Question

How do you extend classes?

Answer 1

The extends keyword is used in class declarations/expressions to create a class which is a child of another class. It can be used to subclass custom classes as well as built-in objects. The syntax would be as below,

class ChildClass extends ParentClass { ... }

Let's take an example of Square subclass from Polygon parent class,

class Square extends Rectangle {
  constructor(length) {
    super(length, length);
    this.name = "Square";
  }

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

  set area(value) {
    this.area = value;
  }
}