Live Demo: JavaScript ES6 Classes

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>JavaScript ES6 Classes</title>
</head>
<body>
    <script>
    class Rectangle {
        // Class constructor
        constructor(length, width) {
            this.length = length;
            this.width = width;
        }
        
        // Class method
        getArea() {
            return this.length * this.width;
        }
    }
    
    // Square class inherits from the Rectangle class
    class Square extends Rectangle {
        // Child class constructor
        constructor(length) {
            // Call parent's constructor
            super(length, length);
        }
        
        // Child class method
        getPerimeter() {
            return 2 * (this.length + this.width);
        }
    }
    
    let rectangle = new Rectangle(5, 10);
    document.write(rectangle.getArea()); // 50
    document.write("<br>");
    
    let square = new Square(5);
    document.write(square.getArea()); // 25
    document.write("<br>");
    document.write(square.getPerimeter()); // 20
    document.write("<br>");
    
    document.write(square instanceof Square); // true
    document.write("<br>");
    document.write(square instanceof Rectangle); // true
    document.write("<br>");
    document.write(rectangle instanceof Square); // false
    </script>
</body>
</html>