captain-yossarian

const fn = <
  T extends typeof Base,
  Instance extends InstanceType<T>
>(Cls: T, prop: keyof Instance,
) => new Cls()[prop];

captain-yossarian

class Base { }

function fn<T extends typeof Base>(
  Cls: T,
  prop: keyof InstanceType<T>,
) {
  return (new Cls())[prop]; // error
}

captain-yossarian

  type UnionToIntersection<U> =
    (U extends any ? (k: U) => void : never) extends (
      k: infer I
    ) => void
    ? I
    : never;

  type ClassType = new (...args: any[]) => any;
  

captain-yossarian

// credits goes to https://stackoverflow.com/a/50375286
type UnionToIntersection<U> =
  (U extends any ? (k: U) => void : never) extends (
    k: infer I
  ) => void
  ? I
  : never;

  function Mixin<T extends ClassType, R extends T[]>(...classRefs: [...R]):
    new (...args: any[]) => UnionToIntersection<InstanceType<[...R][number]>> {

captain-yossarian

type ClassType = new (...args: any[]) => any;

function Mixin<T extends ClassType, R extends T[]>(...classRefs: [...R]): InstanceType<R[number]> {
    return null as any
}


class Foo {
    foo() { }
}

captain-yossarian

// added extra R generic
function Mixin<T extends ClassType, R extends T[]>(...classRefs: [...R]){
    return merge(class { }, ...classRefs);
}

captain-yossarian

function Mixin<T extends ClassType>(...classRefs: T[]) {
    return merge(class { }, ...classRefs);
}

captain-yossarian

type ClassType = new (...args: any[]) => any;

function merge(derived: ClassType, ...classRefs: ClassType[]) {
    classRefs.forEach(classRef => {
        Object.getOwnPropertyNames(classRef.prototype).forEach(name => {
            const descriptor = Object.getOwnPropertyDescriptor(classRef.prototype, name)
            // just add condition statement
            if (name !== 'constructor' && descriptor) {
                Object.defineProperty(
                    derived.prototype,

captain-yossarian

type ClassType = new (...args: any[]) => any;

// add ClassType
function merge(derived: ClassType, ...classRefs: ClassType[]) {

    // same logic from previous version
    return derived 
}

captain-yossarian

type ClassType = new (...args: any[]) => any;