How JavaScript is Single Threaded

How JavaScript is Single Threaded

What is Single Threaded JavaScript?

JavaScript is a single-threaded language, meaning all code must be executed in a single-thread or a single line of execution. This thread is known as the main thread, or the "main loop." This single thread is responsible for all the operations of a website, including the execution of code and the loading of resources.

In other words, all tasks in JavaScript must be completed in their order of arrival. If a task is pending, no other task can proceed until the pending task is completed. Such a mechanism is known as single-threaded execution, and it helps keep resources in check.

Example of Single Threaded JavaScript

Consider the following code:

const request = new XMLHttpRequest();'GET', '');

setTimeout(() => {
  console.log('The request finished!');
}, 1000);

Here we have two separate tasks that need to be completed. First, an XMLHttpRequest is sent to fetch data from a server. Then, we have an asynchronous function (setTimeout) that will run after one second. However, due to the single-threaded nature of JavaScript, the second task will not run until the first one is completed.

In other words, the setTimeout callback won't fire until the XMLHttpRequest completes. Even if the setTimeout callback is scheduled to run after the XMLHttpRequest is initiated, the callback cannot run until the request is finished.

Pros and Cons of Single Threaded JavaScript

The single-threaded nature of JavaScript has both advantages and disadvantages. On one hand, it allows us to write code in a more simplified manner since there is no need to worry about thread synchronization. On the other hand, it can cause performance issues when faced with long-running tasks or heavy computations.

  • Pros:
  • Simplifies code by removing thread synchronization
  • Improved performance for short operations
  • Cons:
  • Poor performance for long operations
  • Can cause web pages to freeze up if blocked by an intense computation


In conclusion, JavaScript is a single-threaded language. This means that all tasks must be completed one after the other, with no two tasks able to run concurrently. This ensures that resources are managed properly and that the code is written in an organized manner. However, it can also lead to performance issues when dealing with long-running tasks or intense computations.

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