TDD vs BDD: Full Comparison

TDD (Test Driven Development) and BDD (Behavior Driven Development) are fairly similar development approaches that both emphasize testing and collaboration, yet have major differences in focus and methodology. Simply put, in TDD, developers test first, then use the test results to guide their development, while in BDD, developers express the system behavior they want to create through Gherkin syntax, then code according to those Gherkin expressions.
 

From a higher perspective, TDD is more developer-centric, revolving around code correctness, with programming language-specific frameworks. On the other hand, BDD is more user-centric, revolves around system behavior, and promotes collaboration between relevant stakeholders with a domain-specific language. 
 

In this article, we will explore in-depth the differences between TDD vs BDD through true-to-life testing scenarios, and learn about the popular TDD and BDD testing frameworks.

What is TDD (Test Driven Development)?

Simply put, the TDD process involves writing automated tests before writing the code. The results from these automated tests provide insights for the developer to improve their code. TDD is a more focused and disciplined approach to development, and is itself a way to provide continuous feedback for faster bug identification and debugging.

How To Do TDD?

TDD is a continuous and iterative process of improving your code through tests. The goal after each test is to make the code incrementally better. 

1. Create Tests

Developers start the TDD process by writing a test. In this test, they specifically define the expected behavior or functionality of a small unit of code, which may include:

• Code input (what parameters the code will receive)
• Code output 
• Preconditions/dependencies that must exist/be true for test execution
• Assertions

The developer can write the test themselves using a programming language or leverage some automation testing tools with low-code features for faster test authoring and test execution. The final decision lies in whether the developer wants to build a test framework or buy a pre-built framework from a vendor, which we have explained quite thoroughly in this Katalon vs Selenium comparison article that you may want to take a look at.

See How You Can Write Tests Faster and Better
 

2. Execute 1 Specific Test

Next, developers run 1 specific test and observe it fail (since no code was developed at this stage, yet). It may sound counterintuitive, but this is actually the fundamental principle behind TDD. There are 4 major reasonings for why we need this step:

1. Running tests on an underdeveloped code ensures that your testing infrastructure, frameworks, and environment are properly set up and functioning correctly. If there are any issues with the test environment, you can address them from the very beginning.
2. A failed test confirms that it is indeed checking the behavior of the functionality you want to code. It is a positive signal that this test is reliable.
3. This test (and future tests that will be executed) act as a checklist that guides your development activities. They become objectives to pursue (think: I am coding this feature in order to pass this test), keeping developers focused on meeting the requirements and specifications laid out in the test plan. 
4. As the cycle repeats, this step provides immediate feedback on the correctness of the code, allowing developers to catch misassumptions early on, minimizing the risk of introducing issues later down the road. 

In other words, in TDD, a failed test is a good test. The real question is: Which specific test should be executed and failed first? Usually we should just start the initial test with a small, focused scope that captures the core requirements of the feature or function. 
 

3. Implement The Code

With the insights from the tests, developers start to write just enough code to make the tests pass. At this stage, the goal is still not to make a complete and fully optimized solution, since, again, the reasoning behind TDD is to use test results to guide development, and not the other way around. Developers should only optimize their code after receiving input from tests.
 

The ingenuity of this step is to promote code minimalism. The code follows what the test results tell them, so developers can write concise, straight-to-the-point code, and therefore are able to solve immediate requirements. It discourages developers from prematurely optimizing or adding unnecessary features (i.e. over-engineering the application). It is a healthy mindset to achieve a lean codebase that is easy to understand, maintain and refactor.
 

4. Run All Tests And Refactor The Code

Now that we have some newly added code, developers can run all of the remaining tests, including the one they’ve just written to further confirm that the new code is working fine, and does not break any existing functionality. It is similar to regression testing where the tests are executed to ensure that the features are still working after a code change. In a way, TDD is about continuously executing regression tests to provide feedback on the quality of newly written code.
 

With this feedback, developers can refactor the code, improve its design, readability, and maintainability. After that, the cycle of Fail - Pass - Refactor begins again, and we call this the Red - Green - Refactor cycle of TDD. 

