TypeScript What does this grammar '-?' (dash question) mean?

Question

guys.

I am new to typescript and saw '-?' grammar in @types/prop-types package

e.g

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

what does '[K in keyof T]-?' mean?

I tried my best to google that, but failed.

Thank you.

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. You can read more details here