doWhileMounted

foo.ts

class Foo {
  a = 1;
  b(): void {}
  c(): boolean {
    return true;
  }
}

// https://medium.com/dailyjs/typescript-create-a-condition-based-subset-types-9d902cea5b8c
type FilterFlags<Base, Condition> = {
  [Key in keyof Base]: Base[Key] extends Condition ? Key : never
};
type AllowedNames<Base, Condition> = FilterFlags<Base, Condition>[keyof Base];
type SubType<Base, Condition> = Pick<Base, AllowedNames<Base, Condition>>;

type Constructor<T> = new () => T;

function doWhileMounted<P, K extends AllowedNames<P, () => void>>(
  Cstr: Constructor<P>,
  functionName: K,
): P {
  return new Cstr();
}

// doWhileMounted(Foo, 'a'); // TypeError
doWhileMounted(Foo, 'b');
doWhileMounted(Foo, 'c');