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Domain-Driven Design in Deno

When to Use This Skill

Use this skill when:

• Building applications with complex business logic
• Implementing hexagonal/clean architecture in Deno
• Structuring large-scale Deno applications
• Separating domain logic from infrastructure
• Working with bounded contexts and aggregates
• Need clear separation between layers

Prerequisites: Always read deno-core.md first for essential Deno configuration.

Project Philosophy

Clean, Modern TypeScript: Embrace Deno's vision of secure, modern JavaScript/TypeScript development without the baggage of Node.js legacy patterns.

Domain-Driven Design: Follow DDD principles with clear separation between domain logic, application services, and infrastructure concerns.

TypeScript-First: Leverage TypeScript's type system for safety and developer experience. No any types in production code.

Core DDD Principles

Ubiquitous Language

• Use domain terminology consistently in code, docs, and conversations
• Type names, method names, and variables should match business concepts
• Avoid technical jargon in domain layer

Bounded Contexts

• Each context has its own models and language
• Clear boundaries between contexts
• Explicit translation between contexts

Layered Architecture

1. Domain Layer - Pure business logic, no dependencies
2. Application Layer - Use cases, orchestration
3. Infrastructure Layer - External services, databases, APIs
4. API Layer - HTTP handlers, CLI, GraphQL resolvers

Project Structure

Recommended Directory Layout

src/
├── domain/                    # Domain layer - core business logic
│   ├── entities/              # Domain entities (Memory, User, Order)
│   │   ├── user.ts
│   │   └── order.ts
│   ├── value-objects/         # Immutable values (Email, Money, Status)
│   │   ├── email.ts
│   │   ├── money.ts
│   │   └── order-status.ts
│   ├── aggregates/            # Consistency boundaries
│   │   └── order-aggregate.ts
│   ├── repositories/          # Repository interfaces (ports)
│   │   ├── user-repository.ts
│   │   └── order-repository.ts
│   ├── services/              # Domain services
│   │   └── pricing-service.ts
│   ├── events/                # Domain events
│   │   └── order-created.ts
│   └── errors/                # Domain-specific errors
│       ├── validation-error.ts
│       └── business-rule-error.ts
│
├── application/               # Application layer - use cases
│   ├── use-cases/             # Use case implementations
│   │   ├── create-order.ts
│   │   ├── update-user.ts
│   │   └── process-payment.ts
│   ├── services/              # Application services
│   ├── dto/                   # Data transfer objects
│   │   ├── create-order-dto.ts
│   │   └── user-response-dto.ts
│   └── errors/                # Application-specific errors
│       ├── not-found-error.ts
│       └── unauthorized-error.ts
│
├── infrastructure/            # Infrastructure layer - technical details
│   ├── persistence/           # Database implementations
│   │   ├── postgres/
│   │   │   ├── user-repository-impl.ts
│   │   │   └── order-repository-impl.ts
│   │   └── migrations/
│   ├── external/              # External service integrations
│   │   ├── payment-gateway.ts
│   │   └── email-service.ts
│   ├── logging/               # Structured logging
│   │   └── logger.ts
│   ├── config/                # Configuration
│   │   └── database.ts
│   └── errors/                # Infrastructure errors
│       ├── database-error.ts
│       └── external-api-error.ts
│
├── web/                       # Web/API layer - HTTP entry points
│   ├── controllers/           # Request handlers
│   │   ├── user-controller.ts
│   │   └── order-controller.ts
│   ├── middleware/            # HTTP middleware
│   │   ├── auth.ts
│   │   ├── validation.ts
│   │   ├── error-handler.ts
│   │   └── logging.ts
│   ├── routes/                # Route definitions
│   │   ├── user-routes.ts
│   │   └── order-routes.ts
│   └── server.ts              # HTTP server setup
│
└── shared/                    # Shared kernel
    ├── types/
    │   └── result.ts
    └── utils/
        └── validation.ts

tests/
├── domain/                    # Domain tests (unit)
│   ├── entities/
│   │   └── user.test.ts
│   └── value-objects/
│       └── email.test.ts
├── application/               # Application tests (integration)
│   └── use-cases/
│       └── create-order.test.ts
└── e2e/                       # End-to-end tests
    └── order-workflow.test.ts

