TDD vs BDD – What’s the Difference Between TDD and BDD?

Anushree Chatterjee

In the world of software development, we often hear about test-driven development (TDD) and behavior-driven development (BDD), but what exactly is the difference between these terms? If you’re looking for a simple answer to the difference between TDD and BDD, this post from our team of software experts contains everything you need to know.

While both approaches aim to improve software quality, TDD focuses on writing low-level tests before code, whereas BDD emphasizes defining application behavior from a user or business perspective. Understanding when to use each approach can help teams align development, testing, and business goals more effectively.

What is Test-Driven Development (TDD)?

Test-driven development is a software development process that involves writing tests describing the results you’ll want to see from the changes in the code before any code is written. Ultimately, the aim of using test-driven development is to reduce bugs and improve the maintainability of the project code over time.

How Does TDD Work?

Here’s how it works:

TDD involves a cyclic activity: red-green-refactor

• Red (Write a Failing Test): You start by writing a single, small automated test for a new piece of functionality or an improvement you want to add. When you run this test, it should fail (turn “Red”) because the code it’s testing hasn’t been written yet. This failure is a crucial signal. It proves that the test is actually working and correctly detecting the missing feature.
• Green (Write the Minimum Code): Now, you write just enough code—the minimum necessary—to make that failing test pass. The test now passes (turns “Green”). Your goal here is purely to satisfy the test, not to write elegant or complete code yet.
• Refactor (Improve the Code): With all tests passing (Green), you can now safely clean up and improve the code you just wrote. This might involve removing duplication, renaming variables, or restructuring methods to make the code clearer and more efficient. Since you have a suite of tests that are all passing, you have a “safety net”. If your refactoring introduces a bug, one of your tests will instantly fail (turn Red), alerting you to the problem.

Why Use TDD?

TDD isn’t just a testing technique; it’s a design discipline that offers significant benefits. It turns the traditional development process on its head, using testing as the primary tool to drive good design, leading to higher-quality, more maintainable software.

Since test-driven development starts with describing the outcome needed from the software, and the tests needed to confirm the software is producing the expected outcome, it often goes hand-in-hand with thought exercises used to design test cases.

Usually, test-driven development also relies heavily on automated tests because the developers regularly run the tests against the code they are developing to track progress towards completion. Once all of the automated tests pass, the developer can confirm that they’ve met the acceptance criteria defined and verified by the tests.

Related reads:

• What is Test Driven Development? TDD vs. BDD vs. SDD
• How to Handle TDD with AI

What is Behavior-Driven Development (BDD)?

Behavior-Driven Development (BDD) is an approach that bridges the communication gap between business stakeholders (like product owners or customers) and the technical team (developers and testers). It achieves this by defining application behavior in a clear, common language that everyone can understand. BDD, same as TDD, is also driven by tests. However, the main difference is that in behavior-driven development, the tester is the one who defines the expected software behavior, that is, a more user-centric perspective. The outcome is in the form of user stories at a higher level of abstraction (we’ll show you an example shortly).

How Does BDD Work?

The heart of BDD is a simple, structured, non-technical language called Gherkin. This language uses a specific format to describe system behavior in the form of Scenarios.

A typical Gherkin scenario follows this three-part structure: Given, When, Then. Gherkin syntax structures plain English text so that a machine can later recognize it, in a way that describes a set of inputs and context that exist to produce an expected result.

{
  Given I am on the password reset page
  When I enter "username" or "email address"
  And I click on "Reset password"
  Then I receive a password reset email
}

Usually, these Gherkin user stories in behavior-driven development are produced as a result of a collaborative effort between product owners, business analysts, developers, and QA teams. This is often referred to as the “Three Amigos” meeting, which includes:

• The Business/Product Owner: Defines what the system should do.
• The Developer: Determines how to build the feature.
• The Tester: Figures out how to verify the feature works correctly.

In this meeting, the team collectively writes the Gherkin scenarios. These scenarios then become:

• The requirement specification for the product owner.
• The unit tests for the developer (who automates the Gherkin statements).
• The test cases for the tester.

Also read: What is Behavior Driven Testing?

What Are the Main Differences Between TDD and BDD?

While TDD and BDD do share a lot of similarities, they still have some important differences that one needs to know about to avoid confusion.

From the above differentiation, we can see that one crucial difference is in what TDD and BDD are aiming to test, respectively. While TDD aims to test the software functionality down to specific, smaller pieces in isolation, BDD aims to test the software from the user perspective to ensure a high-level outcome is achieved.

In addition, TDD is usually performed by a software developer in isolation, whereas BDD is normally a collaborative process where several team members from different disciplines write the user stories together.

How Does testRigor Take BDD to a New Level?

Typical BDD is a 3-step process:

• Teams create specifications using the Gherkin language.
• Engineers develop the desired feature.
• QA people write automated tests based on Gherkin specifications from step 1.

BDD generally faces the following obstacles, which make it difficult to adopt:

• The Gherkin syntax becomes difficult to use for complex features, further obstructing non-technical folks from participating in the QA process.
• There’s a need for a middleman, someone who can write executable code corresponding to each Gherkin statement. This means that for every update or write, you need an engineer as well. This can even be cost-intensive for companies.

If you’re somewhat familiar with testRigor, you know that you can easily create executable specifications in plain English, which is easier for everyone to understand than Gherkin. This truly makes test cases accessible to all, as one no longer needs to depend on code to write test cases that actually work. By using testRigor, you can streamline the process and eliminate one of the steps completely.

The process with testRigor looks like this:

1. Teams create executable specifications in plain English using testRigor
2. Engineers develop the feature

Unlike other BDD-supporting tools like Cucumber and Selenium, this tool eliminates the need to rely on the technical implementation details like XPaths and CSS selectors of UI elements. You can simply write what you see on the screen, and testRigor’s AI engine does the rest. For example, if you want to click on the login button, you can simply write click on “Login”. This ability to refer to UI elements in simple language not only reduces complications arising when trying to create test cases, but also slashes test maintenance efforts to a bare minimum. testRigor’s AI can adapt to changing UI and keep your test cases running smoothly.

In the end, testRigor is one of the most powerful tools in the market for any team using a BDD process for their software project. It takes out many of the imperfections of BDD and saves resources. We’ve created a separate article that further explains the benefits of incorporating testRigor into the BDD framework (we even shaped a new term for it, SDD).

If you’re interested in learning more about how testRigor and BDD can streamline your software development process, get in touch with us, and our friendly team of software development experts will gladly show you how your team can get up and running in a short time.

Summing it Up

The primary difference between TDD and BDD is their focus and who they’re for. TDD is a technical practice for developers that focuses on testing small, individual units of code. BDD is a collaborative methodology for the entire team that focuses on testing the application’s behavior from a user’s perspective.

Think of it like this: TDD helps you build the software right, while BDD helps you build the right software.

By using intelligent test automation tools, you can simplify your QA endeavors further.

“We spent so much time on maintenance when using Selenium, and we spend nearly zero time with maintenance using testRigor.”

Keith Powe VP Of Engineering - IDT