Class Circle<CustomDataType>

Without custom data (JS/TS):

const circle = new Circle({ 
  x: 100, 
  y: 100, 
  r: 32,
});

With custom data (JS/TS):

const circle = new Circle({ 
  x: 100, 
  y: 100, 
  r: 32, 
  data: { 
    name: 'Jane', 
    health: 100,
  },
});

With custom data (TS):

interface ObjectData {
  name: string
  health: number
}
const entity: ObjectData = {
  name: 'Jane',
  health: 100,
};

// Typescript will infer the type of the data property
const circle1 = new Circle({ 
  x: 100, 
  y: 100, 
  r: 32, 
  data: entity,
});

// You can also pass in a generic type for the data property
const circle2 = new Circle<ObjectData>({ 
  x: 100, 
  y: 100, 
  r: 32,
});
circle2.data = entity;

With custom class extending Circle (implements CircleGeometry (x, y, r)):

// extending inherits the qtIndex method
class Bomb extends Circle {
  
  constructor(props) {
    // call super to set x, y, r (and data, if given)
    super(props);
    this.countdown = props.countdown;
  }
}

const bomb = new Bomb({
  countdown: 5,
  x: 10, 
  y: 20, 
  r: 30,
});

With custom class and mapping CircleGeometry:

// no need to extend if you don't implement CircleGeometry
class Bomb {
  
  constructor(countdown) {
    this.countdown = countdown;
    this.position = [10, 20];
    this.radius = 30;
  }
  
  // add a qtIndex method to your class
  qtIndex(node) {
    // map your properties to CircleGeometry
    return Circle.prototype.qtIndex.call({
      x: this.position[0],
      y: this.position[1],
      r: this.radius,
    }, node);
  }
}

const bomb = new Bomb(5);

With custom object that implements CircleGeometry:

const player = {
  name: 'Jane', 
  health: 100,
  x: 10, 
  y: 20, 
  r: 30,
  qtIndex: Circle.prototype.qtIndex,
});

With custom object and mapping CircleGeometry:

// Note: this is not recommended but possible. 
// Using this technique, each object would have it's own qtIndex method. 
// Rather add qtIndex to your prototype, e.g. by using classes like shown above.
const player = {
  name: 'Jane', 
  health: 100,
  position: [10, 20], 
  radius: 30,
  qtIndex: function(node) {
    return Circle.prototype.qtIndex.call({
      x: this.position[0],
      y: this.position[1],
      r: this.radius,
    }, node);
  },
});