Test-Driven Development vs Behavior-Driven Development: Key Differences You Need to Know

Working on software development can be overwhelming, especially when your team needs to debug code that has low cohesion and high coupling—two qualities that aren’t hard to find in MVPs.

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by Matias Emiliano Alvarez Duran


To get really high-quality solutions and reduce editing and maintenance hours, your development team should follow some Agile practices and methodologies. For example, testing code is a crucial component of the software development lifecycle

The question is never whether to test or not, but when to do it. And according to both test-driven development and behavior-driven development, your developers should test before they write a single line of functional code.

In this article, we’ll go through the similarities and differences between test-driven development vs behavior-driven development, and how to use them for your project.

Table of contents

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What is test-driven development?

Test-driven development, or TDD, is an Agile technical practice that focuses on creating unit tests for all software requirements before coding. TDD has three stages:

  1. Red: write a failing unit test with the expected code functionality

  2. Green: write enough code to pass the test

  3. Refactor: rewrite the code to make it generable, usable, modular, and simple. 

Test-driven development Agile cycle

What are the benefits of TDD? 

Agile test-driven development has many benefits, the most relevant ones are:

  • High-quality code documented. Coding unit tests and running them individually makes your code modular and documented. As a result, it’s easier to maintain, debug, and scale.

  • Less technical debt. Doing software development without TDD usually creates a large technical debt because programmers value time to market over software quality. TDD does the opposite; it values quality and functionality over everything else.

  • High code coverage. Testing isolated units of code allows you to have higher code coverage, simplify continuous integration, and increase the quality of your source code. 

  • Fewer bugs are pushed to production. TDD makes it easy to find bugs and fix them before pushing your code to production. 

What is behavior-driven development?

Behavior-driven development, or BDD, is an Agile team methodology that allows developers to test their software against desired behaviors before coding. BDD uses simple English to describe the desired behavior. So, when using BDD, you’d refer to:

  • Test suite as specifications

  • Individual tests as scenarios

“With BDD, you should define the behavior before you start coding,” explained Martin Aliverti, Senior Tech Leader at NaNLABS.“It’s similar to TDD but it goes further. Defining the behavior of the system doesn’t require technical knowledge, anybody can do it. The code can only be written after it passes the test.”

For example, a Product Owner can write the definition for an expected behavior following this structure:

  1. Given some starting conditions,

  2. When an action occurs, 

  3. Then we expect these results.

BDD uses simple English to describe the desired behavior and outcomes. Source: HyreCar

Based on the definition, the development team transforms the structure into the following test:

Source: HyreCar

The behavior-driven development phases

BDD has three phases:

  1. Discovery. During this stage, technical and business teams meet to discuss how the system should behave. The outcome of his phase is agreement on the desired system behavior. 

  2. Formulation. Based on the previous agreements, the technical team comes up with specific examples and use cases that can be used for guidance and test automation. The outcome of this phase is documented examples.

  3. Automation. Build automated tests one by one to guide the development, and then code the documented examples.

Here’s an example of how those phases work. Imagine your business need is to get more leads but you don’t have a landing page to direct your funnel traffic. Following BDD, you should: 

  1. Call a meeting with key stakeholders(i.e. the head of marketing, sales, and finances, your development team’s project manager, QA automation engineer, and a senior developer). 

During the meeting, you’ll share your need: a responsive landing page that has a contact form for people to request demos. The development team will share their take on it, whether or not it’s possible, and come up with an agreed idea. That’s the discovery phase. 

  1. Your development team will come up with specific examples that show what your landing page should do. Once you agree on those examples, they’ll document them and use this as a guide for the development process. This is the formulation phase.

  2. Your development team will code automated tests based on the documented examples, and then code the responsive landing page to pass the tests. That’s the automation phase. 

BDD aims for team collaboration between the business and technical teams to come up with a shared understanding of the desired behavior of the application. That interaction allows teams to come up with documentation that’s accessible and understood by all team members and stakeholders

Behavior driven development process

What are the benefits of BDD?

Implementing BDD in your Agile software development lifecycle can bring several benefits such as: 

  • Focusing on the end-user and increasing the perceived value of your software solution

  • Promoting team collaboration and alignment

  • Being easy to adopt by all stakeholders due to the use of a conversational language 

  • Reaching a wider audience due to it not being highly technical

What’s the difference between TDD vs BDD?

