Oppiskele Generic Repository Pattern | Advanced Type Patterns and Best Practices

The repository pattern is a design approach used to manage collections of objects in a consistent and abstract manner. By acting as a mediator between the data source and the rest of your application, a repository provides a simple interface for adding, retrieving, and searching for items. This pattern helps you separate concerns, making your code easier to maintain, test, and scale. With TypeScript, you can combine interfaces and generics to create repositories that work with any type of data, ensuring both flexibility and strong type safety.


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// Define a generic interface for a repository
interface Repository<T> {
  add(item: T): void;
  getAll(): T[];
  findById(id: number): T | undefined;
}

Suppose you want to manage collections of users and products. With a generic Repository<T>, you can create specific repositories for each type without duplicating code. Consider these simple User and Product types, and their corresponding repository implementations:


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// 1. Define a generic interface for a repository
interface Repository<T> {
  add(item: T): void;
  getAll(): T[];
  findById(id: number): T | undefined;
}

// 2. Define types for User and Product
type User = { id: number; name: string };
type Product = { id: number; title: string };

// 3. Create a single generic repository class
class GenericRepository<T extends { id: number }> implements Repository<T> {
  private items: T[] = [];

  add(item: T): void {
    this.items.push(item);
  }

  getAll(): T[] {
    return this.items;
  }

  findById(id: number): T | undefined {
    return this.items.find((item) => item.id === id);
  }
}

// 4. Use the repository for Users and Products
const userRepo = new GenericRepository<User>();
userRepo.add({ id: 1, name: "Alice" });
userRepo.add({ id: 2, name: "Bob" });

const productRepo = new GenericRepository<Product>();
productRepo.add({ id: 101, title: "Laptop" });
productRepo.add({ id: 102, title: "Phone" });

// 5. Display results
console.log("All users:", userRepo.getAll());
console.log("User #2:", userRepo.findById(2));

console.log("All products:", productRepo.getAll());
console.log("Product #101:", productRepo.findById(101));

With a generic repository interface, you avoid code duplication;
Each repository class provides methods for adding, retrieving, and searching for items;
TypeScript enforces that only the correct type of object can be added or retrieved from each repository.

By combining generics and interfaces, you can design reusable and scalable code structures. The Repository<T> interface defines a contract that any repository must follow, while generics allow you to specify the type of objects being managed. This means you can create repositories for any data type without rewriting logic, reducing code duplication and making your application easier to extend and maintain.

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              123456
            
// Define a generic interface for a repository
interface Repository<T> {
  add(item: T): void;
  getAll(): T[];
  findById(id: number): T | undefined;
}


              12345678910111213141516171819202122232425262728293031323334353637383940414243
            
// 1. Define a generic interface for a repository
interface Repository<T> {
  add(item: T): void;
  getAll(): T[];
  findById(id: number): T | undefined;
}

// 2. Define types for User and Product
type User = { id: number; name: string };
type Product = { id: number; title: string };

// 3. Create a single generic repository class
class GenericRepository<T extends { id: number }> implements Repository<T> {
  private items: T[] = [];

  add(item: T): void {
    this.items.push(item);
  }

  getAll(): T[] {
    return this.items;
  }

  findById(id: number): T | undefined {
    return this.items.find((item) => item.id === id);
  }
}

// 4. Use the repository for Users and Products
const userRepo = new GenericRepository<User>();
userRepo.add({ id: 1, name: "Alice" });
userRepo.add({ id: 2, name: "Bob" });

const productRepo = new GenericRepository<Product>();
productRepo.add({ id: 101, title: "Laptop" });
productRepo.add({ id: 102, title: "Phone" });

// 5. Display results
console.log("All users:", userRepo.getAll());
console.log("User #2:", userRepo.findById(2));

console.log("All products:", productRepo.getAll());
console.log("Product #101:", productRepo.findById(101));

With a generic repository interface, you avoid code duplication;
Each repository class provides methods for adding, retrieving, and searching for items;
TypeScript enforces that only the correct type of object can be added or retrieved from each repository.

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Kiitos palautteestasi!

Osio 3. Luku 2