mixins.ts

export type Constructor<T> = new(...args: any[]) => T;

export interface B {
    fromB(): void;
}

export function withB<T extends Constructor<object>>(superclass: T): Constructor<B> {
  return class B extends superclass {
    fromB() {

    }
  };
}

export class A extends withB(C) {

}

export class C { }

const a = new A();
a.fromB();