TDD vs BDD: Full Comparison

TDD and BDD are 2 popular software development approaches.

TDD is a developer-centric technique that doesn't require input from other team members, while BDD is a cross-functional collaboration technique. TDD focuses on ensuring code correctness, while BDD focuses on defining how certain features behave. TDD involves coding a lot of unit tests, while BDD only uses plain human 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)?

TDD, or Test-Driven Development, is the process of writing and executing automated tests before writing code. Insights from the tests help developers improve the code. TDD reflects the spirit of continuous feedback, resulting in faster bug identification and debugging.

There are 3 stages to TDD: 

1. Write a small, focused test that describes the intended behavior of a function or feature. When you run it, this test should fail because the functionality hasn’t been implemented yet.
2. Implement just enough code to make the test pass.
3. Clean up and optimize the code without changing its functionality. 

Over time, TDD builds up your software functionality through this continuous cycle of writing tests, implementing code, and refactoring.

What is BDD (Behavior Driven Development)?

BDD is an Agile approach to software testing where testers write test cases in simple language that even people without technical expertise can understand. 

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 - And statements. These statements show developers how to develop the code that fulfills the behaviors described. 

Here's a simple example of a BDD test using a login feature: 

Feature: User Login

Scenario: Successful login with valid credentials

• Given the user is on the login page
• When the user enters a valid username and password
• And clicks the "Login" button
• Then the user should be redirected to the dashboard
• And see a welcome message

The goal of BDD testing is to increase collaboration between the technical side and the business side of the organization. Non-technical people can have an idea of what's going in the testing team, while the technical team can better communicate their ideas to stakeholders.
 

How To Do TDD?

1. Write one specific test

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
• Assertion

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 the test

Next, developers run that test and observe it fail. It must fail since no code was developed at this stage, yet. 

You may find that it sounds counterintuitive, but this is actually the fundamental principle behind TDD. 

There are 4 major reasonings for why we need this step:

• Running tests on underdeveloped code ensures that your testing infrastructure, frameworks, and environment are properly set up and functioning. Addressing any issues with the test environment early on helps establish a solid foundation for testing.
• A failed test confirms that it’s actively checking the intended functionality, providing a positive signal that the test is reliable.
• These tests—and any future tests—serve as a checklist guiding your development activities. They act as objectives (think: "I am coding this feature to pass this test"), helping developers stay focused on meeting the requirements and specifications laid out in the test plan.
• As the cycle repeats, testing provides immediate feedback on code correctness, allowing developers to catch misunderstandings early and reducing the risk of issues later on.

In other words, in TDD, a failed test is a good test. 

The real question: which test should be executed and failed first? It is recommended that we only start the initial test with a small, focused scope that captures the core requirements of the feature or function. A small scope is simpler, more focused, and allows for faster feedback.

3. Implement the code

With the insights from the tests, developers start to write just enough code to make the tests pass. Try to address the immediate requirements indicated by the test.

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). 

This is a healthy mindset to achieve a lean codebase that is easy to understand, maintain and refactor.
 

4. Run all tests and refactor 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.

TDD Real-life Example

Scenario: You’re tasked with adding a login feature to an app. The requirements are simple: users should log in with a username and password, and if the credentials are incorrect, they should see an error message. 

Here's the TDD Workflow:

• Step 1: Start by writing a test to simulate a login attempt with valid credentials. You write a test that calls the login function with a correct username and password, then checks if the response indicates success (e.g., user.isAuthenticated == true).
• Step 2: Run the test—it fails, which makes sense because you haven’t written the code for login yet.
• Step 3: Write the minimum code in the login function to check if the provided credentials are correct. Run the test again, and it passes.
• Step 4: Next, write a test for the failure scenario. This time, the test calls login with incorrect credentials and checks for an error message (e.g., error == "Invalid credentials"). Run the test, and it fails because you haven’t handled this scenario yet.
• Step 5: Modify the login code to return an error message when credentials are incorrect. Run the test—it now passes.

How To Do BDD?

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.

Key Differences 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 been rigorously tested with C#, Visual Basic .NET, Managed C++, and J#. A notable advantage of cSUnit over NUnit is its flexible license, which allows seamless integration into commercial closed-source products without incurring any costs.

• Pros: Inherits stability of NUnit; familiar syntax; free for commercial use.
• Cons: Limited community support; less active maintenance.

2. PyUnit (unittest)

PyUnit, also known as unittest, is the standard unit testing framework for Python. Included in the Python standard library, it is readily accessible to Python developers without additional installation. Known for its simplicity and user-friendly syntax, PyUnit makes it easy to create and execute test cases.

• Pros: No additional installation required; clean, easy-to-use syntax; widely adopted in Python community.
• Cons: Limited advanced features compared to third-party frameworks; additional libraries needed for mocking and parameterized testing; some boilerplate code.

3. TestNG

TestNG is a testing framework for Java that builds on JUnit, adding features like test grouping, dependencies, parallel execution, and advanced test configuration. It excels at providing robust reporting and integrates well with Java IDEs and build tools, making it popular among Java developers.

• Pros: Advanced features for complex testing needs; excellent support for configuration and reporting.
• Cons: Learning curve for newcomers; smaller community than JUnit.

Popular BDD Frameworks

1. Cucumber

Cucumber is a BDD tool that allows defining, automating, and executing test cases in a natural language format, often using Gherkin syntax. This format (Given, When, Then) is accessible to both technical and non-technical stakeholders, enabling business analysts and product owners to participate in defining test scenarios.

• Pros: Intuitive, human-readable format; supports multiple programming languages; enables code reusability.
• Cons: Complexity from Gherkin syntax; Given-When-Then can lead to redundant steps, increasing maintenance.

2. SpecFlow

SpecFlow is a BDD framework for .NET that promotes collaboration across teams, involving business analysts, designers, developers, and stakeholders in the development process. It supports living documentation, capturing development progress and failures in real time.

• Pros: Encourages team collaboration; low cost for bug fixes; provides living documentation.
• Cons: Time-intensive for short projects; requires frequent team involvement; may be costly for beginners.

Perform TDD and BDD with Katalon

With up to 3 modes of test creation available in Katalon Studio, TDD is a breeze.

1. No-code mode: launch the Test Recorder and manually interact with UI elements. Katalon captures your actions and turn them into an executable test script. Any UI elements/web pages you interacted with are stored in a centralized Object Repository for future usage.
2. Low-code mode: get into the Katalon IDE and choose from hundreds of keywords available in the Keyword Library. Each keyword is a little code snippet to automate a certain action. For example, the keyword Click automates the action clicking. You only need to specify which element to click on.
3. Full-code mode: want full customization? Switch to Scripting mode and write the tests you want. Switch back to the No-code and Low-code mode whenever you want and enjoy the best of both worlds.

What does this mean for TDD? You can use No-code and Low-code modes to quickly create initial tests that kick-starting the TDD cycle. Not just that, Katalon also integrates with your CI/CD pipeline, allowing you to streamline the entire process.

Envision this: you craft your tests without having to write any code, then run them across environments, browsers, OS. These tests fail, and you start coding. These tests are all stored in a hierarchical fashion, so you can re-execute them whenever you want. That's a lot of time and effort saved!

Start testing better with Katalon

Turning to BDD, 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's a really insightful and detailed BDD testing tutorial with Katalon from Automation Step By Step that you really should check out:

Start BDD Testing With Katalon