The event loop is what makes Node.js non-blocking and efficient. It handles asynchronous operations by delegating tasks to the system and processing their results through callbacks, allowing Node.js to manage thousands of concurrent connections with a single thread.

How the Event Loop Works:

• Execute the main script (synchronous code)
• Process any microtasks (Promises, process.nextTick)
• Execute timers (setTimeout, setInterval)
• Run I/O callbacks (file system, network operations)
• Process setImmediate callbacks
• Handle close events (like socket.on('close'))

This demonstrates the execution order:

console.log('First');
setTimeout(() => console.log('Third'), 0);
Promise.resolve().then(() => console.log('Second'));
console.log('Fourth');
        

Execution Order:

• Sync code runs first: 'First', 'Fourth'
• Microtasks (Promises) run before the next phase: 'Second'
• Timers execute last: 'Third'

The event loop processes different types of callbacks in this order:

• Timers: setTimeout, setInterval
• I/O Callbacks: Completed I/O operations
• Poll: Retrieve new I/O events
• Check: setImmediate callbacks
• Close: Cleanup callbacks (like socket.on('close'))

Note: Between each phase, Node.js runs microtasks (Promises) and process.nextTick callbacks.

console.log('1. Start');

// Next tick queue
process.nextTick(() => console.log('2. Next tick'));

// Microtask queue (Promise)
Promise.resolve().then(() => console.log('3. Promise'));

// Timer phase
setTimeout(() => console.log('4. Timeout'), 0);

// Check phase
setImmediate(() => console.log('5. Immediate'));

console.log('6. End');
        

Expected Output:

1. 1. Start
2. 6. End
3. 2. Next tick
4. 3. Promise
5. 4. Timeout
6. 5. Immediate

This shows the priority order: sync code > nextTick > Promises > Timers > Check phase.

The event loop enables Node.js to handle thousands of concurrent connections with a single thread, making it perfect for:

• Real-time applications
• APIs and microservices
• Data streaming
• Chat applications

• Node.js uses an event loop to handle async operations
• Different types of callbacks have different priorities
• Microtasks (Promises) run before the next event loop phase
• This non-blocking model enables high concurrency