Introduction to TDD with Node.js

Overview

This TDD with Node.js course is about building a backend service from behavior, not implementation. We will use Test-Driven Development (TDD) — writing a failing test before production code — to build a URL shortener API with TypeScript, Fastify, PostgreSQL, Jest, and Docker.

By the end, you should be able to take a feature from a failing test to production-ready code without losing control as the system grows.

Why TDD with Node.js

Early in my career, I kept hearing that tests were important. In real projects, they were often the first thing cut when deadlines got tight. That gap between what teams say and what teams do is one reason I made this course.

Node.js is a good place to learn TDD because the feedback loop is short. You write a test, run it in seconds, and see whether the behavior is correct. That makes the Red-Green-Refactor cycle easy to feel, not just understand.

This course stays practical. We will build one service from start to finish, keep the scope clear, and add real problems only when we need them.

TDD does feel slower at first. You will sometimes wonder whether the test-first step is worth it. That is normal. The goal of this course is to shorten that awkward phase by giving you repeated practice on one project.

Who This Course Is For

This course is for engineers who know the basics of backend development and want a better testing workflow. You do not need Fastify or PostgreSQL experience, but you should already be comfortable with:

• Node.js fundamentals
• JavaScript or TypeScript, including async and await
• REST API basics
• SQL and relational database basics
• Git commands such as clone, checkout, and commit

If you are brand new to Node.js, this course will move too fast. If you can already read backend code and want a more disciplined way to write it, this course is for you.

How To Use the Repository

All source code for the course lives on GitHub:

https://github.com/mt26691/test-driven-development-tdd-with-nodejs-course

Most hands-on chapters have two branches:

• xx-chapter-name-start — the point before we write the code for that chapter
• xx-chapter-name-finish — the completed state after we finish the chapter

Start from the start branch, write the code yourself, and use the finish branch only to compare or recover if you get stuck.

This introduction chapter does not have a coding branch. The first branch appears in the next chapter as 01-what-is-tdd-start. That numbering is intentional. The branch numbers track the hands-on chapters, not the markdown file numbers.

The main branch contains the final version of the project.

What We Will Build

We are going to build a URL shortener API — a service that accepts a long URL, returns a short code, and later redirects users to the original address.

The idea is simple, which makes it a good TDD project. The behavior is easy to understand, but the implementation still gives us 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 will move into the real project: a Fastify API in TypeScript, backed by PostgreSQL, and built step by step as the system grows.

The progression is deliberate:

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.

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

What To Expect

This is not a course where we write code first and add tests later. The test comes first whenever the behavior is clear enough to describe.

You will see failing output often. You will see small steps. You will also see code that is simple before it becomes elegant. That is not a flaw in the process. It is the process.

In production, this gives you more than coverage. It gives you fast feedback. When a change breaks behavior, you know quickly. When a refactor is safe, you know quickly.

In the next chapter, we will make TDD concrete with a tiny Node.js example before we move into the Fastify application.