
});

CPU-heavy work blocks all requests

Event Loop Monitoring

// Monitor event loop lag
const start = process.hrtime();

setImmediate(() => {
  const [s, ns] = process.hrtime(start);
  const lag = s * 1000 + ns / 1e6;
  console.log(`Event loop lag: ${lag}ms`);
});

// High lag = event loop blocked
// Target: < 100ms for responsive server

Monitor lag to detect blocking issues