Greetings! In today's lesson, we'll unravel the concept of polymorphism in JavaScript's Object-Oriented Programming (OOP). Grasping polymorphism enables us to use a single entity (a method, class, or interface) to represent different types in various scenarios. Let's proceed.

Polymorphism, a pillar of OOP, allows one object to embody multiple forms. Visualize a button in software; depending on its type (for instance, a submit button or a radio button), the action resulting from pressing it varies. This dynamic encapsulates the spirit of polymorphism!

Let's observe polymorphism in action within a simple application involving shapes. The base Shape class has an area method, which calculates the area for shapes. This method is uniquely implemented in the subclasses Rectangle and Circle.