Prevent changes or manipulate to front end/client side codes!

Because front end codes are exposed to the users/site visitors and I can’t hide it, can a malicious user/visitor some how break or change my website’s/web application’s codes or ui through something like developer tools or some other similar tools?
And when they some how break or change the website/web application using those tools, are they only change or break their own copy of the website/web application or my original website/web application (which when other good users/visitors visit my site they’ll see those changes that malicious user/ visitor did )?

(Like I mentioned above, front end codes are exposed to the users/site visitors and I can’t hide it, so malicious user/visitor can easily change or add or remove any kinds of codes that is running on my website/ web application, like variables, arrays, objects, etc… or change, remove html element’s classes and etc… so any ways that I can implement to protect my front end/ client side codes?)

Right now, I am not talking about any kinds of back end things or codes, only talking about front end/client side things and codes.

Doesn’t matter. Its not a security issue, if it was, don’t you think someone would have noticed and done something about it?

Even when your website only consists of html, css and JS. You will still need a server to serve these files. Once the client receives the page, it can make any changes it likes. Doesn’t matter, a new request will still serve your original files. What is send to the client can be viewed as a “copy”. Any changes made to a copy, doesn’t affect the original (the files on the server)

it becomes a security issue when someone is able to modify the files on the server, because then other clients will get the malicious version.