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Jan 05, 2024 15 min read

Leonid Mayevskiy
CTO, Senior QA

How To Do Unit Testing: A Step-by-Step Guide to Excellence

Every QA engineer needs to have a fundamental understanding of unit testing. The unit-level testing volume constitutes between 50% to 70% or even more, depending on the project. Therefore, modern software development and testing cycles invariably include unit testing.

How To Do Unit Testing

Unit testing is the first level of the testing pyramid and an essential part of the complete testing cycle. Its principal value lies in the ability to identify defects even before changes enter the database with the product code, which allows them to be easily and quickly corrected long before they go to market. 

How to do unit testing is up to the customer. Unit testing in software engineering is typically carried out by developers. Best practice involves developing unit tests alongside the specific units (module or component) under examination. Modern development highly automates this process, especially in continuous integration and deployment/delivery. The system won't accept a unit with defects but will return it to the developer for correction. However, manual unit testing also exists.

The unit-level testing volume constitutes between 50% to 70% or even more, depending on the project. Therefore, modern software development and testing cycles invariably include unit testing. Every QA engineer needs to have a fundamental understanding of unit testing. From our experience, we can confidently state that testers sometimes write unit tests for code created by other developers in small companies and projects. 

In this one, you will find answers to frequently asked questions: "How to do unit testing?". 

What are the basics of unit testing? 

Unit or module testing is the "foundation level" of functional testing, positioned at the beginning of the testing hierarchy. The primary purpose of module testing is to verify each part of the product, often referred to as a "unit" or "module," at the early stages of development. This is done to prevent the spread of errors and defects to higher application levels. 

To grasp the concept of unit testing, it's essential to first define a "unit" from a tester's perspective. A unit is the smallest and simplest product component that undergoes testing. It can be a single method, process, object, or code. Typically, unit testing only uses one or a few input parameters and receives a single result, which is then compared to the expected value. It may also not require input parameters. It all depends on what the object of testing needs to work. 

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Why is unit testing indispensable?

Unit or modular testing is fundamental for verifying software at its lowest level. The primary goal of this type of testing is to ensure that each component of the product, referred to as a "unit," does not contain errors or defects that could propagate to other parts of the system. 

Now, let's explore why conducting unit testing is highly significant: 


Product quality: Unit testing helps prevent errors at the early stages of development and ensures a high-quality product or its components. Writing unit tests improves quality and speeds up the testing process. 


Code Reusability: Unit testing creates reliable and validated code modules that can be reused in other products or components. This is valuable in any project, as reusability helps save time and resources.


Accelerated Development: Implementing unit testing promotes a smoother and more organized development process. Developers who implement unit tests find it easier to refactor their code and improve the product. 


Enhanced Documentation: Unit tests improve product documentation, making it clearer and more accessible for both developers and testers.


Integration and Compatibility: When a product is well-covered by unit tests at a low level, it simplifies integration with other product components and external tools and technologies. 

In summary, despite questions about the necessity of such low-level testing, unit testing remains an indispensable tool for enhancing the quality and efficiency of software development. 

Importance of unit testing 

Unit testing offers many benefits that make it an integral part of development. Here's why it is: 

  • Increasing developer skills.  
  • Unit testing frameworks have been around for decades, are familiar to developers, and are usually reliable and well-documented.  
  • Learning static testing techniques (walkthrough/review/inspection) is more accessible, which will help increase your level in the NF. 
  • Good unit testing saves the company time and money by finding bugs early in development. It prevents the need to spend resources on fixing hard-to-reproduce issues just before a release.  
  • Such testing allows the team to adopt and implement the Test Driven Development (TDD) methodology, in which test cases are created before the code is written. (Traditional approach: write code first, then test). 

Unit testing is an essential tool that helps improve developer skills, saves companies time and money, and promotes the adoption of modern development techniques such as TDD. This approach also makes it easier to learn static testing techniques and is considered a standard in the software development world. 

How to do unit tests? 

A unit test is a program code that checks that a module works correctly. By "correctly", we mean that the module returns the desired result (performs the desired functionality and outputs the expected data).   

It the test produces the expected result, it passes. You can now proceed to the next tests or stages. 

Who conducts unit testing? 

As said above, unit tests were originally and always the developers' responsibility. They write unit tests because they know their code better, so tests are faster and more reliable.

Here are other reasons why developers write unit tests:   

  • Developers know better which parts of the code are critical and need to be tested more thoroughly  
  • They are much more competent when it comes to mock test data  
  • They do it faster, having much more programming experience than any QA automation engineer  
  • And most importantly, they save time, which is critical in today's "Agile" projects. 

Indeed, unit tests can also be written by testers. Some project managers in smaller IT companies believe that testers should even write unit tests if required. Perhaps this approach is justified in terms of the software quality strategy written by the project manager.  From his point of view, unit tests can be the responsibility of testers as part of their overall QA task.  

