Class Rectangle<CustomDataType>

Without custom data (JS/TS):

const rectangle = new Rectangle({ 
  x: 10, 
  y: 20, 
  width: 30,
  height: 40,
});

With custom data (JS/TS):

const rectangle = new Rectangle({ 
  x: 10, 
  y: 20, 
  width: 30,
  height: 40,
  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 rectangle1 = new Rectangle({
  x: 10, 
  y: 20, 
  width: 30,
  height: 40,
  data: entity,
});

// You can also pass in a generic type for the data property
const rectangle2 = new Rectangle<ObjectData>({ 
  x: 10, 
  y: 20, 
  width: 30,
  height: 40,
});
rectangle2.data = entity;

With custom class extending Rectangle (implements RectangleGeometry (x, y, width, height)):

// extending inherits the qtIndex method
class Box extends Rectangle {
  
  constructor(props) {
    // call super to set x, y, width, height (and data, if given)
    super(props);
    this.content = props.content;
  }
}

const box = new Box({
  content: 'Gravity Boots',
  x: 10, 
  y: 20, 
  width: 30,
  height: 40,
});

With custom class and mapping RectangleGeometry:

// no need to extend if you don't implement RectangleGeometry
class Box {
  
  constructor(content) {
    this.content = content;
    this.position = [10, 20];
    this.size = [30, 40];
  }
  
  // add a qtIndex method to your class
  qtIndex(node) {
    // map your properties to RectangleGeometry
    return Rectangle.prototype.qtIndex.call({
      x: this.position[0],
      y: this.position[1],
      width: this.size[0],
      height: this.size[1],
    }, node);
  }
}

const box = new Box('Gravity Boots');

With custom object that implements RectangleGeometry:

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

With custom object and mapping RectangleGeometry:

// 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], 
  size: [30, 40], 
  qtIndex: function(node) {
    return Rectangle.prototype.qtIndex.call({
      x: this.position[0],
      y: this.position[1],
      width: this.size[0],
      height: this.size[1],
    }, node);
  },
});