An introduction to the JavaScript event loop

Node.js is an event-driven JavaScript runtime that allows you to build scalable and efficient server-side applications. One of the key features that makes this possible is the event loop.

In this article, we'll take a closer look at the event loop and how it works in Node.js. We'll also explore some of the key concepts and terminology that are important to understand when working with the event loop.

But first, let's define what the event loop is and why it's so important.

The event loop is a central mechanism in JavaScript that allows the runtime to execute code in a non-blocking manner. It does this by continuously checking for new events or tasks that need to be processed, and then executing them asynchronously. This allows Node.js to handle multiple requests concurrently and efficiently, without having to wait for one request to finish before moving on to the next.

The event loop operates in a single thread, but it uses multiple internal components to manage the flow of events. These components include the task queue, the microtask queue, and the event loop phases.

The task queue is where events or tasks that need to be processed by the event loop are placed. This could include things like I/O operations, timers, or other async events.

The microtask queue is a separate queue that holds microtasks, which are smaller tasks that need to be executed after the current task has been completed. This could include things like promise callbacks or process.nextTick callbacks.

The event loop phases refer to the different stages that the event loop goes through in order to process events and tasks. These phases include the following:

Timers phase: This phase is where timers that have been scheduled with setTimeout or setInterval are executed.

I/O callbacks phase: This phase is where callbacks for I/O operations (such as reading from a file or making an HTTP request) are executed.

Idle, prepare phase: This phase is a short, non-blocking phase that allows the event loop to do some internal bookkeeping.

Poll phase: This phase is where the event loop checks the task and microtask queues for events and tasks that are ready to be executed. If there are any tasks in the queues, they are executed in this phase.

Check phase: This phase is similar to the poll phase, but it's specifically for executing microtasks that are in the microtask queue.

Close callbacks phase: This phase is where callbacks for any closed resources (such as sockets or file descriptors) are executed.

After the event loop has gone through all of these phases, it will start again at the timers phase and repeat the process. This allows the event loop to continuously process events and tasks as they come in.

Examples of Async Code

One common use case for the event loop is to schedule timers or intervals. This is done using the setTimeout and setInterval functions, which allow you to specify a callback function that will be executed after a certain amount of time has passed.

For example, the following code sets a timer that logs a message to the console after 2 seconds:

setTimeout(() => {
  console.log('Hello, world!');
}, 2000);

Another common use case is to process I/O operations asynchronously. This is done using the fs module, which provides functions for reading and writing files.

For example, the following code reads a file asynchronously and logs its contents to the console:

const fs = require('fs');

fs.readFile('my-file.txt', (err, data) => {
  if (err) throw err;
  console.log(data);
});

Finally, Node.js also provides the process.nextTick function, which allows you to execute a callback function as soon as possible. This is useful if you need to execute a callback before any other I/O operations that may be pending.

For example, the following code will log a message to the console before any I/O operations that may be pending:

process.nextTick(() => {
  console.log('This will be executed first!');
});

As you can see, the event loop is a powerful and essential feature of Node.js that allows you to build scalable and efficient server-side applications. Now that you have a better understanding of how the event loop works, you'll be better equipped to use it in your own applications.