In today's browsers, it is possible to run parallelized code with the help of so-called web workers. The event loop of an individual browser window is, however, still only handled by a single thread.

Currently, JavaScript engines are single-threaded , which means that they cannot execute code in parallel. As a result, it is a requirement in practice to use a non-blocking model for executing IO operations. Otherwise, the browser would "freeze" during, for instance, the fetching of data from a server.

Another consequence of this single-threaded... See more