In this article, by Ravi Gupta, Harmeet Singh, and Hetal Prajapati, authors of the book Test-Driven JavaScript Development explain how testing is one of the most important phases in the development of any project, and in the traditional software development model. Testing is usually executed after the code for functionality is written. Test-driven development (TDD) makes a big difference by writing tests before the actual code.

You are going to learn TDD for JavaScript and see how this approach can be utilized in the projects. In this article, you are going to learn the following:

Complexity of web pages
Understanding TDD
Benefits of TDD and common myths

(For more resources related to this topic, see here.)

Complexity of web pages

When Tim Berners-Lee wrote the first ever web browser around 1990, it was supposed to run HTML, neither CSS nor JavaScript. Who knew that WWW will be the most powerful communication medium? Since then, there are now a number of technologies and tools which help us write the code and run it for our needs. We do a lot these days with the help of the Internet. We shop, read, learn, share, and collaborate... well, a few words are not going to suffice to explain what we do on the Internet, are they? Over the period of time, our needs have grown to a very complex level, so is the complexity of code written for websites. It's not plain HTML anymore, not some CSS style, not some basic JavaScript tweaks. That time has passed. Pick any site you visit daily, view source by opening developer tools of the browser, and look at the source code of the site. What do you see? Too much code? Too many styles? Too many scripts? The JavaScript code and CSS code is so huge to keep it in as inline, and we need to keep them in different files, sometimes even different folders to keep them organized.

Now, what happens before you publish all the code live? You test it. You test each line and see if that works fine. Well, that's a programmer's job. Zero defect, that's what every organization tries to achieve. When that is in focus, testing comes into picture, more importantly, a development style, which is essentially test driven. As the title says for this article, we're going to keep our focus on test-driven JavaScript development.

Understanding Test-driven development

TDD, short for Test-driven development, is a process for software development. Kent Beck, who is known for development of TDD, refers this as "Rediscovery." Kent's answer to a question on Quora can be found at https://www.quora.com/Why-does-Kent-Beck-refer-to-the-rediscovery-of-test-driven-development.

"The original description of TDD was in an ancient book about programming. It said you take the input tape, manually type in the output tape you expect, then program until the actual output tape matches the expected output. After I'd written the first xUnit framework in Smalltalk I remembered reading this and tried it out. That was the origin of TDD for me. When describing TDD to older programmers, I often hear, "Of course. How else could you program?" Therefore I refer to my role as "rediscovering" TDD."

If you go and try to find references to TDD, you would even get few references from 1968. It's not a new technique, though did not get so much attention yet. Recently, an interest toward TDD is growing, and as a result, there are a number of tools on the Web. For example, Jasmine, Mocha, DalekJS, JsUnit, QUnit, and Karma are among these popular tools and frameworks. More specifically, test-driven JavaScript development is getting popular these days.

Test-driven development is a software development process, which enforces a developer to write test before production code. A developer writes a test, expects a behavior, and writes code to make the test pass. It is needless to mention that the test will always fail at the start.

Need of testing

To err is human. As a developer, it's not easy to find defects in our own code and often we think that our code is perfect. But there are always some chances that a defect is present in the code. Every organization or individual wants to deliver the best software they can. This is one major reason that every software, every piece of code is well tested before its release. Testing helps to detect and correct defects.

There are a number of reasons why testing is needed. They are as follows:

To check if the software is functioning as per the requirements
There will not be just one device or one platform to run your software
The end user will perform an action as a programmer you never expected

There was a study conducted by National Institute of Standards and Technology (NIST) in 2002, which reported that software bugs cost the U.S. economy around $60 billion annually. With better testing, more than one-third of the cost could be avoided. The earlier the defect is found, the cheaper it is to fix it. A defect found post release would cost 10-100 times more to fix than if it had already been detected and fixed.

The report of the study performed by NIST can be found at http://www.nist.gov/director/planning/upload/report02-3.pdf.

If we draw a curve for the cost, it comes as an exponential when it comes to cost. The following figure clearly shows that the cost increases as the project matures with time. Sometimes, it's not possible to fix a defect without making changes in the architecture. In those cases, the cost, sometimes, is so much that developing the software from scratch seems like a better option.

overview-tdd-img-0

Benefits of TDD and common myths

Every methodology has its own benefits and myths among people. The following sections will analyze the key benefits and most common myths of TDD.

