Introduction to Domain Driven Design using Nodejs

Domain Driven Design (DDD) is an approach to software development that emphasizes the importance of the business domain. The goal of DDD is to create software that models the business domain explicitly, making it easier to understand and maintain over time.

At the heart of DDD is the Domain Model, which is a representation of the business domain in code. The Domain Model consists of entities, value objects, aggregates, and domain services, which are used to model the key concepts and behaviors of the business domain.

DDD also includes other concepts such as the Ubiquitous Language, which is a shared language between business and development teams, and the Application Services Layer, which is responsible for coordinating the behavior of the Domain Model.

class Cart {
  private readonly id: string;
  private readonly items: CartItem[];

  constructor(id: string) {
    this.id = id;
    this.items = [];
  }

  public getId(): string {
    return this.id;
  }

  public addItem(item: CartItem): void {
    const existingItem = this.items.find((i) => i.getProduct().getId() === item.getProduct().getId());
    if (existingItem) {
      existingItem.setQuantity(existingItem.getQuantity() + item.getQuantity());
    } else {
      this.items.push(item);
    }
  }

  public removeItem(productId: string): void {
    const index = this.items.findIndex((i) => i.getProduct().getId() === productId);
    if (index !== -1) {
      this.items.splice(index, 1);
    }
  }

  public getTotal(): Money {
    const total = this.items.reduce((acc, item) => acc.add(item.getTotal()), Money.ZERO);
    return total;
  }
}

class CartItem {
  private readonly product: Product;
  private quantity: Quantity;

  constructor(product: Product, quantity: Quantity) {
    this.product = product;
    this.quantity = quantity;
  }

  public getProduct(): Product {
    return this.product;
  }

  public getQuantity(): Quantity {
    return this.quantity;
  }

  public setQuantity(quantity: Quantity): void {
    this.quantity = quantity;
  }

  public getTotal(): Money {
    return this.product.getPrice().multiply(this.quantity);
  }
}

class Product {
  private readonly id: string;
  private readonly name: string;
  private readonly description: string;
  private readonly price: Money;

  constructor(id: string, name: string, description: string, price: Money) {
    this.id = id;
    this.name = name;
    this.description = description;
    this.price = price;
    }

    public getId(): string {
        return this.id;
    }

    public getName(): string {
        return this.name;
    }

    public getDescription(): string {
        return this.description;
    }

    public getPrice(): Money {
        return this.price;
    }
}

class Money {
    private readonly amount: number;
    private readonly currency: string;

    public static readonly ZERO = new Money(0, 'USD');

    constructor(amount: number, currency: string) {
        this.amount = amount;
        this.currency = currency;
    }

    public getAmount(): number {
        return this.amount;
    }

    public getCurrency(): string {
        return this.currency;
    }

    public add(other: Money): Money {
        if (this.currency !== other.currency) {
            throw new Error('Cannot add money with different currencies');
        }
        return new Money(this.amount + other.amount, this.currency);
    }

    public multiply(multiplier: Quantity): Money {
        return new Money(this.amount * multiplier.getValue(), this.currency);
    }
}

class Quantity {
    private readonly value: number;

    constructor(value: number) {
        if (value < 0) {
            throw new Error('Quantity must be non-negative');
        }
        this.value = value;
    }

    public getValue(): number {
        return this.value;
    }
}

In this example, we have defined the entities Cart, CartItem, and Product, as well as the value objects Money and Quantity. We have also implemented some basic behavior for the Cart entity, including adding and removing items, and calculating the total price.

Blog Post 3: The Application Services Layer in Node.js

The Application Services Layer is responsible for coordinating the behavior of the Domain Model. It acts as a bridge between the presentation layer and the Domain Model, and is responsible for executing business logic in response to user input.

To implement the Application Services Layer in Node.js, we can define a set of classes that encapsulate the behavior of the Domain Model. These classes are responsible for handling requests from the presentation layer, executing the necessary business logic, and returning the results.

Here is an example of how we can implement the Application Services Layer for an e-commerce system in Node.js:

class AddItemToCartRequest {
  public readonly productId: string;
  public readonly quantity: number;

  constructor(productId: string, quantity: number) {
    this.productId = productId;
    this.quantity = quantity;
  }
}

class AddItemToCartResponse {
  public readonly cartId: string;
  public readonly total: Money;

  constructor(cartId: string, total: Money) {
    this.cartId = cartId;
    this.total = total;
  }
}

class RemoveItemFromCartRequest {
  public readonly productId: string;

  constructor(productId: string) {
    this.productId = productId;
  }
}

class RemoveItemFromCartResponse {
  public readonly cartId: string;
  public readonly total: Money;

  constructor(cartId: string, total: Money) {
    this.cartId = cartId;
    this.total = total;
  }
}

class GetCartRequest {
  public readonly cartId: string;

  constructor(cartId: string) {
    this.cartId = cartId;
  }
}

class GetCartResponse {
  public readonly cartId: string;
  public readonly items: CartItem[];
  public readonly total: Money;

  constructor(cartId: string, items: CartItem[], total: Money) {
    this.cartId = cartId;
    this.items = items;
    this.total = total;
  }
}

class CartService {
  private readonly repository
this.cartRepository.getCartById(query.cartId);
return new CartDto(cart.getId(), cart.getItems(), cart.getTotal());