Why doesn't JavaScript support multithreading?

What is Multithreading?

Multithreading is a sort of execution model that enables different threads to exist inside the setting of a process with the end goal that they execute autonomously yet share their process resources. A thread keeps up a rundown of data important to its execution including the priority schedule, exception handlers, a set of CPU registers, and stack state in the location space of its hosting process. 

Threads in JavaScript:

JavaScript is a single threaded language which does not support multithreading. All the operations in JavaScript are non-blocking and are executed in the event loop. This single-threaded nature along with event loop allows JavaScript to handle concurrent operations at once.

Why doesn’t JavaScript support multithreading? 

JavaScript is a scripting language so it was only made to support the browsers. The borwsers in the past were not fast as compared to modern day browsers so JavaScript was kept single-threaded so that operations in the browsers does not get blocked due to multiple requests. The asynchronous behaviour of JavaScript helps prevent event blocking. Though modern day web browsers are fast and can even handle multi-thread operations but JavaScript is still single-threaded.

How to use JavaScript in a multithreaded manner?

Though there is a way in which we can write JavaScript in a multi-threaded manner which is by using the web workers. Web workers were introduced in 2009 when browsers became faster but there are some limitations to web workers as Web Workers are not allowed to interact with UI. Also, we can only pass values as copy in the function and use of global variables is not allowed

To learn more about web workers in JavaScript, refer to the article Web Workers in JavaScript