When to go for Behavior Driven Development or Test Driven Development (BDD vs TDD)?

Since the middle of the 2000s, there has been a debate between “test driven development” and “behavior driven development” (TDD vs BDD) related blog posts, hackathons, and corporate IT departments. Even today, there are many enthusiastic supporters on both sides. 

Given the extent to which both the business and software landscapes have evolved from choosing the waterfall to an agile framework. It is time to revisit the subject and ask: How do these frameworks significantly contribute to the agile, software-centric, client-obsessed state that so many businesses hope to achieve through digital transformation initiatives in 2022?

TDD or BDD – is one more important than the other in today’s business world?

BDD vs TDD

Before we reach any conclusion, it is significant to understand both the frameworks metaphorically and technically.

Understanding TDD & BDD Metaphorically

Test-Driven Development (TDD) Metaphor

Let’s assume you are going out for a ride in your Toyota, you push the engine start button, and there’s no reaction (the car doesn’t start). The first thing you should do is open the hood and check if the car battery is charged.

Here you performed a test on an individual component of the car, e.g., the battery. You are pretty sure that if the battery works, the vehicle will start. The outcome (if the car starts) depends on the battery test.

If the battery test fails, you can install a new battery (Unit), and the car (system) will start.

With this framework, we are more concerned about testing and making sure that the individual units of the system work correctly.

Often called an inside-out approach.

Behavior-Driven Development (BDD) Metaphor

Keeping the same scenario, with the behavior-driven test, we are only concerned with the inputs (pushing the engine start button) and output (the sound of the engine).

Here we are testing the system’s working (Car) as a whole. We do not test the individual components and focus on testing the system.

Often called an outside-in approach.

Understanding BDD & TDD Technically

Test Driven Development (TDD)

TDD developers consider the application to be a product that can be broken down into separate features or units. He determines the minimally acceptable functionality that a feature must have before writing a test.

This is the most crucial step: he writes the test before writing any code. The test naturally fails because neither the code nor the feature exists. With the failed test as a guide, he writes the smallest amount of code necessary to produce a feature that can pass the test. If necessary, he will return to the code and refactor it to remove any duplication and non-essential material. When he is satisfied, he moves on to the next feature on the list.

The cycle of writing a test for a feature, failing it, and writing code until the feature passes the test, then refactoring, is known as Red-Green-Refactor in TDD parlance.

TDD is an engineering/development practice.

Let’s take a look at TDD using JavaScript language – we are solving String Calculator TDD Kata – designed by Roy Osherove to help programmers improve their TDD skills.

Here’s an example test — 

var assert = require(‘assert’);

var calc = require(‘../src/calculator.js’)

describe(‘StringCalculator’, function() {

describe(‘#add()’, function() {

var calc = new StringCalculator();

it(‘should return 0 when passing an empty string’, function() {

assert.equal(calc.add(”), 0);

});

});

});

The test is quite basic. It requires the existence of the calculator.js file, which is currently empty. When passing an empty string, it verifies if the add function returns to 0.

The next task is to pass the test. You don’t need to get to the final solution at this stage. Instead, do the simplest thing possible to pass the test.

StringCalculator = function() {};

StringCalculator.prototype.add = function(stringNumbers) { return 0; }

We just made the test pass by returning to 0.

The next stage is optional, which is to refactor the code. We generally do two things here:

refactoring the code as needed (removing duplication, improving readability, or increasing efficiency)

rerunning the test and making sure it passes.

Behavior Driven Development (BDD)

BDD begins with the developer envisioning the application as more of an experience – something with which the end-user interacts. He imagines himself in the shoes of that user, conjuring up possible scenarios and results, and then considers what app requirements would be required to reach those ends. 

These scenarios are transformed into explicit user stories that map out the user’s journey through the program. User stories are then converted into test cases, which simply ask the app to perform particular things under specific scenarios, and coding is done to match those requirements.

BDD is a whole team activity.

Many a time, BDD involves a test engineer, product manager, developer, and sometimes other stakeholders. The group gets together to brainstorm tangible cases of acceptance criteria in user stories. 

These cases are then converted into behaviors using plain-English language (using the Given/When/Then template) to boost collaboration among non-technical participants. 

When using tools like Cucumber, FitNess, or JBehave, these cases are described into a feature file using domain-specific language like Gherkin. This feature file can be converted into executable behaviors, from which a developer can implement the required functionality by writing the production code and then running behavior tests to verify everything is correct. If it’s not, we change the production code until the behavior tests pass.

Here’s an example BDD test in JavaScript –

Firstly, we’ll describe the behaviors using scenarios. 

Feature: Is it Saturday yet?

Everyone wants to know when it’s Saturday.

Scenario: Sunday isn’t Saturday

Given today is Sunday

When I ask if it’s Saturday yet

Then I should be informed “Nope”

Here, we start by defining a feature named “Is it Saturday yet?” According to the scenario, if it is Sunday and I question if it is Saturday, the answer should be no. The scenario is then defined using the Given-When-Then syntax.

Now we are going to convert the scenario into executable tests. The following code snippet represents the codification of our scenario’s steps:

const assert = require(‘assert’);

const { Given, When, Then } = require(‘cucumber’);

const isItSaturday = today => {

return ‘Nope’;

}

Given(‘today is Sunday’, () => {

this.today = ‘Sunday’;

});

When(‘I ask whether it\’s Saturday yet’, () => {

this.actualAnswer = isItSaturday (this.today);

});

Then(‘I should be told {string}’, expectedAnswer => {

assert.equal(this.actualAnswer, expectedAnswer);

});

Here, we just coded the response “nope” for the given constant “isItSaturday”. It will pass the behavior(test) even if it does not entirely solve the problem. The steps should pass at this point.

