ES6 class variable alternatives

Question

Currently in ES5 many of us are using the following pattern in frameworks to create classes and class variables, which is comfy:

Answer 1

What about the oldschool way?

class MyClass {
     constructor(count){ 
          this.countVar = 1 + count;
     }
}
MyClass.prototype.foo = "foo";
MyClass.prototype.countVar = 0;

// ... 

var o1 = new MyClass(2); o2 = new MyClass(3);
o1.foo = "newFoo";

console.log( o1.foo,o2.foo);
console.log( o1.countVar,o2.countVar);

In constructor you mention only those vars which have to be computed. I like prototype inheritance for this feature -- it can help to save a lot of memory(in case if there are a lot of never-assigned vars).