Another fundamental principle of object-oriented design is the concept of abstract classes. An abstract class is a definition of a class that cannot be instantiated. In other words, it is a class that is designed to be derived from. The abstract classes, sometimes referred to as abstract base classes, are often used to provide a set of basic functionality or properties that are shared among a group of similar classes. They are similar to interfaces in that they cannot be instantiated, but they can have function implementations, which interfaces cannot.
Abstract classes are a technique that allows for code reuse among groups of similar objects. Consider the following two classes:
class Employee { public id: number | undefined; ...
