How can you implement an abstract class in TypeScript?

In TypeScript, an abstract class is a class that cannot be instantiated directly, but can be used as a base class for other classes. Abstract classes can have abstract methods, which are declared without an implementation, and they must be implemented by derived classes. Abstract class is defined using the ‘abstract‘ keyword.

Here’s an example of how to implement an abstract class in TypeScript:

1. Create an abstract class:

abstract class Shape {
  protected x: number;
  protected y: number;

  constructor(x: number, y: number) {
    this.x = x;
    this.y = y;
  }

  abstract getArea(): number;
  abstract getPerimeter(): number;
}

In this example, we have an abstract class ‘Shape‘ with two abstract methods, ‘getArea()‘ and ‘getPerimeter()‘. The class also has a constructor that takes two arguments, ‘x‘ and ‘y‘.

2. Define a derived class:

Now, let’s define a class ‘Rectangle‘ that derives from the abstract class ‘Shape‘ and implements the required abstract methods:

class Rectangle extends Shape {
  private width: number;
  private height: number;

  constructor(x: number, y: number, width: number, height: number) {
    super(x, y);
    this.width = width;
    this.height = height;
  }

  // Implement the abstract methods from the base class
  getArea(): number {
    return this.width * this.height;
  }

  getPerimeter(): number {
    return 2 * (this.width + this.height);
  }
}

In this example, we extend the base class ‘Shape‘, implement the required abstract methods ‘getArea()‘ and ‘getPerimeter()‘ and provide our own private properties (‘width‘ and ‘height‘) and constructor.

3. Instantiate the derived class and use the implemented methods:

Finally, we can create an object of the derived class ‘Rectangle‘ and use its methods:

const myRectangle = new Rectangle(0, 0, 5, 8);
console.log(`Area: ${myRectangle.getArea()}`);
console.log(`Perimeter: ${myRectangle.getPerimeter()}`);

Here, we create a ‘Rectangle‘ with ‘x‘, ‘y‘, ‘width‘, and ‘height‘ as ‘0‘, ‘0‘, ‘5‘, and ‘8‘, respectively. Then, we call the ‘getArea()‘ and ‘getPerimeter()‘ methods to calculate and print the area and perimeter of the rectangle.

In this way, abstract classes can be implemented in TypeScript.