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TDD v/s BDD: How Differences Complement

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Would you compare apples and oranges?

Your answer might be no for obvious reasons. 

But what if we reframe the question to this – Would you compare apples and oranges to understand how they complement each other in providing complete health benefits?

You will probably say a yes here. The rationale given in the second question about exploring their complementary roles puts our analysis in a different light. No more are we going to do a versus analysis. Instead, we will find if they have a complementary relationship in our diet. The same goes for different approaches used for software development, especially TDD and BDD.

In this blog, through the differences that exist between Test Driven Development (TDD) and Behavior Driven Development (BDD), we will understand if they are competing against each other or can have a harmonious collaboration in software development:

Difference in How: Their Approach and Dev Philosophy

TDD and BDD differ significantly in their approach to software development. While TDD focuses on developing code that validates a test successfully, ensuring unit-level code functionality, BDD takes a more holistic approach by defining the user behavior expected from an application and then coding for it.

In TDD, developers write tests in a technical style, most often using a programming language. But, since BDD requires cross-functional collaboration, it requires test language to be more user-centric and easy to comprehend, preferably plain English. One of the most common formats used to describe user behavior is Given-When-Then scenarios which incorporate perspectives of different stakeholders, both technical and non-technical.

While TDD primarily focuses on testing code in isolation to validate its functionality, BDD takes a broader approach by involving critical participants in developing a shared understanding of the system’s behavior from a user’s point of view and validating it.

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Difference in When: This or That

As per one of the active dev communities on the internet, knowing when to use TDD comes with practice. TDD usually works better when product goals and requirements are clearly defined. But there may be better approaches than TDD when you are doing a spike in Agile programming in which the intent is to explore how a particular code will work out.

Also, TDD is not recommended in situations requiring complete functional testing. But besides these exceptional scenarios, TDD is a great way to achieve high-quality codes, deeper test coverage and a more significant safety net in refactoring.

BDD is useful for end-to-end functional testing of the application, ensuring that the overall integrated build displays the behavior aligned with user expectations and business logic. This may not be very useful for unit-level testing since a unit code showcases a tiny part of the application functionality. When several thousand lines of such codes are integrated to form the application, the complete application behavior comes to life (which, in the background, results from a complex interaction among these unit codes).

Difference in Consequences: Quality, Collaboration and Speed

TDD-driven software development promotes a lesser need for regression, greater confidence in refactoring and deeper test coverage. In 2008, Microsoft conducted a famous experiment to show how TDD-led teams from IBM and Microsoft created software with fewer defects than their non-TDD counterparts (40% fewer defects in the case of IBM and 60-90% in the case of Microsoft). However, the time spent on writing the codes was 15-33% more for TDD teams for both organizations. This shows that while TDD approach ensures clean code but at the cost of a longer development time.

BDD fosters seamless collaboration, shorter development cycles and a uniform understanding of the application requirements and behavior across groups. This results in faster iterations, reduced rework and better alignment with business goals.

As we can now see that TDD and BDD do not work as competing alternatives for software development. Their focus areas are different but complementary. Hence it is advisable to use them in tandem to ensure cleaner and more maintainable codes at the unit level and a high-quality software product meeting the business and user needs at the broader level.

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