Two methodologies dominate the conversation about building high-quality software: Test-Driven Development (TDD) and Behavior-Driven Development (BDD). Each focuses on delivering reliable, well-tested code but takes a different approach. One leans on the technical details of testing from a developer’s perspective. The other prioritizes collaboration and user-centric outcomes.
How do you choose the strategy that aligns with your goals? This post explains the core differences between TDD and BDD, highlights the unique advantages of each, and helps you decide which methodology suits your team and project. Whether you want to improve your development process or tackle your next big build, this guide will point you in the right direction.
What is test-driven development (TDD)?
TDD is a software development methodology in which tests are written before the code. This approach’s primary goal is to verify that the software meets its requirements, typically through frequent test cases and validation.
How TDD works
The typical TDD approach to software is as follows:
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- List scenarios. First, the test engineers will outline the software’s desired features or functionality.
- Write a test. Then, they write an automated test that would meet the acceptance criteria in this scenario.
- Run the tests. Since no new functionality has been implemented yet, this test should fail.
- Write simple code. Write the most straightforward code possible to achieve a test pass by meeting the acceptance criteria.
- Run the test again. This should result in a test pass, thanks to the new code.
- Refactor the code. Now that the acceptance criteria have been met, the code is optimized to improve performance.
- Repeat the process.
Benefits of TDD
Thanks to the refactoring process, TDD can result in better code. Refactoring can be a more reliable and efficient form of coding, as the testing methods mean the code always works before it’s optimized. Another benefit of TDD is the development team’s ability to document system behavior through continual tests. A TDD approach can result in a more efficient solution over time.
When to use TDD
TDD works well with unit testing and is best applied to smaller, modular components. Thanks to its reliance on refactoring, it can also be effective when dealing with legacy systems.
What is behavior-driven development (BDD)?
Behavioral-Driven Development (BDD) is an agile software development methodology focusing on user behaviors. This means the developers build a solution by focusing on its functionality from the end user’s perspective.
How BDD works
A typical process for BDD feature development would work as follows:
1. “Given when then.” The software engineer outlines a scenario within the domain model in a simplified “given when then” format using a non-technical language like Gherkin. An example might be:
Given the user logs in
When they use their login details
Then, they should gain access to their account
2. These scenarios are often gathered into documents, given a write-up, and shared with developers, stakeholders, and other teams.
3. The developer writes and automates tests in the domain model to check whether the software works with these Gherkin scenarios.
Benefits of BDD
BDD is a collaborative approach that unifies developers, testers, and other professionals . Its scenarios are also created in plain language rather than code, furthering its benefits as a collaborative process. BDD also focuses directly on user experience and behaviors. This means the end-user is always prioritized during software development.
When to use BDD
Behavior-driven development is an ideal solution for full-stack applications. It’s also an incredibly useful strategy when developers need to collaborate directly with product designers, sales teams, or stakeholders. As a methodology, it’s also well-suited to complex systems that require behavior validation, such as systems with user profiles and authentications.
Key differences between TDD vs BDD
Test vs. behavior
The main difference between TDD and BDD is that TDD focuses entirely on testing, whereas BDD factors in user behaviors. Therefore, TDD focuses on a system’s functionality based on internal logic and process optimization. BDD instead prioritizes the behavior of end users. One way of looking at this is understanding that TDD is focused on how software is developed, whereas BDD focuses on why.
Technical vs non-technical collaboration
Test-driven development revolves entirely around the software development process. As a methodology, it mainly involves the developers and other technical departments. BDD is a broader and more collaborative approach. It can involve critical individuals and teams with non-technical roles in addition to developers.
Test style
One of these methodologies’ key differences is the testing process. TDD involves software unit tests, usually focused on specific methods or functions to achieve a desired result. BDD uses an acceptance test system written in simple English or Gherkin.
Advantages of TDD over BDD
Faster for smaller codebases
Test-driven development can be a lightweight and quicker alternative to BDD if you’re working with a more minor system. A smaller codebase means a test can be written and returned faster, and developers have more opportunities to iterate.
