Introduction to TDD with Node.js

If you are brand new to Node.js, this material will move a bit too fast. But if you can already read backend code and want a more disciplined way to write it, you are in the right place.

How to Use the Repository

All source code lives on GitHub:

👉 test-driven-development-tdd-with-nodejs-course

Most hands-on chapters have two branches:

For the best experience, start from the start branch and write the code yourself. Use the finish branch only to compare your work or to recover if you get stuck.

A few notes on how the repository is organized:

• This introduction has no coding branch.
• The first branch appears in the next chapter as 01-what-is-tdd-start.
• Branch numbers track the hands-on chapters, not the markdown file numbers.
• The main branch holds the final version of the project.

What We Will Build

We are building a URL shortener API. This is a service that:

1. Accepts a long URL
2. Returns a short code
3. Later redirects users to the original address

The idea is simple, which is exactly what makes it a great TDD project. The behavior is easy to describe, but building it still leaves plenty of room to practice real backend work:

• Request and response design
• Route testing
• Application structure
• Database integration
• Input validation
• Concurrency and collision handling
• Test isolation and CI

We will start with a tiny example in plain Node.js so the TDD loop is easy to see. Then we move into the real project: a Fastify API in TypeScript, backed by PostgreSQL.

The progression looks like this:

1. Learn the Red-Green-Refactor cycle with a small Node.js example.
2. Set up the Fastify project and write the first failing integration-style test.
3. Implement URL shortening with the smallest possible code, then replace the fake with real logic.
4. Move from in-memory storage to PostgreSQL without breaking existing behavior.
5. Expand the API with redirects, click tracking, listing, stats, and deletion.
6. Harden the service with validation, collision handling, better test infrastructure, and CI.

The project stays understandable from start to finish. We add complexity in layers, and the test suite tells us if each step was safe.

What to Expect

We will not write code first and bolt tests on later. The test comes first whenever the behavior is clear enough to describe.

Expect to see:

• Failing output, often
• Small, incremental steps
• Code that is simple before it becomes elegant

This is exactly how the process is supposed to work. In a production environment, this approach gives you much more than just test coverage. It gives you fast feedback. When a change breaks your app, you know immediately. When a refactor is safe, you know immediately.

Next, we will make TDD concrete with a tiny Node.js example before moving into the main Fastify application.