WhatsApp has been working on a standalone desktop app for Windows for quite some time now and it has finally launched it in beta version. It's available for download from Microsoft's Windows app store.
The app itself is based on the Universal Windows Platform (UWP) and notifications work even if the app is closed. There's also a new writing pad feature, with support for multi-device functionality, which means you don't need a phone to log into your account.
Of course, the app is still in beta so some are expected and more features will be implemented in the future.