[KevinH]

Here is the easiest way to deal with a set of js files that need a specific home:

1. Use Add Existing to add all of the js files. They will show up in the OEBPS/Misc folder.

2. Select them in BookBrowser and right click to select Move ...

3. It will prompt you for epub bookpath for the destination folder. You can use the pull down to pick any existing folder or simply type in a full bookpath for your new destination folder:

In this case I typed "OEBPS/js"

Once the move is complete, the js files will be present in only that OEBPS/js/ folder.
The opf will be appropriately updated.

They will still appear under the Misc virtual Folder in BookBrowser but showing the full book relative path will show you where those files are physically located.