In companies with a TDD policy, developers don’t have to be afraid of show-stopping bugs slipping into production, since quality checks happen way before code implementation. Over time, QA teams can even develop a comprehensive set of test suites that can be reused across scenarios and environments, allowing TDD to unfold at a much faster rate.

Test Driven Development Examples

Let’s imagine a scenario where hundreds of angry clients are reporting a critical bug that stops them from using your application, and this bug has a terrible impact on their business. Your team provides a hot-fix, but unfortunately this hot-fix causes an even worse bug in the system, leading to a wide level crash. 
 

With TDD, this won’t have much of a chance to happen. On a daily basis, the QA teams execute a list of automated unit tests and functional tests for the soon-to-be implemented feature. After watching the test fail, the developers can code with confidence, knowing full well that they can check their code later with that test and it won’t produce a false positive.

What is BDD (Behavior Driven Development)?

Simply put, BDD is an agile testing methodology that uses system behavior to guide the development activities. Instead of starting with a test like in TDD, BDD starts with analyzing the desired behavior that developers want to create. After that, they’ll express the desired behavior using the Gherkin syntax, which consists of Given - When - Then statements. These statements show developers how to develop the code that fulfills the behaviors described.

How To Do Behavior Driven Development?

1. Write Scenarios Using Gherkin

In this stage, stakeholders, testers, and developers sit together to gather the requirements and define the expected outcome they want to create. They will use a unique business-readable language called Gherkin (available in 37 languages) to describe business behaviors. It consists of Given - When - Then statements, each describing a specific aspect of the system.

For example, below is an example of Gherkin describing the Checkout process:
 

Feature: Checkout Process
  As a customer
  I want to be able to add items to my cart and complete the checkout process
  So that I can purchase products online
 
  Scenario: Add item to cart
    Given I am on the product page
    When I click on the "Add to Cart" button for product "ABC"
    Then the item "ABC" should be added to my cart
 
  Scenario: Remove item from cart
    Given I have "ABC" item in my cart
    When I click on the "Remove" button for item "ABC"
    Then the item "ABC" should be removed from my cart
 
  Scenario: Update quantity of item in cart
    Given I have "XYZ" item with quantity "2" in my cart
    When I update the quantity of item "XYZ" to "3"
    Then the quantity of item "XYZ" in my cart should be “3”
 
  Scenario: Proceed to checkout
    Given I have items "ABC" and "XYZ" in my cart
    When I click on the "Checkout" button
    Then I should be redirected to the checkout page
 
  Scenario: Enter shipping details
    Given I am on the checkout page
    When I enter my shipping information
    Then my shipping details should be saved
 
  Scenario: Enter payment details
    Given I am on the checkout page
    When I enter my payment information
    Then my payment details should be saved
 
  Scenario: Place order
    Given I have entered my shipping and payment details
    When I click on the "Place Order" button
    Then my order should be placed successfully
    And I should receive an order confirmation
 

2. Apply TDD

TDD can be a standalone or complementary practice to BDD. Now with the Gherkin statements written out, developers can start implementing automated tests based on them. Depending on the programming language or techstack of the company, developers can choose between Cucumber, Behat, SpecFlow as their BDD testing frameworks.
 

The first test execution is supposed to fail, but it will provide valuable direction to the developers. After developers have implemented the feature, we can run the automated BDD scenarios again to show that the feature is complete. The cycle then repeats until the application is fully developed. 
 

The ingenuity behind BDD is that it translates complex, highly technical jargon from requirements into readable language so that even non-technical professionals can also join hands and contribute to the project. BDD promotes collaboration and reduces misunderstandings.

What Is The Difference Between TDD vs BDD?

Popular TDD Testing Frameworks

1. cSUnit

csUnit is an open-source unit testing framework tailored for the .NET Framework, ensuring compatibility with any .NET compliant language. It has undergone rigorous testing with C#, Visual Basic .NET, Managed C++, and J#. A notable advantage of cSUnit compared to NUnit  is its flexible license, which enables it to be seamlessly integrated into commercial closed-source products without incurring any costs.
 

Pros:

• Inherits the maturity and stability of NUnit.
• Familiar syntax for developers familiar with NUnit.

