typescript - Get argument types for function / class constructor

Question

I am trying to do something I am not sure is possible in TypeScript: inferring the argument types/return types from a function.

For example:

function foo(a: string, b: number) {
  return `${a}, ${b}`;
}

type typeA = <insert magic here> foo; // Somehow, typeA should be string;
type typeB = <insert magic here> foo; // Somehow, typeB should be number;

My use case is to try to create a config object that contains constructors and parameters.

For example:

interface IConfigObject<T> {
    // Need a way to compute type U based off of T.
    TypeConstructor: new(a: U): T;
    constructorOptions: U;
}

// In an ideal world, could infer all of this from TypeConstructor

class fizz {
    constructor(a: number) {}
}

const configObj : IConfigObj = {
    TypeConstructor: fizz;
    constructorOptions: 13; // This should be fine
}

const configObj2 : IConfigObj = {
    TypeConstructor: fizz;
    constructorOptions: 'buzz'; // Should be a type error, since fizz takes in a number
}

Answer 1

Typescript now has the ConstructorParameters builtin, similar to the Parameters builtin. Make sure you pass the class type, not the instance:

ConstructorParameters<typeof SomeClass>

ConstructorParameter Official Doc

Parameters Official Doc