JavaScript has ~sane~ fun rules around where variables can be redeclared or not,
and which kinds of variables this applies to. Actually the rules are pretty
straightforward:
* `var` can be redeclared any number of times.
* In strict mode, other declarations cannot be redeclared within the same scope.
This implies that a `var` declaration cannot conflict with these other forms,
which must take into account hoisting. So you can't have a `var a; let a` in the
same scope, but you also can't have a `let a` at a scope and then a `var a`
which will hoist to that same scope.