Have you ever wondered what is an “Event Loop”

In most of the recent interviews this question is asked and if you wonder what an “Event loop” is, here is an answer.

The event loop is a fundamental concept in JavaScript that governs the way that the language handles asynchronous code. Understanding the event loop is essential for writing efficient and scalable JavaScript applications.

The Basics of the Event Loop

At a high level, the event loop is a mechanism for handling and prioritizing code execution in JavaScript. When a script is executed in a browser or a Node.js environment, it runs on a single thread. This means that only one task can be executed at a time, and any blocking operation will prevent other tasks from being executed until the blocking operation is complete.

To avoid blocking, JavaScript uses an event loop to handle asynchronous operations. The event loop is essentially a loop that runs continuously and checks for tasks that are waiting to be executed. These tasks are added to a queue, and the event loop runs through the queue, executing each task in order.

const one = () => console.log('one');
const two = async () => {
    console.log('Before executing three')
    await three();
    console.log('After executing three');
}
const three = () => console.log('three');

one();
two();
three();

// In this example three() will be executed twice before logging 'After executing three'

Tasks can be added to the queue in a variety of ways. For example, when an asynchronous function is called, its callback function is added to the queue. Similarly, when an event listener is triggered, its callback function is added to the queue. In this way, the event loop can handle a wide variety of asynchronous operations, allowing JavaScript to be used for complex and dynamic applications.

The Phases of the Event Loop

The event loop is made up of several different phases, each of which is responsible for a different aspect of handling asynchronous code. These phases include:

1. Timer Phase: This phase handles setTimeout() and setInterval() functions, which add timed callbacks to the queue.
2. Pending Callback Phase: This phase handles callbacks that are generated by I/O operations, such as network requests or file system operations.
3. Idle, Prepare, and Poll Phases: These phases are responsible for waiting for events to occur, such as user input or other system events.
4. Check Phase: This phase handles setImmediate() functions, which add immediate callbacks to the queue.
5. Close Callbacks Phase: This phase handles callbacks that are generated by close events, such as when a server connection is closed.

By breaking down the event loop into these phases, JavaScript can efficiently manage asynchronous code and ensure that tasks are executed in the correct order.

The event loop is a critical component of JavaScript that allows the language to handle asynchronous code efficiently and effectively. By understanding how the event loop works and following best practices for working with it, you can write more efficient and scalable JavaScript applications.