How to prevent html/JavaScript code modification

Solution 1:

It is not possible (which is a very good thing).

Solution 2:

To defeat the method described in the question:

• You can use the keyboard shortcuts to the console/tools (Ctrl+Shift+I in Chrome)
• You can use the resource/net panel to see the source
• You can see it at any other level, e.g. Fiddler
• You can use a bookmarklet for easier access

No, you can't prevent people seeing or modifying your source/script if they want to...the ones you most want to prevent are the most able to circumvent any deterrent (and that's all anything you do is, a deterrent, not a stop) you put in place.