I think on mutable distros, or at least arch, you can run a command to reinstall all installed packages, which will verify integrity of the package files (signatures) and then ensure the files in the filesystem match package files? And I think it takes minutes at most, at least for typical setups.
I do think it’s also possible to just verify integrity of all files installed from a package, but I don’t remember if it required an external utility, pretty sure it’s on the arch wiki under pacman/tips and tricks
JSX is JavaScript, but you can also just put HTML in it (with bonus syntax for embedding more JS expressions inside) and it can get transpiled into function calls, which means it’ll result in an object structure representing the HTML you wrote. It’s used so that you can write a component as a function that returns HTML with properties already computed in and any special properties, like event listeners, passed as function references contained in the structure.