not operator (anything but)

tl;dr:

(param: Any) => not String
// shorthand
(param: Any) => !String

// union case
(param: Any) => not(String | Array)
// short hand
(param: Any) => !(String | Array)

Sometimes it’s way easier and convenient to define the type by discarding one or several. There’s also the possibility to define it as a substraction—e.g. Any - String—but that would not be very conventional.

TypeScript approaches

• substraction: https://github.com/Microsoft/TypeScript/issues/4183
• exclude: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#predefined-conditional-types
• negation: https://github.com/Microsoft/TypeScript/issues/18280

[ericelliott]

I like it.

[Mouvedia]

newly introduced types may be considered acceptable by the type system, even if they’re not acceptable types in practice

That’s to be expected. Let’s say you had !Array before typed arrays were introduced and it doesn’t cover Int16Array, I wouldn’t blame rtype. The type will have to be updated to !Array & !TypedArray; it’s such a rare occurence that I don’t mind.