Abstract classes allow us to create base classes from which other classes may be derived. They've been part of JavaScript since years 2015. Abstract classes cannot be instantiated themselves, but they can provide names for the required class variables and functions. Functions can have a concrete implementation, or they can be made abstract. If that's the case, then the class using the abstract as a base must provide the implementation for the function. A good example for an abstract class is a shape class. All shapes, it could be argued, will have a name, a number of sides and an area. But we'd never want to create an instance of an arbitrary shape. We have a constructor that defines the name, number of sides and a value for the area of any class that uses this as a base. There's an abstract get function called calc area. This has no implementation as each shape will have its own method of calculating its area. It defines a concrete implementation of a print shape info method. Beneath that, we have a circle class that extends the shape class, having its own property of radius. An implementation of get calc area is provided, that calculates sets and returns the area. On the right hand side of the screen we have a triangle class that does the same thing, but uses a different implementation for calc area. On line 50, we try and instantiate the shape class. We get the error I show. we cannot create an instance of an abstract class. The instantiations of the circle and triangle classes are fruitful. And we can use calc area and the print shape info method.

Lectures

• Inheritance
• Access Modifiers
• Encapsulation Getters & Setters
• Structural Typing