Why doesn't JavaScript support multithreading?

JavaScript used to be single-threaded. It runs using what is called an event loop.

The Event Loop has one simple job — to monitor the Call Stack and the Callback Queue. If the Call Stack is empty, it will take the first event from the queue and will push it to the Call Stack, which effectively runs it.

JS in browsers doesn't support multithreading in the event loop as it is not needed for 99.999% of the websites. The event loop handles everything seamlessly.

For the remaining apps, devs can use web workers. Web Workers are a simple means for web content to run scripts in background threads. The worker thread can perform tasks without interfering with the user interface. In addition, they can perform I/O using XMLHttpRequest (although the responseXML and channel attributes are always null). Once created, a worker can send messages to the JavaScript code that created it by posting messages to an event handler specified by that code (and vice versa). This article provides a detailed introduction to using web workers.