Easier for a development team without much business interaction
By applying TDD, developers can focus entirely on development from a technical standpoint. They don’t need to involve other departments or create user stories before performing tests. This can be an advantage for development teams that don’t often need to collaborate with the broader business, as they can stay focused on testing and optimizing.
Advantages of BDD over TDD
Enhanced collaboration
BDD allows developers to share technical information with other teams in a less technical way. This means developers can gain insights from several different departments and then apply them to their code. This shared understanding also means other departments benefit from the development process. For example, marketing teams will better understand how the software works, which they can use to create superior targeted campaigns.
Focus on user outcomes
By applying BDD, developers always align code optimizations with their end users. This allows developers to build solutions that cater to customers’ needs and behavior, which typically results in higher customer satisfaction.
Challenges of using TDD
Potential for over-testing
Since it’s such a test-focused process, TDD methodology can sometimes lead to over-testing. One example might be developers conducting redundant tests for simple features or designing unnecessarily complicated testing suites. This can sometimes make it a needlessly time-consuming strategy.
Difficult to use with legacy systems
Due to their age, some legacy systems lack a modular structure. This can challenge test-driven development since the TDD approach depends on modular components. This is a particularly frustrating challenge since TDD’s refactoring process can be very useful for updating legacy systems’ code if they’re modular.
Challenges of using BDD
Requires more upfront planning
BDD fosters better team collaboration, but only when planned effectively. BDD demands a lot of effort from developers, particularly in the early stages of development. At this stage, all use cases must be written and distributed to relevant teams before developers can test and optimize.
Time-consuming for simple features
Since it involves many different teams, BDD tests can be expensive if applied to simple features. This is because multiple teams, including business stakeholders, are often involved in the workflow.
When to choose TDD
You should choose TDD if your project is smaller in scope and has clearly defined technical requirements, where unit-level testing is the main goal. You should also consider test-driven development if you need a faster development cycle without involving stakeholders.
When to choose BDD
You should apply BDD methodology if your project involves multiple stakeholders and your focus is on user experience or customer behavior. It’s also worth choosing BDD if collaborating between technical and non-technical teams is crucial for the project’s success.
Tools for TDD
Some of the most popular frameworks that support test-driven development (TDD) include:
- JUnit for Java.
- NUnit for .NET.
- RSpec for Ruby.
- PyTest for Python.
Tools for BDD
Some of the most popular frameworks that support behavioral-driven development (BDD) include:
- Cucumber for Java, Ruby, and JavaScript.
- SpecFlow for .NET.
- Behat for PHP.
- Jasmine for JavaScript.
Choosing the Best Path for Your Project
In the argument of TDD vs BDD, you need to determine which development technique makes sense for your particular solution. Where TDD development will help you improve code quality, BDD will help you collaborate within your business and meet your customer’s needs. Make sure to choose your methodology based on these critical differences, your project’s size, your teams, and what you want to achieve.
FAQ
What is the main difference between TDD and BDD?
One key difference between TDD and BDD is that TDD is a more technical approach. It often uses only code or a domain-specific language, with test results delivered through an executable specification. Meanwhile, BDD focuses on behavioral data and overall user experience and can be communicated to non-technical teams.
Can TDD and BDD be used together?
Yes, applying a hybrid approach to your software test cases is possible, particularly if you manage complex domains. Unit tests conducted as test-driven development can be combined with BDD behavioral tests or spec flow.
Is BDD only for large teams?
No. Behavior-driven development implementation can be useful regardless of your business requirements, but it’s particularly useful if diverse stakeholders are involved. Its effectiveness will also depend on your business domain model and goals.
Do TDD and BDD require specific tools?
While the TDD methodology and BDD development have specific solutions, their implementation can be handled using general testing frameworks. This makes their different test cases highly adaptable, depending on your code, the needs of your development, and your business goals.
Is BDD more difficult than TDD?
Implementing behavioral-driven development is relatively easy but requires more forward planning. Development complexity depends on your software project, domain experts, business domain model, test cases, and team dynamics.
