Class Line<CustomDataType>

Without custom data (JS/TS):

const line = new Line({
  x1: 10, 
  y1: 20, 
  x2: 30,
  y2: 40,
});

With custom data (JS/TS):

const line = new Line({
  x1: 10, 
  y1: 20, 
  x2: 30,
  y2: 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 line1 = new Line({ 
  x1: 10, 
  y1: 20, 
  x2: 30,
  y2: 40,
  data: entity,
});

// You can also pass in a generic type for the data property
const line2 = new Line<ObjectData>({
  x1: 10, 
  y1: 20, 
  x2: 30,
  y2: 40,
});
line2.data = entity;

With custom class extending Line (implements LineGeometry (x1, y1, x2, y2)):

// extending inherits the qtIndex method
class Laser extends Line {
  
  constructor(props) {
    // call super to set x1, y1, x2, y2 (and data, if given)
    super(props);
    this.color = props.color;
  }
}

const laser = new Laser({
  color: 'green',
  x1: 10, 
  y1: 20, 
  x2: 30,
  y2: 40,
});

With custom class and mapping LineGeometry:

// no need to extend if you don't implement LineGeometry
class Laser {
  
  constructor(color) {
    this.color = color;
    this.start = [10, 20];
    this.end = [30, 40];
  }

  // add a qtIndex method to your class  
  qtIndex(node) {
    // map your properties to LineGeometry
    return Line.prototype.qtIndex.call({
      x1: this.start[0],
      y1: this.start[1],
      x2: this.end[0],
      y2: this.end[1],
    }, node);
  }
}

const laser = new Laser('green');

With custom object that implements LineGeometry:

const player = {
  name: 'Jane', 
  health: 100,
  x1: 10, 
  y1: 20, 
  x2: 30,
  y2: 40,
  qtIndex: Line.prototype.qtIndex,
});

With custom object and mapping LineGeometry:

// 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,
  start: [10, 20], 
  end: [30, 40],
  qtIndex: function(node) {
    return Line.prototype.qtIndex.call({
      x1: this.start[0],
      y1: this.start[1],
      x2: this.end[0],
      y2: this.end[1],
    }, node);
  },
});