TypeScript: Inferring function arguments from a constructor signature

Question

As a tool for composability, I have a function that takes a class constructor argument and returns a function that uses the arguments it is passed to construct an instance of the class I initially passed in.

class Thing {
  constructor (metadata) { this.metadata = metadata; }
}

const A = (C) => (...a) => new C(...a);
const X = A(Thing);

When I hover over X , I want it to tell me that the function signature is essentially (metadata) => Thing . Instead, it says that the signature of X is (...a) => any . What annotations are required in order to get TypeScript (latest version) to infer the signature of X correctly?

Answer 1

const A = <T extends unknown[], K>(C: new (...args: T) => K) => (...args: T) => new C(...args);

Playground link

Breaking it down, we have a T array type, and we annotate C to have a constructor (using the new keyword) that takes in the args denoted by T , and returns a value K . We then return a function that essentially undoes the constructor.

As a side note, please compile with --noImplicitAny ; it'll give you helpful warnings for things like this where arguments have an any type due to not specifying their type.