Are WinJS/Metro Apps single threaded?-Collection of common programming errors

It’s a multi part answer.

Traditionally – yes it is single threaded. Async is done behind the scenes with timers. However with the advent of web workers, you can run background processing on multiple threads (or use a Windows Runtime component that does threading and relies on .NET for the threading here).

Web Workers are supported in Windows Store HTMl/JS applications so yes, you can have more than one thread.

Web Workers