Architecture, Dependency Injection & Patterns — Node.js Course

What you'll learn

Architecture controls dependency direction and the cost of change. Build use cases around domain language, express infrastructure as ports and adapters, and wire everything once in a composition root.

By the end of this lesson, you'll be able to:

Define layer responsibilities
Apply dependency inversion
Use explicit dependency injection
Choose modular-monolith boundaries

Core mental model

Node.js becomes easier when you separate the JavaScript language from the runtime and the operating-system capabilities it exposes. Use this table as a decision guide.

Concept	What it means	Decision rule
Use case	Application behavior coordinating domain and ports	Name it after user intent, not framework mechanics
Port	Interface required by inner code	Define it where it is consumed
Composition root	The single place constructing the graph	Keep new and configuration outside business logic

Professional workflow

Build and verify Node.js programs from the terminal in small, observable steps.

Define the application architecture boundary: inputs, outputs, invariants, ownership, and expected failures.
Design the data or message contract before choosing implementation details.
Implement the smallest correct path with dependencies passed explicitly.
Add validation, failure translation, cleanup, and concurrency behavior.
Verify the boundary with realistic data and at least one adversarial case.
Measure or observe the behavior before optimizing or extracting abstractions.

Keep the feedback loop short

Run the smallest useful command after every meaningful change. Read the complete error message before editing again, and keep inputs and outputs visible while you learn.

Guided code lab

Wire ports to adapters

The use case remains independent of Express, PostgreSQL, Redis, and the system clock.

main.js

const taskRepository = createPostgresTaskRepository({ pool });
const publishEvents = createQueuePublisher({ queue });
const createTask = makeCreateTask({
  tasks: taskRepository,
  publishEvents,
  clock: systemClock,
  ids: uuidIds,
});
const app = createHttpApp({ createTask });

Production practice

Contract

Dependencies point inward: transports translate, use cases coordinate, domain rules decide, and adapters perform I/O.

Verification

Unit-test use cases with fakes, contract-test every adapter, and add architecture checks for forbidden imports or cycles.

Operations

Keep modules independently observable and configurable, define ownership, and document cross-module events and failure modes.

Common failure mode

Adding layers without a dependency rule creates ceremony, while framework objects still leak into every function.

Independent workshop

Refactor the API into a modular monolith with task and identity modules.

Your finished workshop must include:

Module map
Use-case services
Ports beside consumers
Composition root
Adapter contract tests
Dependency rule

Definition of done

Run the happy path and at least two edge cases, keep responsibilities separated, and add a short README explaining how to run the program.

Recap & quick check

Key takeaways

Architecture manages change
Dependencies point inward
DI is explicit construction
Ports express needs
Start with a modular monolith

Quick check

1. Where should a port be defined?

2. What belongs in the composition root?

3. Why prefer a modular monolith first?

Next: Performance, Memory & Profiling