The best bits of WinRT can be accessed from JavaScript thanks to WinJS — a new library for Windows 8 programming from Microsoft. The WinJS library gives HTML5 programmers the ability to get to all of the cool stuff in WinRT that might otherwise go against the grain of JavaScript development, like async and data binding.

Namespace What It Does
WinJS.Application Gives easy access to the higher-level DOM objects like storage and app eventing.
WinJS.Binding Data binding, HTML style.
WinJS.Class JavaScript might not have classes, but WinRT does! Makes your JavaScript more class-driven.
WinJS.Namespace If you have classes, you need namespaces to keep them in, right?
WinJS.Navigation The navigational model of Windows Store apps is wide and deep. Get around with this set of classes.
WinJS.Resources Core resources and localization can be found here.
WinJS.UI This is the biggest bit. All of the HTML controls that have styles for touch are in here, along with animation and the page model.
WinJS.Utilities Contains all of the helpers that smooth the transition from web development to Windows 8 development.