How Does the Node.js Event Loop Work? How to Avoid Blocking?

Node.js Event Loop

Core Concept

Node.js is single-threaded but non-blocking, using the Event Loop for async I/O.

Six Event Loop Phases (in order)

1. timers: Execute setTimeout / setInterval callbacks
2. pending callbacks: Execute deferred I/O callbacks
3. idle, prepare: Internal use
4. poll: Retrieve new I/O events; run I/O callbacks
5. check: Execute setImmediate callbacks
6. close callbacks: Run socket.on("close") etc.

Microtasks Run Before the Next Phase

• Promise.then / queueMicrotask → highest priority
• process.nextTick → higher than Promise (Node.js-specific)

Execution Order

process.nextTick → Promise.then → setTimeout → setImmediate

Common Blocking Causes and Fixes

Problem	Solution
CPU-intensive sync operations	Worker Threads / child_process
Large JSON parsing	Split + setImmediate to yield control
Synchronous fs calls	Switch to fs.readFile (async)