When should you use interfaces or classes in TypeScript?

In TypeScript, Interfaces and Classes are both crucial concepts that play distinct roles in different scenarios. Here are some guidelines and practical use cases for when to use Interfaces or Classes:

Using Interfaces

1. Defining the Shape of an Object: Interfaces are primarily used to define the structure of an object, specifying its properties and methods without providing implementations. This is particularly useful for establishing contracts between different components of a system.

   Example: Suppose we are developing a system that needs to define a user object with properties like name, age, and a method to display information.

   typescript
   interface IUser {
       name: string;
       age: number;
       displayInfo(): void;
   }

2. Improving Code Reusability: Interfaces can be implemented by multiple classes, allowing you to define a standard behavior that different classes can adhere to, thereby promoting code reuse.

   Example: If we have multiple user types, such as administrators and visitors, they can both implement the IUser interface, though the specific implementation of displayInfo may vary.

3. Defining Common Protocols Between Components: When multiple components need to interact, interfaces serve as the communication protocol between them.

   Example: In large projects, a function might handle various user types, all of which implement the same interface.

Using Classes

1. Creating Concrete Instances: Classes serve as blueprints for creating concrete instances, defining both the structure and implementation of members. They are ideal for generating multiple similar objects.

   Example: To create multiple user objects with unique names and ages, you can use a class.

   typescript
   class User implements IUser {
       constructor(public name: string, public age: number) {}
   
       displayInfo() {
           console.log(`Name: ${this.name}, Age: ${this.age}`);
       }
   }

2. Encapsulation and Inheritance: Classes support encapsulation and inheritance, enabling you to hide internal implementation details and extend functionality through inheritance.

   Example: You can create an AdminUser class that extends User, adding specific features like management permissions.

   typescript
   class AdminUser extends User {
       manageSystem() {
           console.log('Adminstrating system');
       }
   }

3. Implementing Interfaces: Classes can implement one or more interfaces, ensuring adherence to a specific structure.

Summary: When deciding between Interfaces and Classes, consider whether you need concrete implementations (use Classes) or only need to define the structure or protocol (use Interfaces). Typically, Interfaces define the 'shape' of behavior, while Classes implement specific behaviors and create concrete instances. Combining both approaches can result in flexible and robust systems.