Visual Studio 2013 Update 2 introduced the concept of a Shared Project for use with "Universal Apps", i.e. Windows + Windows Phone, though the mechanism is also used by Xamarin to share code for iOS and Android apps. It can also be used independently of universal apps by installing the Shared Projects Resource Manager Visual Studio extension.
Using the Shared Projects Resource Manager, can Shared Projects be used to share resources such as Javascript, rather than .NET code, between different Web projects in the same solution?