Import Map Configuration

Configure deno.json for clean imports across all layers:

{
  "imports": {
    "@/": "./src/",
    "@/domain/": "./src/domain/",
    "@/application/": "./src/application/",
    "@/infrastructure/": "./src/infrastructure/",
    "@/web/": "./src/web/",
    "@/shared/": "./src/shared/"
  }
}

Layer Dependencies

Understanding and enforcing layer dependencies is critical for maintaining a clean DDD architecture.

Allowed Dependencies

• domain → (no external dependencies - pure business logic)
• application → domain
• infrastructure → domain + application
• web (or api) → domain + application + infrastructure

Forbidden Dependencies

NEVER allow these dependencies:

• domain → application, infrastructure, web
• application → infrastructure, web
• infrastructure → web

Dependency Flow Visualization

    ┌─────────────┐
    │     web     │  (HTTP handlers, routes, middleware)
    └──────┬──────┘
           │
    ┌──────▼──────┐
    │infrastructure│  (Database, external APIs)
    └──────┬──────┘
           │
    ┌──────▼──────┐
    │ application │  (Use cases, orchestration)
    └──────┬──────┘
           │
    ┌──────▼──────┐
    │   domain    │  (Entities, value objects, business rules)
    └─────────────┘

Key Principle: Dependencies flow inward. Inner layers have no knowledge of outer layers.

Domain Layer

Entities

Entities have identity and lifecycle. Use classes with private constructors.

// src/domain/entities/user.ts
import type { Email } from "@/domain/value-objects/email.ts";
import type { UserId } from "@/domain/value-objects/user-id.ts";

export class User {
  private constructor(
    private readonly id: UserId,
    private name: string,
    private email: Email,
    private readonly createdAt: Date,
  ) {}

  // Factory method - ensures valid construction
  static create(name: string, email: Email): User {
    if (name.trim().length === 0) {
      throw new Error("User name cannot be empty");
    }
    return new User(
      UserId.generate(),
      name,
      email,
      new Date(),
    );
  }

  // Reconstruct from persistence
  static reconstitute(
    id: UserId,
    name: string,
    email: Email,
    createdAt: Date,
  ): User {
    return new User(id, name, email, createdAt);
  }

  // Business logic methods
  changeName(newName: string): void {
    if (newName.trim().length === 0) {
      throw new Error("User name cannot be empty");
    }
    this.name = newName;
  }

  // Getters
  getId(): UserId { return this.id; }
  getName(): string { return this.name; }
  getEmail(): Email { return this.email; }
  getCreatedAt(): Date { return this.createdAt; }
}

Value Objects

Value objects have no identity, compared by value.

// src/domain/value-objects/email.ts
export class Email {
  private constructor(private readonly value: string) {}

  static create(value: string): Email {
    if (!Email.isValid(value)) {
      throw new Error(`Invalid email: ${value}`);
    }
    return new Email(value.toLowerCase());
  }

  private static isValid(value: string): boolean {
    const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
    return emailRegex.test(value);
  }

  getValue(): string { return this.value; }
  equals(other: Email): boolean { return this.value === other.value; }
  toString(): string { return this.value; }
}

// src/domain/value-objects/money.ts
export class Money {
  private constructor(
    private readonly amount: number,
    private readonly currency: string,
  ) {}

  static create(amount: number, currency: string): Money {
    if (amount < 0) {
      throw new Error("Money amount cannot be negative");
    }
    if (!["USD", "EUR", "GBP"].includes(currency)) {
      throw new Error(`Unsupported currency: ${currency}`);
    }
    return new Money(amount, currency);
  }

  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);
  }

  multiply(factor: number): Money {
    return new Money(this.amount * factor, this.currency);
  }

  getAmount(): number { return this.amount; }
  getCurrency(): string { return this.currency; }
  equals(other: Money): boolean {
    return this.amount === other.amount && this.currency === other.currency;
  }
}

Aggregates

Aggregates enforce consistency boundaries and business invariants.

