If good test coverage is the key to writing quality code, test-driven development (TDD) is the simplest means of achieving it. TDD is a software development approach that flips conventional wisdom on its head: First you write a test that fails, and then you code a solution. Rinse and repeat.
This incredibly simple development methodology may seem perplexing to some—how can you test something that does not yet exist? Others might dismiss TDD as being ideal in theory but not practical in reality.
Whether you’re a complete beginner or a seasoned nonbeliever, in this article we’ll strive to answer any questions and dispel any misconceptions you may have about this agile approach to software development. Let’s dive into TDD, what it is, and how to implement it.
What Is Test-Driven Development?
For most developers, priority number one is to get things done. When you have an impending deadline, you want tangible results in the form of production code. Writing unit tests up front feels counterproductive. If you’re in the camp that feels TDD is not realistic for your workflow, I have good news: Writing unit tests does not have to feel like a chore.
The secret to using TDD is to understand what it means to be “test driven.” When you write a unit test, you’re typically trying to test the implementation of some production code you already wrote. TDD flips the sequence: You write the test for the intended implementation first, then write the bare minimum amount of code required to pass that test.
Here is where most people get led astray; they continue to write unit tests as they would post-production code, the only difference being the order in which they do it. The trick is to realize that the unit test in TDD is more than a test. It’s also a tool you can use to flag the next step in development.
To do this, each unit test must be:
- Modular—targeted at a specific requirement or feature you want to implement.
- Small—just enough code to fail.
- Descriptive—when you hit “run” after being away awhile, it should clearly tell you how to pick up where you left off.
It’s that last bullet that will help you get things done. I highly recommend you check out this GopherCon 2017 video presentation, “TDD for those who don’t need it” by Chew Choon Keat, for an entertaining introduction to using TDD in a Golang workflow. Even if you’re not a Go programmer, you’ll still get value from seeing how a combination of simple unit tests and clear error messages can not only ensure good test coverage but also make you more productive.
Benefits of Test-Driven Development
TDD comes with many benefits. These are just a few:
- It places planning front and center.
- Quality is built into your application.
- Faster feedback leads to faster iteration.
- It results in lower development costs over time.
- It encourages the development of small, clean, and efficient units of code.
- It’s maintainable, flexible, and extensible.
- Unit tests double as documentation (with fewer comments in code).
- It maximizes test coverage, greatly reducing bugs at launch.
- It helps you write code that meets all the acronyms: YAGNI, SOLID, KISS, and DRY.
How to Implement Test-Driven Development
Implemented correctly, TDD keeps you focused on writing efficient code that’s free of scope creep or bulk. The production code you create from refactoring the solution to the unit test you just wrote is 100% tested. This development cycle is often divided into three phases:
- Red—the first phase of TDD, where you determine what you want to develop. You write a unit test that assumes some implementation of a feature you want already exists. It will naturally fail because you have yet to create it, putting you “in the red.”
- Green—the second phase of TDD, where you write the bare-minimum production code required to pass the unit test. You are “in the green” if you pass the test.
- Refactor—before you put yourself back “in the red,” it’s a good idea to check and see if you can’t implement your code better or more efficiently.
Each new test you write gets added to your test suite. Your test suite grows with your application and allows you to automatically check your code for breaks in software with each new addition to the codebase.
Test-Driven Development Is a Long-Term Investment
A little planning up front goes a long way. TDD helps you identify the smallest unit of work required to move a project forward, giving you the modularity and flexibility to adapt your product to a changing environment. Your application grows along with your test suite which guides development and ensures complete test coverage throughout development. TDD helps you build quality into an app from the ground up.
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