How to bolt on mixins to a base class in ES2015

ES2015 Class Mixins

// combine properties to base prototype
function Combine(target, fns = []) {

  let protoCopy = Object.assign(target.prototype);
	
  let extendedProto = (Array.isArray(fns) ? fns : [fns]).reduce((proto, fn) => {
    Object.getOwnPropertyNames(fn.prototype).forEach(prop => {
      if(!(protoCopy.hasOwnProperty(prop))) {
        proto[prop] = fn.prototype[prop];
      }
    });
    return proto; 
  }, protoCopy);
	
  target.prototype = Object.assign(extendedProto);
  return target;
	
}

// base class
class ABC {
  constructor(a) {
	this.a = a;
  }
  methABC() {
	return `ABC ${this.a}`;
  }
}

class A {
  methA() {
    return `A ${this.a}`;
  }
}

class B {
  methB() {
	return `B ${this.a}`;
  }
}

class C {
  methC() {
    return `C ${this.a}`;
  }
}

class D {
  init(value) {
    this.sharedProp = value;
  }
  methD() {
    return `D ${this.sharedProp}`;
  }
}

class E extends D{
  methE() {
    return `E ${this.sharedProp }`;
  }
}

// practical usage - compose from whatever functionality needed 
// export var ComboClass = Combine(ABC, [A, B, C]);

var abc = new (Combine(ABC, [A, B, C]))(10);
console.log(abc.methC());  // C 10

var ab = new (Combine(ABC, [A, B]))(10);
console.log(ab.methB());  // B 10

var a = new (Combine(ABC, A))(10);
console.log(a.methA());  // A 10

var baseClassOnly = new (Combine(ABC))(10);
console.log(baseClassOnly.methABC()); // ABC 10

var d = new (Combine(ABC, [A, B, C, D]))(10);
d.init(100)
console.log(d.methD()); // D 100 

var e = new (Combine(ABC, [A, B, C, E]))(10);
e.init(100);
console.log(e.methE()); // E 100

var _e = new (Combine(ABC, [A, B, C, E]))(10);
console.log(_e.methE()); // undefined