Test-driven development (TDD) and behaviour-driven development (BDD) terms are often used interchangeably when discussing software development. However, these two approaches have notable distinctions that should be considered. This article offers an overview of the differences between TDD and BDD.

Test-Driven Development (TDD) is a practice which involves writing unit tests for a unit of code before the unit of code itself is written. The philosophy behind this practice is that well-written unit tests are a strong indicator of good design and high quality because they isolate small units of functionality from being broken by the refactoring of other units of code.

TDD’s goal is to keep code minimal and fit-for-purpose whilst ensuring good regression test coverage throughout continued cycles of application development.

BDD (Behavior-Driven Development) is a methodology that expands on TDD by prioritising communication and collaboration among stakeholders in the software development process.

Unlike TDD's focus on individual unit tests, BDD aims to bridge the gap between business requirements, customer expectations, and technical implementation by defining the system's behaviour from a user's perspective.

Test-Driven Development Explained

Test-driven development is a software development approach that involves writing tests that describe the expected outcomes of units of code before any code is actually written. The test will be executed and it will immediately fail as there will be no production code to satisfy it. Then, the production code is written as cleanly and concisely as possible in order for the test to pass the next time it is executed.

Ultimately, test-driven development aims to produce just the right amount of good quality, test supported, fit-for-purpose code for a particular function.

TDD enhances the maintainability of project code over time and the ongoing execution of the unit tests help to ensure code refactoring does not break functions that were not scheduled for change.

Automated testing plays a significant role in test-driven development, with developers frequently running tests against their code to monitor progress. Once all the automated tests have been successfully passed, the developer can confirm that the software satisfies the acceptance criteria and specifications established by the tests.

Behaviour-Driven Development Explained

Behaviour-driven development, like test-driven development, begins and is guided by tests. However, the critical difference is that a developer, tester and product owner or business-analyst collectively define the expected software behaviour in the form of easily readable business language driven scenarios. The desired outcome is in the form of user stories at a higher level of abstraction.

BDD enables the use of plain English sentences to clearly describe the business process under development and test. The Gherkin parser used by Cucumber is the middleware that directly links a BDD scenario with an underlying automated regression test at test run time, detailing specific inputs and contexts necessary for achieving the desired outcome.

Here is an example BDD scenario:

‘Given I am on the home page

When I sign into my account

Then I should be redirected to my account dashboard’

User stories in behaviour-driven development are often produced via a collaborative effort between product owners, business analysts, developers, and QA teams.

What are the Key Differences Between TDD and BDD?

In general, Test-Driven Development (TDD) focuses on writing tests before code to ensure functionality meets specifications. Behaviour-Driven Development (BDD) extends TDD by using natural language to describe the desired behaviour of an application, making it more accessible to non-developers.

Despite their similarities, BDD versus TDD have some crucial differences that anyone working with either methodology should be aware of.

One of the critical distinctions between TDD and BDD is the scope of their intended test coverage. The former focuses on individual units, while the latter concentrates on a user's experience with the software to guarantee satisfactory results

Furthermore, TDD is undertaken solely by the developer writing application code, while BDD involves multiple people from different team roles coming together to draft user stories.

How does Test Evolve Support BDD and Behaviour-driven-testing?

At Test Evolve, we strongly advocate using BDD for project discoveries and generating quality requirements from business conversations between product owners, developers, and testers.

Test Evolve provides excellent transparency of process and traceability to source requirements at all levels of test reporting for your supporting test automation. Ruby, JavaScript or TypeScript can be used as core development languages for the tests - all of them being very accessible to new automation testers and experienced developers alike. 

Testers who have never coded before can contribute to the automation effort within a sprint or two, and industry best practice page-object models provide ultimate code maintainability. 

Ultimately, the comprehensive Test Evolve Automation solution is ideal for any team utilising a BDD methodology for their software project delivery.