Unit Testing vs. Functional Testing: Which Testing is Better for Software Development?

Unit Testing vs. Functional Testing

Before I start the actual discussion, let me clear out the misinterpretation you might have from the blog’s title. I’m not encouraging anyone to choose between these two software testing types as both of them have significantly unique benefits in the development process.

A lot of developers might have told you that unit testing is a wasted effort and not really useful for the software. While those developers’ words may not be true, the effectiveness of unit testing has become a subject of controversy on Hacker News and Reddit.

So now, there are two factions in the developer community – unit testing haters and unit testing defenders. On the other hand, functional testing has not gone through such disputes. Developers run this test from the user’s viewpoint, and everyone acknowledges its usefulness. In the end, it’s the UI and functionality of the software users prioritize.

Any application or software testing aims to ensure you deliver a top-notch product to the users. Besides, you wouldn’t want your software project to belong to that 66% of tech projects that partially or completely fail. Hence, my aim in this blog is to explain the key difference between unit testing and functional testing so that you can understand what to employ in your app development initiative and when. In addition, I’ll also discuss how you can keep a fine balance between these two testing methods to help you develop quality apps.

Let’s walk through with our checkpoints:

A Quick Analogy of Unit Testing vs. Functional Testing

Let’s suppose the development of software is like building a house. This analogy might not be accurate, but we can use it to understand how one testing type differs from the other in detail.

You can compare unit testing with a building inspector. His job is to inspect different internal systems of the house, such as the foundation, roof, frame, electrical, plumbing, etc. He tests every system and ensures everything is working correctly and safely.

In such a scenario, functional testing is like the owner of the house paying a visit to the construction site. The house owner wants to know how living there will feel and mostly cares about the house look, size of the rooms, interior decoration, etc.

So, the building inspector is running a unit test on the house, and from a user’s perspective, the house owner is running the functional test. In other words, unit tests ensure the code is doing what the developer wishes, and functional tests ensure the developer is doing what the client wishes.

It’s time to understand them in more detail.

What’s Unit Testing?

Definition of Unit Testing

Unit testing is extremely helpful if you want to eliminate bugs in code. It’s a well-liked testing practice embraced by many organizations. As the name says, you can test each and every component. The goal is to check all the units of the code and ensure they’re running as expected without any bugs.

Usually, complex apps have a handful of variables, and practical, you can’t test every one of them. But if you break them into smaller units, you can reduce the development and maintenance costs.

You can also produce bug reports that will answer your questions, such as –

  • Which component is under test?
  • What’s the predicted behavior?
  • What’s the actual outcome?

and many more.

What’s the Key Purpose of Unit Testing?

Opting for a unit test of your software will serve the following purposes:

  • Testing each functionality and procedure
  • Separating a section of code
  • Evaluating the precision of code
  • Identifying early bugs and fixing them during the development process
  • Helping developers understand the code base and allowing them to make changes quickly
  • Saving development and maintenance costs
  • Enabling to reuse code

Benefits of Unit Testing

Unit testing comes with a set of advantages that can be exceptionally helpful for your app development. Have a look at them:

  • With unit testing, your developers will be able to refine the code and ensure and correct functioning of the module.
  • Unit testing helps developers comprehend the functionality of a unit and its utilization for obtaining the needed command over unit API.
  • It enables developers to check parts of the project without depending on others to complete them.

So, there you go, the key aspects of unit testing. Considering the software testing and quality assurance trends in 2021, the objectives and benefits of unit testing carry considerable weight in the development process and are not just a waste of time.

Now it’s time to analyze functional testing. Let’s explore!

What’s Functional Testing?

Definition of Functional Testing

If you want to evaluate the compliance of your software system against the functional needs you have determined, you must perform functional testing. It will provide you with accurate input-output verification, and thus, you can test every functionality of your app.

You can detect bugs or glitches that developers might have missed while building the app by running a functional test. So functional testing provides crucial information about the software quality and prevents potential faults so that you can fulfill your users’ and business requirements.

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What’s the Key Purpose of Functional Testing?