You must be wondering, “Isn’t it true that TDD developers must also consider the app’s behavior?” And does the BDD developer have to develop features in the same way that the TDD developer does? Good news: You’re on the right trajectory.

These developmental approaches do not differ in the same way as cats and dogs. They differ in the way that Cocker Spaniels and Beagles do. Naturally, they share the same basic DNA. TDD developers must consider the behavior of the app they’re creating, just as BDD developers must test features constantly to ensure correct functionality.

Although the difference is significant in the heads of the development team, the path chosen can affect the project’s practicality and cost, communication inside the organization or with clients, and shared digital transformation goals, such as pushing IT closer to the business.

So, what’s the right development approach? BDD or TDD?

Let us compare both approaches over individual metrics like Cost, Quality, and other delivery parameters.

Cost

Anyone who follows the TDD path should expect to have a lot of practice creating various tests. While automated testing with tools like Cyprus.IO can increase the pace and streamline processes, there isn’t much you can do about it: TDD testing may take up as much as half of a developer’s time will be expensive.

While BDD is also largely test-driven, and developers will undoubtedly test code before writing it, they do not test every function as rigorously as TDD developers do. They may, for example, execute an end-to-end test for a specific application behavior rather than individual unit tests of each, perhaps minor, component. Testing what is vital in terms of the application’s behavior usually takes less time and is thus the more cost-effective strategy.

Quality

Which technique produces better code is frequently determined by the particular developer and how well-suited they are to one or the other. TDD supporters often argue that allowing upfront automated feature testing rather than user stories results in the cleanest, most minimal code. BDD developers argue that coding exclusively for specified desired behaviors ensures that nothing extraneous is included in the code. However, it is crucial to note that BDD testing may necessitate more heuristics or judgment calls than TDD.

Proper testing is necessary to keep a working, bug-free application. Some may argue that TDD has an advantage in this situation because a team has all the tests they’ll ever need (until modifications are committed) as soon as they finish coding. Supporters of BDD, on the other hand, may claim that many of the recorded tests aren’t worth repeating because 100 percent code coverage isn’t required to verify that an application is working correctly. Many developers and quality analysts believe that 70% code coverage is adequate for testing.

Communications and Feedback 

BDD outperforms TDD in communication and feedback. Because the behavior descriptions and scenarios are written in plain English, it is easier for non-technical team members and clients to comprehend the testing process and share their observations and comments with the technical teams.

This opens up the door to more fluid two-way communication lines, which can speed up the process of sending, receiving, and implementing feedback, eventually improving the quality of software design and testing. TDD restricts the ability to understand the test to expert programmers. While this technique reduces communication outreach but the quality of code compensates. 

Testing Methodology

TDD is a more traditional approach to software testing that is carried out only by developers and QA engineers, with no input from stakeholders such as product managers. On the other hand, BDD is more inclusive and encourages partnerships between product managers, test engineers, developers, and other stakeholders to accomplish the needed functionality.

Complexity and Learning Curve

TDD is a bit complex and has a steeper learning curve requiring a high level of discipline because it requires that all phases be followed as written, making it excessive for developers. The most common TDD faults are not running tests at the Red stage and covering complex features with a single test. On the other hand, BDD is less complex with a shallow learning curve, making it easier for non-technical teammates to collaborate and be on the same page during the development process.

TDD vs BDD — Key Differences 

CriteriaTest-Driven Development (TDD)Behavior-Driven Development (BDD)  
LanguageTDD test cases are technical in nature. These are analogous to the test cases that are often written during the testing process.The test scenarios created by BDD are written in plain English.
Implementation LevelTDD is comprised of three major stages: test creation, implementation, and code refactoring.BDD involves several stages, involving feature discussion, scenario building, testing, implementation, and code reworking.
Primary FocusThe key focus of TDD is the development of desired functionality based on test cases.BDD is related to the relationship between implemented features and expected behavior.
Bug TrackingTDD makes bug tracking easier because the tests indicate whether they passed or failed.Bug tracking in BDD demands the integration of various tools across the different departments.
DocumentationRequires documentation in order to create correct test cases.Thrust is placed on documents created throughout the scenario design process.
ToolsJUnit, TestNG, NUnit, etc. These tools are used to execute test cases.In BDD, Gherkin is used to write scenarios. Cucumber, SpecFlow, and other popular test automation frameworks are extensively used.

Conclusion 

While comparing both the testing approaches, you should always look at the bigger picture and decide which approach is best suited for your software development requirements. Understanding how these approaches work can assist developers and other individuals involved in software development in determining which test strategy is most suited to their needs. 

Depending on the type of project and the intended outcomes, the best method (or possibly a combination of methods) can be used to meet specific needs most effectively. AnAr Solutions has extensive experience working with both methodologies. 

To get more insights into testing tools and approaches, you can take look at some of our detailed articles that will assist you in deciding what’s the best development approach for a given technology stack.

Further Reading

How BDD acceptance criteria can be used in User stories
unit testing
Automated Unit Testing: MSTest vs XUnit vs NUnit
Agile Metrics for Successful Project Management
BDD
Introduction to Behaviour Driven Development (BDD)
TDD
Introduction to Test Driven Development (TDD)
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