whole_code.ts

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

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

  function merge(derived: ClassType, ...classRefs: ClassType[]) {
    classRefs.forEach(classRef => {
      Object.getOwnPropertyNames(classRef.prototype).forEach(name => {
        // you can get rid of type casting in this way
        const descriptor = Object.getOwnPropertyDescriptor(classRef.prototype, name)
        if (name !== 'constructor' && descriptor) {
          Object.defineProperty(
            derived.prototype,
            name,
            descriptor
          );
        }
      });
    });

    return derived;
  }

  class Foo {
    foo() { }
  }

  class Bar {
    bar() { }
  }

  class Baz {
    baz() {
      console.log('baz');
    }
  }

  class MyClass extends Mixin(Foo, Bar, Baz) { }

  const my = new MyClass();
  my.foo() // ok
  my.bar() // ok
  my.baz(); // ok