Deep Dive into Node.js: Event Loop & Asynchronous Programming

Understanding the Single Thread

Node.js runs on a single thread, yet it handles thousands of concurrent connections. How? The secret lies in the Event Loop.

The Event Loop Phases

1. Timers: Executes setTimeout() and setInterval() callbacks.
2. Pending Callbacks: Executes I/O callbacks deferred to the next loop iteration.
3. Idle, Prepare: Internal use only.
4. Poll: Retrieves new I/O events; execute related callbacks.
5. Check: Executes setImmediate() callbacks.
6. Close Callbacks: e.g., socket.on('close', ...).

Microtasks vs Macrotasks

Understanding the priority of tasks is key to preventing blocked threads. process.nextTick() and Promises (Microtasks) have higher priority than setTimeout (Macrotasks).

console.log('Start');

setTimeout(() => console.log('Timeout'), 0);

Promise.resolve().then(() => console.log('Promise'));

process.nextTick(() => console.log('Next Tick'));

console.log('End');
// Output: Start, End, Next Tick, Promise, Timeout

Best Practices

• Don't block the Event Loop with heavy computation.
• Use Worker Threads for CPU-intensive tasks.
• Understand async/await thoroughly to avoid callback hell.