If you're willing to use dev tools, then in principle users can probably do pretty much anything to your project. You can try to hide things, but then you can use the debugger to break inside engine code and run more commands. You can even use dev tools or browser extensions to rewrite the JavaScript code and remove protections. You can even go lower-level and use advanced tools to identify values in RAM like for the score or lives and overwrite them.
In general normal players just don't do that, so I don't think it's ever been a major concern. Trying to remove various interfaces could also break your project, so it's risky to do that. If you're concerned about cheating, yes, people can cheat in single-player games, and have always been able to do so pretty much regardless of the technology, unless you run on a totally secured and locked down console platform. If you're concerned about cheating in multiplayer games, in a well-designed system with an authoritative server, peers cannot cheat regardless of what the client does. That's about how it is and how it's always been for the industry.