Something you run into when adding Typescript annotations to existing JS / libraries in the wild is when the signature turns out to be:

    function transform(a: number | string): number | "" | false

And it's usually all by accident. And it works because in practice it's used with a `if (!transform(x))` check.

Static typing doesn't just tell you what's going on, but it also helps you keep your implementation honest with your intent. It's also why it's a good habit to annotate top-level functions instead of relying on their inference.