What is the dash-question mark syntax in TypeScript?

Question

I don't mean the question mark syntax, rather, I'm asking about -? in, for example:

type Required<T> =
  T extends object
    ? { [P in keyof T]-?: NonNullable<T[P]>; } // <---------- "-?" here
    : T;

via this 2018 GitHub comment. I cannot find this syntax in the TypeScript handbook's chapters on advanced types nor utility types.

-? as above still compiles in TypeScript 3.8, and appears to be the opposite of ?, i.e., making the key required? Is it equivalent to Required? If not, what is this syntax called and where can I read more about it?

Answer 1

You are basically right: it removes the optional property modifier ? from a mapped type. It was introduced in TypeScript 2.8 as part of improvements to control over mapped type modifiers (see this link for the docs you want). It's not identical to the Required utility type, but Required is implemented with it (see library definition here) and could not exist without it.

It's a sad fact that TypeScript documentation is kind of spread out between the handbook, release notes, FAQ, outdated spec, and GitHub issues, without a clear canonical place to look for any particular thing. The language has been evolving more quickly than the documentation can keep up.

Okay, hope that helps; good luck!