Node.js Event Loop Explained: How It Works

Understanding the Node.js Event Loop

Node.js uses the event loop to carry out asynchronous, non-blocking I/O operations, whereas JavaScript is only single-threaded. Whenever feasible, it assigns these tasks to the system kernel. The majority of contemporary operating system kernels are multi-threaded, which enables them to manage several tasks concurrently. The kernel alerts Node.js when an operation is finished so that the relevant callback can be added to the poll queue for execution.

How the Node.js Event Loop Works

The event loop starts as soon as Node.js is initialized, either by running a .js script or in REPL mode. It follows a specific sequence of operations:

1. Timers Phase
   • Executes callbacks scheduled by setTimeout() and setInterval().
2. Pending Callbacks Phase
   • Executes I/O callbacks that were deferred to the next loop iteration.
3. Poll Phase
   • Determines how long to block and wait for I/O events.
   • Processes events in the poll queue and executes I/O-related callbacks.
4. Check Phase
   • Executes callbacks scheduled by setImmediate().
   • If the poll phase is idle and scripts are queued with setImmediate(), the event loop moves directly to this phase.
5. Close Callbacks Phase
   • Handles callbacks registered with the close event, such as socket.on('close', function).
   • If a socket is closed abruptly, the close event is triggered. Otherwise, process.nextTick() defers execution to the next iteration of the event loop.

Role of Libuv and V8

• The V8 engine executes JavaScript code.
• The Libuv library enables Node.js to handle asynchronous operations using the system’s native mechanisms.

Event Loop Execution and Shutdown

Node.js looks for scheduled timers or pending asynchronous I/O activities before beginning a new iteration. The runtime ends cleanly if there are none.

The Significance of the Event Loop

Developing scalable and effective Node.js apps requires an understanding of the event loop. It guarantees that the single-threaded execution architecture of JavaScript stays non-blocking, enabling applications to manage concurrent tasks with ease.

It might be helpful:

What’s New in Node.js 21/22? 

Best Practices for API Design in Full-Stack Development

What is REST API with an example?