// src/domain/aggregates/order-aggregate.ts
import type { OrderId } from "@/domain/value-objects/order-id.ts";
import type { Money } from "@/domain/value-objects/money.ts";
import type { OrderLine } from "@/domain/entities/order-line.ts";

export enum OrderStatus {
  PENDING = "PENDING",
  CONFIRMED = "CONFIRMED",
  SHIPPED = "SHIPPED",
  DELIVERED = "DELIVERED",
  CANCELLED = "CANCELLED",
}

export class Order {
  private constructor(
    private readonly id: OrderId,
    private status: OrderStatus,
    private readonly lines: OrderLine[],
    private readonly createdAt: Date,
  ) {}

  static create(lines: OrderLine[]): Order {
    if (lines.length === 0) {
      throw new Error("Order must have at least one line");
    }
    return new Order(
      OrderId.generate(),
      OrderStatus.PENDING,
      lines,
      new Date(),
    );
  }

  // Business logic - enforce invariants
  confirm(): void {
    if (this.status !== OrderStatus.PENDING) {
      throw new Error(`Cannot confirm order with status ${this.status}`);
    }
    this.status = OrderStatus.CONFIRMED;
  }

  cancel(): void {
    if (this.status === OrderStatus.SHIPPED || this.status === OrderStatus.DELIVERED) {
      throw new Error("Cannot cancel shipped or delivered order");
    }
    this.status = OrderStatus.CANCELLED;
  }

  calculateTotal(): Money {
    return this.lines.reduce(
      (total, line) => total.add(line.getSubtotal()),
      Money.create(0, "USD"),
    );
  }

  getId(): OrderId { return this.id; }
  getStatus(): OrderStatus { return this.status; }
  getLines(): ReadonlyArray<OrderLine> { return this.lines; }
}

Repository Interfaces (Ports)

Define interfaces in domain layer, implement in infrastructure.

// src/domain/repositories/user-repository.ts
import type { User } from "@/domain/entities/user.ts";
import type { UserId } from "@/domain/value-objects/user-id.ts";
import type { Email } from "@/domain/value-objects/email.ts";

export interface UserRepository {
  save(user: User): Promise<void>;
  findById(id: UserId): Promise<User | null>;
  findByEmail(email: Email): Promise<User | null>;
  delete(id: UserId): Promise<void>;
}

Application Layer

Use Cases

Use cases orchestrate domain logic without containing business rules.

// src/application/use-cases/create-order.ts
import type { UserRepository } from "@/domain/repositories/user-repository.ts";
import type { OrderRepository } from "@/domain/repositories/order-repository.ts";
import { Order } from "@/domain/aggregates/order-aggregate.ts";
import { OrderLine } from "@/domain/entities/order-line.ts";
import type { CreateOrderDto } from "@/application/dto/create-order-dto.ts";
import { Result } from "@/shared/types/result.ts";

export class CreateOrderUseCase {
  constructor(
    private readonly userRepository: UserRepository,
    private readonly orderRepository: OrderRepository,
  ) {}

  async execute(dto: CreateOrderDto): Promise<Result<Order>> {
    try {
      const user = await this.userRepository.findById(dto.userId);
      if (!user) {
        return Result.fail("User not found");
      }

      const lines = dto.items.map((item) =>
        OrderLine.create(item.productId, item.quantity, item.price)
      );

      const order = Order.create(lines);
      await this.orderRepository.save(order);

      return Result.ok(order);
    } catch (error) {
      return Result.fail(error.message);
    }
  }
}

Error Handling by Layer

Domain Layer - Domain Errors

// src/domain/errors/validation-error.ts
export class ValidationError extends Error {
  constructor(message: string) {
    super(message);
    this.name = "ValidationError";
  }
}

Application Layer - Application Errors

// src/application/errors/not-found-error.ts
export class MemoryNotFoundError extends Error {
  constructor(id: string) {
    super(`Memory with id ${id} not found`);
    this.name = "MemoryNotFoundError";
  }
}