Developers perform functional testing types for the following objectives:

  • Recognizing errors that developers might have overlooked earlier
  • Acquiring information about the quality of your software
  • Avoiding possible defects
  • Make sure the final product is up to the mark as per the business need

Benefits of Functional Testing

Developers well acknowledge the benefits of this software testing method. Let me present a few of them:

  • You can effectively verify the functionality and performance of your software before releasing it to the users.
  • Developers run functional tests considering the user’s viewpoint. Such an approach helps them create test scenarios and display real-life use scenarios.
  • Developers can cater to both the user and client requirements with functionality testing.
  • Functional tests enhance the system usage.
  • And last but not the least, it improves the software quality.

The Essential Difference Between Unit Testing and Functional Testing

Although the aim of this blog is to solely deal with the key differences between these two software testing types, allow me a minute to introduce the fundamentals of integration testing in this scenario. Trust me, it’ll be worth your understanding.

Let’s assume you need a login page with a username and password for your web app. You can run a unit test to check if the login page is working properly and the field length of the username and password. And you can run an integration test to ensure the user reached the home page after providing correct credentials and clicking the login option.

So, how does functional testing come into this picture?

Well, it depends on the past two tests that you have conducted. Let me present some functional testing ideas for you.

  • Confirmation test of a “happy path”: in both fields and login, will an authentic user be able to enter authentic values?
  • After successful login, should the homepage feature a customized greeting?

In a nutshell, both unit and integration tests are the subtypes of functional tests that reestablish the importance of software maintenance. And there’s just a subtle difference between these two.

Now, let’s discover how unit testing will be going head-to-head with functional testing. Here’s a table featuring the key differentiating factors that set these two testing methods apart from each other.

Factors Unit TestingFunctional Testing
Purpose Isolating each unit of code and evaluating themEvaluating the software’s functionality from the user’s viewpoint
TechniqueWhite box techniqueBlack box technique
Focus Isolated units or modulesEntire software
Complexity Developers can easily write and performComparatively more complicated than unit testing
Test CasesHigher number of test cases than other testsLower number of test cases than unit and integration
Covered issueCode errors and edge casesSoftware performance and functionality
Written byDevelopersTesters
Programming languageUsually needs the same programming languageNo need for the same programming language
Change rateFrequently changingNot much changing
Cost and maintenance LowHigh
Test coverage Number of code linesNumber of requirements
AutomationAutomation is a mustBoth automation and manual test
When to perform At the beginning of the development processAfter developers build the features
Popular tools NUnit, JUnit, TestNG, MockitoUFT, Selenium, SahiPro

Now, I will explain each factor in more detail to help you understand the importance of software testing and the contrast between these testing types.

Essential Difference for Testing Types

1. Purpose

  • Unit Testing
  • Unit tests make up a solid foundation, and you should build the rest of the testing process on it. Developers can easily create such tests with minimal cost. Because of the huge volume of unit tests, developers generally use certain testing tools to automate the tests based on the framework or coding language.

    With this test, you can verify the functionality of the smallest modules of an API in isolation. Apart from ensuring that the system under test (SUT) generates accurate results, unit testing also verifies distinct behavioral aspects of the SUT.

    So, the purpose of unit testing is to help developers build a powerful codebase with low cost and provide information for high-end tests like functional and integration tests.

  • Functional Testing
  • With functional testing types, you can test the functionality of the entire software, from its networking architecture to hardware, back-end databases, front-end UI, and so on. So, we can say that functional and integration testing are quite similar as both of them ensure the proper working of the components.

    Let me explain this in simpler words. While unit testing informs you what you should fix in the codebase and where, functional testing just informs something you should fix. It could mean a race condition, the browser, or the test.

2. Testing Technique

Talking about the testing techniques, let me first introduce the two major categories of software testing – white box technique and black box testing.

In the white box technique, the tester knows the design/internal structure/implementation of the software being tested. Developers design the test cases using programming skills and an internal viewpoint. White box testing has two specific criteria – data structure sensitization and the execution path. People also call these loop testing, path testing, data flow testing, etc.

