Abstract classes and inheritance in TypeScript

Abstract classes

An abstract class cannot be instantiated directly. It defines structure that subclasses must implement:

abstract class Shape {
  abstract area(): number;
  abstract perimeter(): number;

  describe(): string {
    return `Area: ${this.area()}, Perimeter: ${this.perimeter()}`;
  }
}

class Circle extends Shape {
  constructor(private radius: number) { super(); }
  area()      { return Math.PI * this.radius ** 2; }
  perimeter() { return 2 * Math.PI * this.radius; }
}

const c = new Circle(5);
console.log(c.describe()); // uses base class method

const s = new Shape(); // Error: cannot create abstract class

abstract vs interface

Use abstract classes when you want to share implementation across subclasses alongside the contract. Use interfaces when you only want to define a contract with no shared implementation. Abstract classes also support constructor logic and access modifiers, which interfaces cannot.