Infrastructure Layer - Infrastructure Errors

// src/infrastructure/errors/database-error.ts
export class DatabaseError extends Error {
  constructor(message: string, public readonly cause?: Error) {
    super(message);
    this.name = "DatabaseError";
  }
}

Web Layer - HTTP Error Handling

// src/web/middleware/error-handler.ts
import { ValidationError } from "@/domain/errors/validation-error.ts";
import { MemoryNotFoundError } from "@/application/errors/memory-not-found-error.ts";
import { DatabaseError } from "@/infrastructure/errors/database-error.ts";

export function errorHandler(error: Error): Response {
  if (error instanceof ValidationError) {
    return new Response(
      JSON.stringify({ error: error.message }),
      { status: 400, headers: { "Content-Type": "application/json" } },
    );
  }

  if (error instanceof MemoryNotFoundError) {
    return new Response(
      JSON.stringify({ error: error.message }),
      { status: 404, headers: { "Content-Type": "application/json" } },
    );
  }

  if (error instanceof DatabaseError) {
    console.error("Database error:", error);
    return new Response(
      JSON.stringify({ error: "Database service unavailable" }),
      { status: 503, headers: { "Content-Type": "application/json" } },
    );
  }

  console.error("Unexpected error:", error);
  return new Response(
    JSON.stringify({ error: "Internal server error" }),
    { status: 500, headers: { "Content-Type": "application/json" } },
  );
}

Common Patterns

Result Type

Avoid throwing exceptions across boundaries.

// src/shared/types/result.ts
export class Result<T> {
  private constructor(
    private readonly success: boolean,
    private readonly value?: T,
    private readonly error?: string,
  ) {}

  static ok<T>(value: T): Result<T> {
    return new Result(true, value);
  }

  static fail<T>(error: string): Result<T> {
    return new Result(false, undefined, error);
  }

  isSuccess(): boolean { return this.success; }
  isFailure(): boolean { return !this.success; }

  getValue(): T {
    if (!this.success) throw new Error("Cannot get value from failed result");
    return this.value!;
  }

  getError(): string {
    if (this.success) throw new Error("Cannot get error from successful result");
    return this.error!;
  }
}

Anti-Patterns to Avoid

Anemic Domain Model

// BAD - No behavior, just data
export class User {
  id: string;
  name: string;
  email: string;
}

// GOOD - Rich domain model
export class User {
  private name: string;

  changeName(newName: string): void {
    if (newName.trim().length === 0) {
      throw new Error("Name cannot be empty");
    }
    this.name = newName;
  }
}

Infrastructure Leaking into Domain

// BAD - Domain depends on infrastructure
import { Pool } from "@db/postgres";

export class User {
  async save(pool: Pool): Promise<void> { /* ... */ }
}

// GOOD - Domain defines interface
export interface UserRepository {
  save(user: User): Promise<void>;
}

Exposing Mutable State

// BAD - Direct access to mutable array
export class Order {
  public lines: OrderLine[] = [];
}

// GOOD - Encapsulation with readonly
export class Order {
  private readonly lines: OrderLine[];

  getLines(): ReadonlyArray<OrderLine> {
    return this.lines;
  }
}

Key Principles Summary

1. Domain layer has no dependencies - pure business logic
2. Follow dependency flow - dependencies point inward
3. TypeScript-first - No any types in production code
4. Use value objects for immutable concepts
5. Entities have identity and lifecycle
6. Aggregates enforce invariants
7. Repositories are interfaces in domain
8. Use cases orchestrate, don't contain business rules
9. DTOs cross boundaries
10. Layer-specific error handling
11. Test domain logic in isolation
12. Encapsulate state
13. Use Result type for expected failures
14. Error translation at boundaries

Additional Resources

• DDD Book (Eric Evans): https://www.domainlanguage.com/ddd/
• Implementing DDD (Vaughn Vernon): https://vaughnvernon.com/
• Martin Fowler's DDD: https://martinfowler.com/tags/domain%20driven%20design.html