So, if we consider the purpose and definition of unit testing, it belongs to the white box testing category. Besides, unit tests sensitize data structures and the execution path, which are criteria of white box testing.

Contrastingly, in the black box technique, the tester barely knows anything about the tested software. Without any knowledge of the code/program/internal structure, testers will check the behavior or functionality of your software based on the expected outcome. So, without a doubt, functional testing belongs to the black box technique.

3. Complexity

Executing a unit test in a TDD (test-driven development environment is quite complicated. Your developers should write unit tests before the production code in a TDD. So based on your software requirements, developers have to think about the code design in advance.

Hence, if your code design is unclear initially, it will evolve eventually and force you to do the same test again. For instance, if you’re dealing with algorithms that often change, it would be best if you postpone unit testing until you gather some idea about the code design.

In reality, there are other integrations and dependencies we have to consider, such as database, UI, and so on. In such scenarios, TDD becomes pretty difficult and includes various abstractions like MVP/MVC pattern, programming to an interface, mock objects, etc. all this requires a good deal of expertise and knowledge as developers need to write huge volumes of unit tests, more than the production code at times.

Excluding such dependencies, testing trivial code becomes very complicated with unit tests. The reason is code units are most likely to gather together and start interacting with other units.

While mocking out all the dependencies is viable in unit testing, the story is something different in functional testing types. Let me give you an example. If you try to test a screen that processes the data from the cellular network, it’ll not work offline. Executing a functional test becomes very complex in such scenarios.

4. Software Quality Improvement

  • Unit Testing
  • Developers write a test to run the new code, and in such a process, they substantially apprehend the work. For instance, if your developer breaks the code in six months, they can run the test to secure the code. That’s how unit testing helps you prevent regression.

    So, unit tests help you identify potential defects in your system through a bug report so that developers don’t need to spend extra time and resources in the long run.

  • Functional Testing
  • Functional testing focuses on a “happy path” – ensuring all the crucial user functionalities, such as signups, logins, purchasing workflows, are working as expected. It has different subsets, each with specific advantages.

    Take integration testing, for example, that ensures compatible working of all the integration. Smoke testing is another type of functional testing that can help you check essential software features, reduce regression, and save time.

5. Test Coverage

Test coverage holds great importance in software project delivery. It’s a crucial metric that has different purposes of serving in both functional tests and unit tests.

With a test coverage tool, you can easily keep a check on what part of the code is exercised and what not during the testing process. That’s why we can call it code coverage in the context of unit testing. So, if your developers get high code convergence, chances are they have built and maintained a good app. In certain cases, high test coverage also enhances the code quality, but that doesn’t mean poorly written code with high test coverage will result in improved code quality.

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In functional testing, you can track the design quality of test cases with test coverage. By establishing traceability between test cases and the requirements, functional test coverage will inform you about what features are still in progress and what features are complete. While you might not get a chance to know the number of executed code lines, this kind of test coverage information can be easily accessible to you.

6. Time to Start Testing

  • Unit Testing
  • You can’t replace unit tests with functional tests, but they formulate the solid base of the software QA process audit. It would be best if your developers write the test and the code at the same time. In this regard, I should let you know about TDD (Test Driven Development), a software development approach that prioritizes writing tests before codes.

    Among many of its advantages, unit testing allows you to refactor your code and inform in case you break anything.

  • Functional Testing
  • After you have successfully executed the unit tests, integration testing comes into the picture. So in this sense, you can run functional tests on the feature when two modules interconnect with each other. You may run a unit test to check a specific function or an integration test to test the compatibility of two functions.

    Keep in mind that you can run as many unit tests as you want, but you can have only a small number of them when it comes to functional testing.

Can You Keep a Fine Balance Between Unit Testing and Functional Testing?

To be honest, I’m not going to present a universal formula to maintain a decent balance between these two software testing types. Based on the conditions and your project requirements, the number of tests can differ. Despite the common software testing myths, what ultimately matters is whether the tests are delivering the value you expected.

