Here is what I would recommend:
Right click the solution and create a New Solution Folder called Common Javascript Files
(or whatever you feel like calling it.


Right click on the Solution, click Open Folder in Windows Explorer,
or navigate there manually for other versions of Visual Studio :(

In the solution directory, create a directory with the same name as the solution folder (solution folders do not normally match directories at the source code level but this will for sanity sake).

In this new directory, add files that need to be shared between solutions.

In Visual Studio, click the solution folder and select Add - Existing Item.

In the file selection dialog, navigate to the directory previous created, select the file(s) added to the directory and click Add.


In each Project that needs a shared file, right click on the project (or directory within the project) and click Add - Existing Item.

Navigate to the shared Directory, Select the files and click the drop down arrow then click Add As Link.

Now the files in the projects are essentially short cuts to the files in the Solution Folder. But they are treated as actual files in the project (this includes .CS or Visual Basic files, they will be compiled as files that actually exist in the project).

PROS
- Files are truly shared across projects at Design time
- Only the files needed for each project can be added, it's not all or nothing
- Does not require any configuration in IIS (virtual directory etc)
- If the solution is in TFS Source control, you can add the Directory to the TFS Source and the shared files will be source controlled.
- Editing a file by selecting it in the Project, will edit the actual file.
- Deleting a Linked file does not delete the file.
- This is not limited to JS files, linked files can be ANY file you might need (Images, Css, Xml, CS, CSHTML, etc)
CONS
- Each deployment gets it's own file.
- There is a small learning curve when understanding that Solution Folders are not Directories that exist in a Solution Directory.