What is TDD (Test-Driven Development), and why should you care? For many developers today, TDD is a concept shrouded in confusion, often seen as outdated or overly cumbersome. But TDD used to be a rockstar—it was the backbone of Agile’s promise of quality software. During Agile’s golden age, TDD was the technique that enabled rapid iteration with confidence.
Anyway, TDD is still incredibly valuable if practiced effectively. In this post, I’ll share why you should give TDD another chance and how it can boost your productivity and code quality, with some concrete examples to guide you.
Why TDD Matters
At its core, TDD means writing tests before writing the actual code. You start by defining what your code should do in the form of test cases, allowing those requirements to guide the development process. The benefits of TDD come from better understanding requirements, writing cleaner code, and catching potential issues early.
Let me illustrate how TDD works with an example:
Suppose you’re developing a basic shopping cart for an e-commerce site. A feature requirement might be: “The shopping cart must be able to add items and calculate the total price, applying discounts when applicable.” Here’s how TDD can help:
- Step 1: Write a Test
- Start by writing a simple test case to add an item to the cart:
- This defines the expected behavior—adding an apple should increase the item count to 1. You haven’t written the
addItem()function yet, but now you know what it must do.
- Step 2: Run the Test and Watch it Fail
- Since you haven’t implemented
addItem(), the test will fail. This failure is intentional—it gives you a clear direction on what to implement.
- Since you haven’t implemented
- Step 3: Write the Code to Pass the Test
- Now, write just enough code to pass the test:
- You implement the
addItem()method, and now the test should pass.
- Step 4: Refactor and Improve
- Once the test passes, you can refactor your code to improve its quality, knowing that the test will catch any mistakes you make.
Why Following TDD Works
The real value of TDD is that it forces you to think about requirements first, and lets those requirements dictate the code. For example, adding a requirement like “apply a 10% discount for orders over $100” would mean adding a corresponding test:
This test makes sure that the discount is applied correctly, and only then do you implement the discount logic.
My Modified TDD Approach
In practice, I don’t always follow the strict “write tests first” mantra, and that’s okay. Instead, I start by breaking down the requirements in detail, considering both positive and negative scenarios. After that, I write the code while manually testing to make sure it fulfills the acceptance criteria. Once the code works as intended, I move on to writing unit tests.
This modified approach allows me to keep the essence of TDD—focusing on requirements and quality—without getting bogged down by the rigidity of writing tests before any code. These unit tests still focus on validating the requirements rather than just ensuring code coverage. For instance, if the acceptance criteria state that the cart should not accept more than 100 items, I write a test for that scenario:
The test here isn’t about covering every line of code, but about ensuring the logic satisfies the expected behavior—in this case, enforcing a limit on items in the cart.
This modified approach balances the benefits of TDD with the flexibility to write code more naturally. It ensures that the focus remains on delivering high-quality, requirement-driven software without feeling overly constrained by process.
Coverage is Not the Goal—Quality Is
Some developers get bogged down by code coverage metrics. In my experience, if you write tests based on the acceptance criteria and ensure all scenarios are covered, you don’t need to worry excessively about coverage percentages. Focusing on testing real, meaningful cases is more important than achieving 100% coverage with superficial tests.
For example, tests that simply verify that a getter method returns a value add little value in practice. Instead, tests that verify complex scenarios—like the interaction between multiple discounts or adding/removing items from a cart—provide more meaningful coverage.
Conclusion
TDD isn’t dead; it’s just misunderstood. It may seem cumbersome at first, but it helps to build software that is more maintainable, reliable, and aligned with requirements. By using TDD, you start every development task with a clear understanding of the desired outcome, which results in fewer bugs, higher-quality code, and a more confident approach to changes and refactoring.
However, you don’t have to be dogmatic about it. If you’ve dismissed TDD before, try incorporating just the essentials: focus on writing tests that validate requirements, not just code. Adapt TDD to suit your workflow—whether it’s writing tests first or after coding. Trust me—your future self, and your teammates, will thank you.