What does -? mean in TypeScript?

Question

I came across this line in the type definitions for prop-types:

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

Without - it would be a pretty standard partial mapped type, but I can't find anywhere in the docs where it talks about -?.

Can anyone explain what -? means?

Answer 1

+ or - allows control over the mapped type modifier (? or readonly). -? means must be all present, aka it removes optionality (?) e.g.:

type T = {     a: string     b?: string }   // Note b is optional const sameAsT: { [K in keyof T]: string } = {     a: 'asdf', // a is required }  // Note a became optional const canBeNotPresent: { [K in keyof T]?: string } = { }  // Note b became required const mustBePreset: { [K in keyof T]-?: string } = {     a: 'asdf',      b: 'asdf'  // b became required  } 

More

I did a lesson on these mapped type modifiers : https://www.youtube.com/watch?v=0zgWo_gnzVI 🌹