Examples

Using extends

The first example creates a class called Square from a class called Polygon. This example is extracted from this live demo (source). 

class Square extends Polygon {
  constructor(length) {
    // Here, it calls the parent class' constructor with lengths
    // provided for the Polygon's width and height
    super(length, length);
    // Note: In derived classes, super() must be called before you
    // can use 'this'. Leaving this out will cause a reference error.
    this.name = 'Square';
  }

  get area() {
    return this.height * this.width;
  }
}


Using extends with built-in objects

This example extends the built-in Date object. This example is extracted from this live demo (source). 

class myDate extends Date {
  getFormattedDate() {
    const months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
    return `${this.getDate()}-${months[this.getMonth()]}-${this.getFullYear()}`;
  }
}