1. Definition

The Event Loop is a core mechanism in Node.js that allows it to handle multiple operations efficiently using a single thread. It continuously checks the call stack and callback queue and executes tasks without blocking the execution.

2. Key Idea

• Runs on a single thread
• Handles asynchronous operations
• Continuously checks task queues
• Executes callbacks when ready

3. How it Works

• Code is executed line by line in Call Stack
• Async tasks are sent to background (libuv)
• Completed tasks go to Callback Queue
• Event Loop pushes callbacks to Call Stack

4. Example

console.log("Start");

setTimeout(() => {
  console.log("Timeout Callback");
}, 0);

console.log("End");

👉 Output:

Start
End
Timeout Callback

👉 Even with 0ms delay, callback runs later because Event Loop processes it after the main code.

5. Event Loop Phases (Simplified)

• Timers (setTimeout, setInterval)
• I/O Callbacks
• Idle/Prepare
• Poll (fetch new I/O events)
• Check (setImmediate)
• Close Callbacks

6. Why This is Important

• Handles multiple requests efficiently
• Prevents blocking of application
• Core of Node.js performance
• Enables asynchronous programming

7. Advantages

• Efficient concurrency
• Better performance
• Handles large number of requests

8. Disadvantages

• Complex to understand
• Blocking code affects entire system