If you rely too much on functional testing, it might affect the software quality. Sure, the world of software development is advancing, but one thing that has not still gained much ground is software quality assurance. This is the reason a lot of established software development companies prefer functional testing over unit testing.

So, what if you decide to completely neglect unit tests and over-depend on functional tests? Let me show you how this decision will decrease the software quality:

  • Developers perform functional testing only after they entirely build the feature. So, there’s no scope to get feedback early in the software development life cycle.
  • Due to the restriction of cost and time, functional testing hardly covers all edge cases. But with a greater range of input-output, unit testing easily covers most edge cases.
  • If you don’t opt for unit tests at all, there’s a high chance of bugs in the production code. Such production issues take extra effort and time for fixing, and even functional tests can’t detect the accurate location of the bugs.
  • You can’t easily refactor or change a code without unit tests, as no one knows what will break once you change a part of the code. Such changes often result in the breaking of existing functionalities which then leads to regression issues. But if a unit test covers the same part of the code, developers will be aware of an impending loss through a breaking test.

I would suggest you fulfill three basic objectives whenever you take on a software development project – efficient workflow, flawless features, and clean and maintainable code. You can find the right balance between functional and unit testing by preparing a set of questions and presenting it to the project manager to meet the goals.

Here are a few considerations you can think about to fulfill these objectives:

Main objectives of Software Testing Types

Objective 1: Efficient Workflow

  • Is my team taking too much time for the tests to execute, affecting their overall productivity?
  • If my team needs to run many high-end, time-consuming functional tests and must wait all day to get a report about accidental regression, is it possible to perform some quick tests, maybe integration testing, for a faster report?
  • Am I getting the feedback as quickly as I expected? When do I get the bug report, and can I get it sooner by any means?

Objective 2: Flawless Features

  • Is my team minutely testing all the edge cases? Such testing calls for quite a number of unit tests.
  • Is my team running sufficient functional tests from the users’ perspective to ensure the proper functionality of the software?
  • Are there any tests to check whether all the components are coordinating correctly? Are there enough integration tests?

Objective 3: Clean and Maintainable Code

  • Is my team confident with the test result and will it have to refactor the code often?
  • Are the tests helpful enough to develop an effective design? If my team runs fewer unit tests and more integration tests, should they write more unit tests for a good design?
  • Is my team able to maintain the coding standard and code quality in software development?

Ultimately, the testing pyramid is the principle visual support to maintain a decent balance between all three kinds of software testing. As you can see, the bottom of the pyramid consists of unit tests, making the most of your tests. As you go up, the tests get bigger but simultaneously, the width of the pureed gets smaller, meaning the number of tests becomes smaller.

Principle Visual Support Pyramid

According to Google, a 70/20/10 split would be the best approach. This means you should go for 70%-unit tests, 20% integration tests, and 10% end-to-end tests. The combination can vary from team to team, but it should maintain the pyramid structure as a general rule.

Let’s build a top class and future-ready software


Today, every client expects software development companies and their developers to create software of prime quality. And such demands have propelled the companies to come up with different testing methods as part of the development process.

If you want to achieve clean and bug-free code faster, unit testing is the key. It will basically tell you where the problem lies in the code. Yes, your developers need to spend extra hours to fix the errors, but in the end, it will be worth all your time and efforts.

On the contrary, functional testing is complex and slow, but it guarantees that your software will function as per the requirements. If there’s an issue with the functionality, it will inform you.

So, both of these software testing methods consist of their assets and liabilities. It’s up to you how you’re going to balance them and deliver the finest software to the end-users. And an experienced software testing company, Radixweb can help you here. Our skilled software testers can enhance the development experience and product quality for you so that you can constantly stay tuned with the latest market trends. Let’s start discussing your testing and QA needs, shall we?

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Varix Patel, delivery & operations head at Radixweb is a tech crystal gazer. He focusses on bringing insights-rich innovation to customers, attracts top talent and strengthens brand value. Varix is a trusted advisor and dynamic leader who has driven significant business growth for our customers. Outside of work, he loves to take vacations to unknown places.