Explain 'Polymorphic this' and its impact on class inheritance.

The 'this' type allows a method to return the type of the current class instance, even if it is a subclass.

When a base class method returns this, TypeScript dynamically resolves that type to the actual instance at the call site. This is essential for 'Fluent APIs' or 'Builder Patterns' where a method is defined in a base class but must return the derived type to keep derived methods accessible in a chain[cite: 1].