Q288: How do you extend classes - JavaScript Internals (521 Qs)

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,

javascript

  class ChildClass extends ParentClass { ... }

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

javascript

  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;
    }
  }