Is JavaScript single-threaded or multi-threaded? How does asynchronous work?

JavaScript is a single-threaded language, meaning all code executes in one thread. However, asynchrony is achieved through the event loop mechanism and the task queue.

When an asynchronous operation (such as setTimeout, fetch, promises) is called, it is handed over to the browser or runtime environment (e.g., Node.js), where it runs in parallel (in another thread or system API). After the operation completes, the result is placed in the task queue. The event loop monitors the main thread and, when it is free, takes tasks from the queue and executes them.

Thus, JavaScript does not create new threads for asynchronous operations but uses a single-threaded model with an event queue, which allows it to avoid blocking the main thread and efficiently handle asynchronous actions.