In the JavaScript "user space" you don't have access to threads (with the exception of web workers), but the JS engines themselves are multi-threaded and fully

In the JavaScript "user space" you don't have access to threads (with the exception of web workers), but the JS engines themselves are multi-threaded and fully concurrent. This is why your UI doesn't freeze when doing XHR (assuming you're not doing synchronous XHR) and represents the need for elegantly handling asynchronicity.