Dependency Injection in TypeScript: Clean Architecture Without a Framework

Dependency injection makes your code testable and your dependencies explicit.
You don't need NestJS to do it well.

The Problem DI Solves

// Tightly coupled — hard to test
class OrderService {
  async createOrder(userId: string, items: CartItem[]) {
    // Direct instantiation — can't swap in tests
    const db = new PrismaClient()
    const stripe = new Stripe(process.env.STRIPE_KEY!)
    const email = new ResendEmailService()

    const user = await db.user.findUniqueOrThrow({ where: { id: userId } })
    const charge = await stripe.paymentIntents.create({ ... })
    await email.send({ to: user.email, subject: 'Order confirmed' })
  }
}

Constructor Injection

// Define interfaces for dependencies
interface IDatabase {
  user: { findUniqueOrThrow: (args: any) => Promise<User> }
  order: { create: (args: any) => Promise<Order> }
}

interface IPaymentService {
  charge(amount: number, customerId: string): Promise<{ id: string }>
}

interface IEmailService {
  send(options: { to: string; subject: string; body: string }): Promise<void>
}

// Inject dependencies via constructor
class OrderService {
  constructor(
    private db: IDatabase,
    private payments: IPaymentService,
    private email: IEmailService
  ) {}

  async createOrder(userId: string, items: CartItem[]) {
    const user = await this.db.user.findUniqueOrThrow({ where: { id: userId } })
    const charge = await this.payments.charge(total, user.stripeId)
    await this.email.send({ to: user.email, subject: 'Order confirmed', body: '...' })
  }
}

// Production usage
const orderService = new OrderService(prisma, stripeService, resendService)

// Test usage
const orderService = new OrderService(
  mockDb,
  mockPayments,  // never hits Stripe
  mockEmail      // never sends email
)

Simple Container

// lib/container.ts
class Container {
  private bindings = new Map<string, () => unknown>()

  bind<T>(token: string, factory: () => T): void {
    this.bindings.set(token, factory)
  }

  resolve<T>(token: string): T {
    const factory = this.bindings.get(token)
    if (!factory) throw new Error(`No binding for ${token}`)
    return factory() as T
  }
}

// Wire dependencies
const container = new Container()

container.bind('db', () => prisma)
container.bind('payments', () => new StripePaymentService(process.env.STRIPE_KEY!))
container.bind('email', () => new ResendEmailService(process.env.RESEND_KEY!))
container.bind('orders', () => new OrderService(
  container.resolve('db'),
  container.resolve('payments'),
  container.resolve('email')
))

// Usage
const orderService = container.resolve<OrderService>('orders')

Singleton Pattern

class Container {
  private singletons = new Map<string, unknown>()

  singleton<T>(token: string, factory: () => T): void {
    this.bindings.set(token, () => {
      if (!this.singletons.has(token)) {
        this.singletons.set(token, factory())
      }
      return this.singletons.get(token) as T
    })
  }
}

// Prisma client should be singleton
container.singleton('db', () => new PrismaClient())

Testing Without a Container

// Mocks
const mockDb = {
  user: {
    findUniqueOrThrow: vi.fn().mockResolvedValue({ id: '1', email: '[e-mail]' })
  },
  order: {
    create: vi.fn().mockResolvedValue({ id: 'order-1' })
  }
}

const mockPayments = {
  charge: vi.fn().mockResolvedValue({ id: 'ch_123' })
}

const mockEmail = {
  send: vi.fn().mockResolvedValue(undefined)
}

// Test
it('creates order and sends confirmation', async () => {
  const service = new OrderService(mockDb as any, mockPayments, mockEmail)
  await service.createOrder('user-1', [])

  expect(mockEmail.send).toHaveBeenCalledWith(
    expect.objectContaining({ to: '[e-mail]' })
  )
})

When to Use a Framework (NestJS)

Manual DI is fine for most apps. Use NestJS when:

• Large team needs strong conventions
• You need decorators and auto-wiring
• Complex module graphs with many dependencies

Otherwise, manual constructor injection keeps things simple.

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