Developers have different ideas on whether or not TDD is the best approach to software development, especially in projects where specifications and requirements are constantly changing. Unit tests alone can fail to test the actual behavior rather than the code functionality. As an effort to close those TDD gaps, BDD was born. 

Developers have different ideas on whether or not TDD is the best approach to software development, especially in projects where specifications and requirements are constantly changing. Unit tests alone can fail to test the actual behavior rather than the code functionality. As an effort to close those TDD gaps, BDD was born. 

Key similarities and differences of TDD and BDD 

While TDD and BDD are both different ways to test code, they have some similarities. BDD is related to the behavior of a system. It can be used to define rules and tests that are not punctual, whereas TDD is principally used to do unit tests, to test small parts of the code or independent units/modules,” shared Martin Aliverti, Senior Tech Leader at NaNLABS. 

Here are some key differences and similarities between the practices:

When to use TDD or BDD in Agile software development 

Choosing between a test or behavior-driven approach to software development is possible. But using both in the same project can also work well, as you’ll see below.

When to use TDD

Test-driven development is a great approach for projects with clear specifications and a wide timeline. Businesses that are hiring an external software development agency or are augmenting their team will find TDD highly valuable because it means that the code will be easier to maintain in the future. 

Also, TDD results in good quality code, which means it’s

  • Functional

  • Modular

  • Cohesive

  • Good for separation of concerns

When to use BDD

BDD is great when you’re launching a new feature of an already existing software and need to test it against a specific behavior. In those cases, it doesn’t make too much sense to do TDD since it’s harder to write unit tests for existing code. 

BDD, on the other hand, is a great solution due to the simplicity of writing behavioral tests. Ensuring that the coded behavior is the same as the expected user behavior will lead to

When to use BDD and TDD together

Since TDD and BDD test software at two different stages and for two different outcomes, you can easily use both in the same software development project by: 

  • Using a TDD approach with a BDD mindset. It allows you to get clear specifications and build unit tests not only around code functionality but expected behavior. Because understanding the specifications allows you to improve the quality of your tests and therefore, the quality of the code.

  • Doing TDD first and BDD later. You build the unit tests and automated behavioral tests beforehand, do the TDD process while coding, and run the automated tests right after to ensure both functionality and behavior are working properly. 

Closing thoughts: BDD or TDD?

The question should not be when to use one or the other, but how the two methods together can improve the overall result. Doing software development without either is also a possibility, but the outcome will not be specific and may require reworking later. 

Agile software development has many frameworks, tools, practices, and team methodologies that make the Agile mindset come to life. BDD and TDD are only two of a wide sea of practices and methodologies to follow.

Trying to implement all of them can be overwhelming and unnecessary at first. If you’re thinking of adopting some new Agile practices like TDD or BDD, a good way to do it is by augmenting your development team with Agile pods and having your programmers learn from others.

At NaNLABS, we’ve discovered that the best way of learning new skills is by trying them out, guided by someone who already has those skills. We can be those experienced guides for your team!

Ready to build your own custom software with a team that cares about you and your processes? We’re not code monkeys, we care about you.

Frequently asked questions about test-driven development vs behavior-driven development

Is TDD better than BDD? 

TDD and BDD are different testing processes. One can’t be better than the other because they test different outcomes. 

Test-driven development focuses on breaking the software into small parts, making unit tests, and running those to check code functionality. It enters the software development lifecycle at the beginning with a test-first approach. Behavior-driven development also comes at the beginning of the lifecycle but tests the software against the desired user behavior.

Is BDD a form of TDD? 

No, BDD is not a form of TDD.BDD is a separate process that tests end-to-end software behavior. TDD is a test-first approach to software development that involves running unit tests on all functionalities. They’re two different automated testing methods that can go together. 

Is Agile TDD or BDD? 

TDD and BDD are both Agile technical practices. Agile is a set of principles for software development and project management. Test-driven development and behavior-driven development are two technical practices that fall into the Agile umbrella. 

What is TestNG and Cucumber?

TestNG is a testing framework for coding and executing test cases. Cucumber is a tool for doing BDD. You can run both in parallel using the Maven extension plugin in Cucumber. 

What is Cucumber?

Cucumber is a tool that supports BDD. This tool allows the team to write test cases (based on the expected behavior of the system) that are easy to understand for non-technical people. 

Here are some of its benefits:

  • You can write tests without any technical knowledge 

  • The test script architecture is simple

  • The test is easy to read and understand for non-technical people

  • The code is reusable. 

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