How do you extend classes

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;
}
}