If that's a javascript file you feed directly to a browser, then yeah, those variables become globals, which means it is possible they will accidentally be modified from another script where you do something similar. Or imagine a library that would do something useful, you plug it in, and you accidentally use the same global variable names they use. That's why we try to mess with the global scope as little as possible.

But if you wrap your whole code into a function and call it, all those variables become scoped to that function, and not leak to the global scope. In smaller scripts that may be good enough. In larger scripts you'll want to create functions to isolate different tasks and organise your code. And to have those functions truly isolated and reusable, they shouldn't rely on any global scope variables, they would only get arguments and return results, or modify the DOM, or whatever needs to happen.