What are Abstract Classes in Typescript

Abstract classes - Summary #

Abstract classes in typescript:

• can't be used to create an object directly - you can't use the new keyword to directly instantiate an abstract class
• can only be used as parent classes
• can contain real implementation for methods and properties, where the implemented methods can refer to abstract properties/methods
• can make child classes promise to implement abstract methods/properties defined on the parent

Abstract classes vs Interfaces #

• when abstract classes get compiled to javascript they're not completely deleted as opposed to interfaces. Which means that we can use instanceof checks with classes as opposed to with interfaces.
• abstract classes can also have implemented methods, whereas interfaces only have the blueprint for which methods should be implemented.

Abstract classes example #

Let's look at a simple example:

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abstract class Animal {
  // no implementation - just the signature
  abstract type: string;
  abstract move(): void;

  // has implementation;
  breathe() {
    console.log(`The ${this.type} takes a breath.`);
  }
}

// Not permitted to create an instance of an abstract class
// const animal = new Animal()

class Dog extends Animal {
  type = 'dog';
  move() {
    console.log('The dog runs.');
  }
}

class Monkey extends Animal {
  type = 'monkey';
  move() {
    console.log('The monkey jumps.');
  }
}

const dog = new Dog();
// can use the implemented method from the abstract class
dog.breathe();

// or the methods for which the child class provides implementation
dog.move();

const monkey = new Monkey();
monkey.breathe();
monkey.move();

When we extend an abstract class we are forced to provide implementation for the abstract methods or properties, in other words to conform to the signature enforced by the abstract class.

We also are able to use the implemented by the abstract class method, just in the way inheritance would normally work, any of the classes that extend animal get access to the breathe method defined in inheritance. Notice that in the breathe method we make use of an abstract property, in implemented methods in abstract classes we can make use of any of the abstract properties or methods, because we have a guarantee that the child classes will have to implement them.

We can think of abstract properties/methods as - existing in the future, or in other words implemented by some child class.

When to use abstract classes #

We use abstract classes when we want to provide reusable implementation of some function, but that function might depend on some other functions, that we can't yet implement, because they are functions that are very specific to the different child classes, that extend the abstract class.

For example we could imagine that the implementation of the move method could be very different between the different child classes that extend the abstract class. However all of the child classes at some future point in time will have a version of the move method that conforms to the type specified in the abstract class.