Not only the source code of the Viewer is available publicly but the source code of all the extensions provided for it as well. It can be useful to have a look inside the code if you are facing some issues when using them.
When you load an extension, it will show up on the "Sources" tab of the browser's Developer Tools. Depending on the site the base URL might be different but they are in the same "extensions" folder
If the minified version of the file is referenced from the given site then you can just remove the ".min" part from the URL to get to the full source code.