Cons:

• Limited community support and documentation compared to NUnit.
• Less actively maintained.

2. PyUnit

PyUnit, also known as unittest, serves as the standard unit testing framework for Python. It offers a built-in framework within the Python standard library, making it easily accessible for Python developers.PyUnit requires no additional installation since it is already included in the standard library. This framework inherits the simplicity and user-friendly syntax that Python is renowned for, making it straightforward to create tests.
 

Pros:

• There is no need to install extra modules since it is included in the standard Python library.
• Allows for simple, clean, and fast execution of test cases.
• Generates test reports swiftly, including XML reports and unittest-sml-reporting.
• Widely used and widely adopted by the Python community.
   

Cons:

• Compared to some third-party Python testing frameworks, PyUnit may be considered relatively basic.
• Additional libraries might be required for advanced features like test mocking or parameterized testing.
• Test code abstraction can occasionally make the intent unclear.
• Requires a significant amount of "boilerplate" code.

3. TestNG

TestNG is a testing framework for Java that draws inspiration from JUnit while adding additional features and capabilities. It provides support for writing and executing tests in Java, offering features such as test grouping, test dependencies, parallel test execution, and advanced test configuration. TestNG also excels in providing powerful reporting and flexible test configuration options. It integrates seamlessly with Java IDEs and build tools, making it a popular choice for Java developers.
 

Pros:

• Provides advanced features beyond what JUnit offers, expanding testing capabilities.
• Offers excellent support for test configuration and reporting, enhancing the testing process.
   

Cons:

• May have a learning curve for newcomers due to the additional features and configurations.
• Has a smaller community compared to JUnit, potentially affecting available resources and support.

Popular BDD Frameworks

1. Cucumber

Cucumber testing is a behavior-driven development (BDD) testing approach that utilizes the Cucumber tool to define, automate, and execute test cases in a format that is easily understandable by both technical and non-technical stakeholders.
 

In Cucumber testing, test scenarios are written using a natural language format, often employing the Gherkin syntax. This syntax employs keywords like Given, When, and Then to describe the steps of a test case. This allows business analysts and product owners to write test scenarios in a language that aligns with their understanding of the system being tested.
 

Pros:

• Provides an intuitive way to express requirements in a human-readable format.
• Supports multiple programming languages, enhancing flexibility and adoption.
• Enables code reusability, allowing for the efficient creation of test cases.
   

Cons:

• Gherkin-induced complexity can arise.
• The Given-When-Then framework, while providing clarity, may result in the creation of redundant steps, potentially increasing maintenance efforts.

Read More: A Guide To Doing Cucumber Testing

2. SpecFlow

SpecFlow is a testing framework that facilitates code development and collaboration among developers. It supports Behaviour Driven Development, making it easier for developers to write and test code.
 

Pros:

• Collaboration drives design progress, involving business analysts, designers, programmers, and producers.
• Low cost of bug fixes due to real-time visibility during development.
• Living documentation of progress and failures during code writing and testing.
   

Cons:

• Unsuitable for short projects due to time-consuming nature and cost.
• Expensive for beginners in coding.
• Dependency on all developers for stable use of Behavior Driven Development.
• Requires continuous effort and frequent stakeholder meetings.

BDD Testing With Katalon

Katalon is a comprehensive solution supporting automated test creation, management, maintenance, execution, and reporting for web, API, mobile across a wide variety of environments, all in 1 place, with minimal engineering and programming skill requirements. 
 

For BDD testing, Katalon Studio has a native integration with the Cucumber framework, allowing you to perform UI & API functional automation tests written in BDD Cucumber format. Katalon Studio comes with many features:

• Quickly create a feature file (i.e. the documentation for system behavior written in Gherkin syntax).
• Create test cases using built-in keywords, which are basically code snippets for common test scenarios.
• Group those steps into test suites and collections for easier management.
• Reuse test cases from previous test runs across multiple environments.
• Execute feature files.
• Generate BDD report files.
• Sharing and collaboration capabilities.

Here is a sample BDD testing project on our GitHub repository that you can take a look at. 

Try BDD Testing With Katalon