1. Definition

The Event Loop in Node.js works in multiple phases. Each phase has a specific purpose and a queue of callbacks to execute. The loop goes through these phases continuously to process asynchronous operations.

2. Phases Overview

• Timers
• I/O Callbacks
• Idle / Prepare
• Poll
• Check
• Close Callbacks

3. Timers Phase

Executes callbacks scheduled by setTimeout() and setInterval(). These callbacks run after the specified delay (not exactly at that time).

4. I/O Callbacks Phase

Executes callbacks for some system operations like TCP errors or file system operations that were deferred from previous cycles.

5. Idle / Prepare Phase

Internal phase used by Node.js. It is not directly accessible by developers.

6. Poll Phase (Most Important)

• Fetches new I/O events
• Executes I/O callbacks
• Waits if no tasks are available
• Core phase where most work happens

7. Check Phase

Executes callbacks scheduled by setImmediate().

8. Close Callbacks Phase

Executes close events like socket.on('close').

9. Example

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

setImmediate(() => console.log("Immediate"));

console.log("Start");

👉 Output may vary depending on the phase execution order.

10. Execution Order (Simplified)

Timers → I/O → Idle → Poll → Check → Close

11. Why This is Important

• Helps understand async behavior
• Important for debugging
• Frequently asked in interviews