It's an easy thing to do in JS, it happens if you forget the `var`.

only when not in strict mode, which is becoming more and more rare, especially in javascript codebases (as opposed to one-off scripts)

I think this is true because they hold a reference in window.

Sure that will prevent gc, but that doesn't mean it's a memory leak

It is if the variable is accidentally in global scope due to hoisting if it is not explicitly defined in a finer scope. Not an issue in strict mode, you'll get a reference error instead of an implied global.

If you're never going to use it again, and you're never going to free it, it's a memory leak.

No, it is not. A memory leak has to grow. Without the leak part, it is just ...allocated memory.