Understanding the event loop

The event loop allows Node.js to continue handling JavaScript while asynchronous I/O is waiting to finish.

Simple mental model

• JavaScript executes on the main thread.
• I/O work can be handled outside the immediate JavaScript execution flow.
• Completed asynchronous work schedules callbacks or promise continuations.
• The event loop processes those tasks when appropriate.

This model makes Node.js effective for APIs and network-heavy applications, but CPU-heavy JavaScript can still block the event loop.

Deep dive

What can block the event loop?

Large synchronous file operations, expensive loops, complex calculations, and poorly designed CPU-heavy code can prevent Node.js from processing other callbacks quickly.

Practical rule

For an API endpoint, avoid unnecessary synchronous operations and expensive calculations. If CPU work is unavoidable, consider worker threads or moving that work to a background service.