The Principles of Object-Oriented JavaScript

Use the new ES2015 setter and getter methods. Also super() has to be the first method call in the constructor method.

class Circle {
  constructor(cx) {
    this.cx = cx;
    …
  }

  get cx() {
    return this.cx;
  }

  set cx(cx) {
    this.cx = cx;
  }

}

class RandomCircle extends Circle {
  constructor() {
    super();
    this.cx = random(10);
  }

  random(n) {
    …
  }
}

Here is a great video tutorial about ES2015 Classes: https://www.youtube.com/watch?v=EUtZRwA7Fqc

I also recommend "The Principles of Object-Oriented JavaScript " by Nicholas C. Zakas https://www.amazon.com/Principles-Object-Oriented-JavaScript-Nicholas-Zakas/dp/1593275404

Members bound to Prototype are shared across instances. There is only one instance of that member.

function Point(value) { this.value = value; }
Point.prototype.moveBy = function() { console.log('from the prototype', this.value);};
var p1 = new Point('p1');
var p2 = new Point('p2');
p1.moveBy === p2.moveBy; //true. Same instance of moveBy function

Members set directly to this inside the constructor or set to a specific instance of Point is available to that specific instance of Point only. If you want to expose logic shared across all instances of Point, it's best to set it on the Prototype.

var p3 = new Point('p3');
p3.moveBy = function() { console.log('from the instance', this.value); };
p3.moveBy === p2.moveBy; //false

var p4 = new Point('p4');
p4.moveBy = function() { console.log('from the instance', this.value) };
p4.moveBy === p3.moveBy; //false

The cool part is that you can always access the prototype even if you have declared it on the instance. If so, pass the instance as context:

Point.prototype.moveBy.call(p3); //will call moveBy set on the prototype. Output: "from the prototype p3"
p3.moveBy(); //will call moveBy set on the instance. Output: from the instance p3

The above snippet illustrates fundamental concepts of JavaScript (the context) and object-oriented JavaScript (prototype). If the subject is of interest to you, take a look at N. Zackas' book on the subject: https://www.amazon.com/Principles-Object-Oriented-JavaScript-Nicholas-Zakas/dp/1593275404