What is the Node.js event loop?

The event loop lets a single Node.js thread serve many clients by pulling events off a queue and running each handler one at a time.

How it works:

1. HTTP requests and system events enter an event queue
2. The event loop takes events from the queue one at a time
3. Each event is processed (handler function runs)
4. Loop waits for next event when queue is empty

Important implications:

• Single event queue for all clients (like browser JavaScript)
• Keep handlers fast - slow handlers block all other requests
• Long-running tasks should be async (database, file I/O)

Diagram:

HTTP Requests → Event Queue → Event Loop → Process Event
System Events ↗                    ↓
                              Wait for next

Tip: Never do CPU-intensive work in a request handler - it blocks the entire server.

Go deeper:

• Node.js: The event loop, timers, and process.nextTick() — official breakdown of the loop's phases and why slow handlers stall everything.