What does the `-?` (dash question) and `-readonly` (dash readonly) syntax mean in TypeScript?

Question

I am new to TypeScript and saw the following in the @types/prop-types package. What does -? mean in [K in keyof T]-??

export type ValidationMap<T> = { [K in keyof T]-?: Validator<T[K]> };

Similarly, what does -readonly [K in keyof T]? mean in immer?

export type WritableDraft<T> = {-readonly [K in keyof T]: Draft<T[K]>}

I tried my best to google that, but failed.

Answer 1

Homomorphic mapped types copy the optionality of the original type field to the mapped type field. The -? is the syntax used to explicitly remove any optional modifier from the resulting mapped type.

Similarly, -readonly removes the "readonlyness".

You can read more details here.