JavaScript's Single-Threaded Bliss: Because Who Needs Multithreading Anyway
Tags: javascript, webdev, interview, beginners
Published: 2024-01-31 08:15:53

JavaScript is a single-threaded, non-blocking, and asynchronous programming language. This means that it has only one execution thread, but it can handle asynchronous operations efficiently without blocking the execution of the entire program. This is achieved through a mechanism called the event loop.

## Event Loop:

![Event Loop](https://media.giphy.com/media/v1.Y2lkPTc5MGI3NjExZTA1ZmZ2aXN4YzdlNDJjZW43Y2pweHpkZnMwMTkwZ2FuaTJ1bTJxNSZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/2Fazad9Hs5xPAF6iQ/giphy.gif)

The event loop is a crucial part of JavaScript's **concurrency model**. It continuously checks the execution queue (also called the message queue) for new messages and processes them one by one.
The execution queue is a data structure that holds messages or events with their associated callback functions.

## Callback Queue:

![Callback Queue](https://media.giphy.com/media/v1.Y2lkPTc5MGI3NjExZndlbDdsYXg5b2czcnhvN3FudWNwbmM4NG93cHEwZ3BkZjR0dndtcCZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/35Kf7OnQCkPj2Ojtf6/giphy.gif)

When an asynchronous operation completes (such as a network request, a file read, or a timer), a message containing the associated callback function is placed in the callback queue.

## Execution Stack:

![Callback Queue](https://media.giphy.com/media/v1.Y2lkPTc5MGI3NjExczdpb2VwOXJ6bTkxaHNya2ZtY3B2OXJmdHZyYThqajE5NTNvb2l5eSZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/eG53NgJaxNHpJhldVx/giphy.gif)

The execution stack is a stack data structure that keeps track of the currently executing functions. When a function is called, it is pushed onto the stack. When it completes, it is popped off the stack.

## Non-Blocking Nature:

![Non-Blocking Nature](https://media.giphy.com/media/v1.Y2lkPTc5MGI3NjExdGdyY2Y5MW5xYTRlYnlrMzUyb3E2dTBwZWFjOWZvbWE1ZTgwaDk4bCZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/Sv9Z6bLestCgsyR7A9/giphy.gif)

JavaScript is non-blocking, meaning that while an asynchronous operation is in progress, the program doesn't wait for it to complete. Instead, it continues to execute other tasks.

JavaScript relies heavily on event-driven programming. Events, such as user interactions or the completion of asynchronous operations, trigger the execution of associated callback functions.

Here's a simple example to illustrate the event loop in action:

```
console.log("Start");

// Asynchronous operation (simulated with setTimeout)
setTimeout(function () {
  console.log("Async operation completed");
}, 2000);

console.log("End");
```

In this example, the "Start" and "End" messages will be logged immediately, and the asynchronous operation in setTimeout will be scheduled. The event loop will continue running, and after approximately 2 seconds, the callback function inside setTimeout will be pushed into the message queue and executed, logging "Async operation completed."

## Conclusion


![Conclusion](https://res.cloudinary.com/practicaldev/image/fetch/s--dhjH4Wt---/c_limit%2Cf_auto%2Cfl_progressive%2Cq_66%2Cw_800/https://devtolydiahallie.s3-us-west-1.amazonaws.com/gif14.1.gif)
(Illustration taken from https://dev.to/lydiahallie/javascript-visualized-event-loop-3dif)

This mechanism allows JavaScript to efficiently handle asynchronous tasks without blocking the main thread, making it suitable for tasks like handling user interactions, making network requests, and managing I/O operations.