Benefits

TDD has its own advantages over regular development approaches. There are a number of benefits, which help make a decision of using TDD over the traditional approach.

Automated testing: If you did see a website code, you know that it's not easy to maintain and test all the scripts manually and keep them working. A tester may leave a few checks, but automated tests won't. Manual testing is error prone and slow.
Lower cost of overall development: With TDD, the number of debugs is significantly decreased. You develop some code; run tests, if you fail, re-doing the development is significantly faster than debugging and fixing it. TDD aims at detecting defect and correcting them at an early stage, which costs much cheaper than detecting and correcting at a later stage or post release.
Also, now debugging is very less frequent and significant amount of time is saved. With the help of tools/test runners like Karma, JSTestDriver, and so on, running every JavaScript tests on browser is not needed, which saves significant time in validation and verification while the development goes on.
Increased productivity: Apart from time and financial benefits, TDD helps to increase productivity since the developer becomes more focused and tends to write quality code that passes and fulfills the requirement.
Clean, maintainable, and flexible code: Since tests are written first, production code is often very neat and simple. When a new piece of code is added, all the tests can be run at once to see if anything failed with the change.
Since we try to keep our tests atomic, and our methods also address a single goal, the code automatically becomes clean.

At the end of the application development, there would be thousands of test cases which will guarantee that every piece of logic can be tested.

The same test cases also act as documentation for users who are new to the development of system, since these tests act as an example of how the code works.
Improved quality and reduced bugs: Complex codes invite bugs. Developers when change anything in neat and simple code, they tend to leave less or no bugs at all. They tend to focus on purpose and write code to fulfill the requirement.
Keeps technical debt to minimum: This is one of the major benefits of TDD. Not writing unit tests and documentation is a big part, which increases technical debt for a software/project. Since TDD encourages you to write tests first, and if they are well written, they act as documentation, you keep technical debt for these to minimum.
As Wikipedia says, A technical debt can be defined as tasks to be performed before a unit can be called complete. If the debt is not repaid, interest also adds up and makes it harder to make changes at a later stage. More about Technical debt can be found at https://en.wikipedia.org/wiki/Technical_debt.

Myths

Along with the benefits, TDD has some myths as well. Let's check few of them:

Complete code coverage: TDD enforces to write tests first and developers write minimum amount of code to pass the test and almost 100% code coverage is done. But that does not guarantee that nothing is missed and the code is bug free. Code coverage tools do not cover all the paths. There can be infinite possibilities in loops. Of course it's not possible and feasible to check all the paths, but a developer is supposed to take care of major and critical paths.
A developer is supposed to take care of business logic, flow, and process code most of the times. No need to test integration parts, setter-getter methods for properties, configurations, UI, and so on. Mocking and stubbing is to be used for integrations.
No need of debugging the code: Though test-first development makes one think that debugging is not needed, but it's not always true. You need to know the state of the system when a test failed. That will help you to correct and write the code further.
No need of QA: TDD cannot always cover everything. QA plays a very important role in testing. UI defects, integration defects are more likely to be caught by a QA. Even though developers are excellent, there are chances of errors. QA will try every kind of input and unexpected behavior that even a programmer did not cover with test cases. They will always try to crash the system with random inputs and discover defects.
I can code faster without tests and can also validate for zero defect: While this may stand true for very small software and websites where code is small and writing test cases may increase overall time of development and delivery of the product. But for bigger products, it helps a lot to identify defects at a very early stage and gives a chance to correct at a very low cost. As seen in the previous screenshots of cost of fixing defects for phases and testing types, the cost of correcting a defect increases with time. Truly, whether TDD is required for a project or not, it depends on context.
TDD ensures a good design and architecture: TDD encourages developers to write quality code, but it is not a replacement for good design practice and quality code. Will a team of developers be enough to ensure a stable and scalable architecture? Design should still be done by following the standard practices.
You need to write all tests first: Another myth says that you need to write all tests first and then the actual production code. Actually, generally an iterative approach is used. Write some tests first, then some code, run the tests, fix the code, run the tests, write more tests, and so on. With TDD, you always test parts of software and keep developing.

There are many myths, and covering all of them is not possible. The point is, TDD offers developers a better opportunity of delivering quality code. TDD helps organizations by delivering close to zero-defect products.