text: 
  const _radius = new WeakMap();
  
  class Circle {
    constructor(radius) {
      _radius.set(this, radius);
    }
  
    get radius() {
      return _radius.get(this);
    }
    set radius(value) {
      if(value <=0) {
        throw new Error('Invalid radius');
      }
      else {
        _radius.set(this, value);
      }
    }
  }
  
  const c = new Circle(3);
  c.radius
  