February 13, 2023

A Guide to Test-Driven Development

There are many ways to approach developing software. In recent years, one that is gaining traction is Test-Driven Development (known as TDD)...

Raz Gaon

What is Test-Driven Development?

There are many ways to approach developing software. In recent years, one that is gaining traction is Test-Driven Development (known as TDD). What is this approach, you ask? In simple words, it is a software development process that emphasizes writing specifications and tests before implementing the actual code. In TDD, the software developer continuously iterates over the specifications and updates the code and tests as needed. The code is tested often to ensure high quality.

Kent Beck is credited with developing the technique. It came about as one of the concepts of Extreme Programming. By writing specifications and tests before the actual code, the programmer can catch design flaws early and fix them before writing a single line of code.

Write specifications
To explain the process of TDD, we need to go over an important concept: specifications. A specification describes the behavior of a module. It specifies the types of the parameters and any constraints on them (known as the precondition). It also specifies the type of the return type and how it relates to the inputs (known as the postcondition). A function specification gives a high-level overview of what the function does. This allows us to write coherent tests that follow the specification.

This step is optional, but we need to know what specifications to follow to write efficient tests. An excellent way to do this is to write the function signatures,
Write Tests
We won't go into detail about how to structure your test suites in this post, but the key idea is to write thorough tests that follow the specifications. It is crucial to cover the most important cases. Otherwise, you might have false confidence that your implementation is valid.
Implement a minimal working solution
This solution should be as simple as possible. The idea is to get a working implementation and then iterate and improve it after tests have validated it.
This step includes refactoring of the different components. You can improve the implementation, add more tests, change specifications (including updating the tests and implementation), etc.


Fewer bugs

Consistently writing tests offers a more robust program with fewer bugs. Writing the tests after most of the code is implemented leaves you with a sizeable invalidated codebase. By validating the code along the way, it is easier to catch bugs when they pop up and feel more confident about making changes to the implementation.

Improved design

By taking the time to design and structure the implementation, the codebase becomes more robust. TDD forces the developer to write specifications and tests before the implementation, significantly reducing wasted code.

Better Coverage
Code coverage is a critical component of writing good software, and TDD forces the developer to cover most of the code they write.
Easy Refactoring

Refactoring code covered by tests is a pleasant experience that allows the developer to make changes while being confident that if something breaks, the tests will find it. The developer can be agile in making changes that could optimize the system.



In some cases, it might be faster to quickly implement a solution and check if it works than to write a whole test suite beforehand. Writing a solid test suite takes time and careful consideration, which might not suit everyone.  


Although writing tests first makes them more productive for some people, it hinders their ability to create and design efficiently. Writing tests is often tedious, and many developers would preferably implement their ideas before writing the tests.


It can be difficult to write tests for a program that doesn't work. On the one hand, it can improve design qualities; on the other hand, it may be difficult to comprehend how everything will work before writing it.


When working in teams, it may be challenging to have all developers use TDD. In reality, many developers have personal preferences, and some don't enjoy writing tests before writing code.


TDD will often force the developer to rewrite many tests, which can become challenging to maintain over time.


Is TDD right for you? It mainly depends on the situation and the type of work. If you are working on a product with customers, TDD might be a good fit. If you are writing a side project for fun, it might be overkill. My best suggestion is to go over the advantages and drawbacks and decide if it is a good fit for your work. Happy coding!

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