Deep Dive into Node.js Event Loop

Node.js event loop is a fundamental concept for understanding how Node.js handles asynchronous operations. It's a programming construct that allows a program to run many operations concurrently without blocking the main thread.

What is an Event Loop?

An event loop is a piece of code that runs in the background. It keeps track of all the events that occur in an application and executes the corresponding callback functions when they are ready. Here's a simple list of what an event loop does:

Receives Events: The event loop receives events from the operating system and other sources.
Queues Events: It queues these events for processing.
Executes Callbacks: When an event is ready (e.g., a network request has completed), the event loop executes the callback function associated with that event.

Node.js Event Loop Phases

The Node.js event loop has several phases, each with its own purpose:

Timers: Executes functions scheduled by setTimeout() and setInterval().
IO Events: Executes I/O-bound and low-level API callbacks.
Poll: Executes callbacks from the process.nextTick() and poll phase.
Idle: Runs cleanup tasks.
Close: Executes when streams are destroyed.

Example

Here's a simple example to illustrate how the event loop works:

setTimeout(() => {
  console.log('Timer callback');
}, 0);

console.log('Immediate execution');

// Output:
// Immediate execution
// Timer callback

In the above example, the console.log('Immediate execution') is executed immediately because it's not within any asynchronous operation. The setTimeout() callback is queued in the event loop and will be executed after the immediate execution.

Learn More

If you want to dive deeper into the Node.js event loop, we recommend reading our comprehensive guide on the topic.