What does the “-?” syntax mean in TypeScript mapped types?

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What does the “-?” syntax mean in TypeScript mapped types?



Can anybody please explain what that "-?" in the following TypeScript type declarations
means, compared to just using a "?" there?


type ValidationMap<T> = [K in keyof T]-?: Validator<T[K]>




1 Answer
1



It's not a wildcard. The -? notation was added in TypeScript 2.8 as a way of removing the optional modifier from mapped types.


-?



Basically, it means that ValidationMap<T> has the same keys as T, but none of the properties of ValidationMap<T> is optional, even if the corresponding property of T is. For example:


ValidationMap<T>


T


ValidationMap<T>


T


type Example = ValidationMap<a: string, b?: number >;
// type Example = a: Validator<string>, b: Validator<number



Here, Example has a required b property even though the type it's mapped from has an optional b property.


Example


b


b



(Note that if you change -? to just ? or to +?, it becomes the opposite... you would be adding the optional modifier. The ? notation was added in TypeScript 2.1 as part of the mapped types feature, while the +? notation was introduced in TypeScript 2.8 along with -?.)


-?


?


+?


?


+?


-?



Hope that helps. Good luck!






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