However, the difficulty is that unit testing is usually performed at early stages of development. When the code base does not exist yet, the testers certainly do not have enough knowledge and understanding of the code, and they simply cannot write quality unit tests. In some situations, testers are brought in to write unit tests, especially for unimportant modules, freeing developers. 

Which unit tests can be considered good?  

The quality of unit tests plays an important role in the software development process. But what kind of unit tests can be considered good quality? Let's take a look at the key characteristics of good unit tests:  

Fast: Hundreds of thousands of individual modules in large enterprise projects may exist. Therefore, a good unit test is executed quickly (milliseconds). Tests need to be repeated many times. So, a unit test, by definition, must be high-speed. 

Easy debug: All unit tests should be simple enough to "explain the module's behavior". Ideally, each test should be isolated and affect only one unit, so it should not work when integrated with another. 

Independence and isolation: Good tests are independent of external factors, such as file system or database type.  

Self-validation: Unit testing should ideally "anticipate" whether they will pass or fail, with minimal human intervention. 

Reproducibility and stability: A good test should produce consistent results every time.

So, quality unit tests combine these key aspects, providing fast validation, easy debugging, independence from external factors, automatic validation, and reliable results. All this contributes to more efficient and reliable software development. 

Approaches and types of unit testing 

Unit testing is an important practice in software development and can be approached in several ways. Let's take a look at the main approaches to unit testing: 

White box testing: Also known as "glass box testing" or "transparent" testing. The developer (or tester) knows the application code and understands its functionality. Therefore, they can verify the module better - by understanding its code and relationships with other modules.   

Black Box: The opposite approach: the developer does not know the code can judge the module only by its behavior, so this technique is also called "behavioral testing". The tester/developer does not know the internal structure of the module and its relationships and tests only the functionality. Such a method is usually used in manual testing and is not suitable for automated unit testing. 

Gray box: So-called "semi-transparent testing" a mixture of the approaches described above. The developer has a limited understanding of the module code.

Property-Based Testing: This method develops tests based on mathematical properties that the program must observe. This approach is used to automatically test many cases and can detect unobvious bugs.  

Data-Driven Testing: Executing tests with different sets of input data that are created from specific data.

Each approach has its advantages and disadvantages and can be applied depending on specific tasks and testing goals. It is important to choose the approach that best suits the project's needs and helps ensure high-quality software. 

Process on how to run unit tests 

The developer creates code to test a module function. He can also isolate that function to test it more thoroughly. When the functions are isolated, some unwanted dependencies between modules may show up, providing an opportunity to eliminate them.   

As mentioned, developers and testers can perform unit tests manually or automate the process. Automated unit testing is preferable based on the many routine tasks involved. Manual unit testing is tedious and time-consuming, but it can also give good results by saving time on writing automatic tests if you quickly need to fix a module. 

Process how to do a unit test:  


The developer writes test code and changes before adding them to the code base. Then, it checks the code. The changes are added to the code base if the tests are successful. If not, it fixes the errors. Tests created by the developer remain in the code to test future changes and are not deleted but commented on. 


The developer can isolate the function to better test it, especially problematic dependencies if needed.   


The framework records all failed autotest results. Some frameworks have handy summarized reports.  

Disadvantages of unit testing  


Complexity and cost when implementing new features into the application  


Slows down prototyping when the application source code is frequently updated



Naming and labeling. Assigning clear names and labels to tests is important for the team.  


Understanding all the code. There are always a lot of unit tests and code, respectively. Writing effective tests requires a thorough understanding of the entire codebase.  


Low level of experience with mocks. It happens that mocks are more complicated than production code.


Debug requires a lot of time. If tests crash frequently, unit testing delays development.

Best Practices  


Unit tests should be self-contained. Any dependencies will be negatively impacted, making it difficult to debug test cases.   


Test many use cases. A single unit can be associated with multiple use cases, so you should test each in different test cases, allowing you to refactor code more efficiently.   


AAA pattern that separates the test object from the arranging stage, making test cases more readable. 


Logical and clear names of variables, test cases, and scenarios. 


One test should test one unit. This will improve the understanding of the test and subsequent debugging. 

Unit testing tools

Many tools and frameworks are designed to simplify the unit testing process. Let's take a look at some of them. 


A framework for unit testing in Python. It provides a rich set of functionality and convenient syntax for writing tests. 


Part of the Python standard library, it provides the basic tools for creating and running tests. It also supports unit testing. 


A popular framework for testing JavaScript code, which can be used in the browser or server-side (using Node.js, for example).  


The open-source IDE plugin that creates, scales, and maintains unit tests in one click. Helps automate the process and saves time by freeing up team time for business logic.


  A free tool for Java. Supports assertions.   


A free open=source framework for. NET. Supports DDT methodology and parallel execution. 


Java code coverage tool that includes writing and syntax-checking functions for API mocks. Counts coverage by strings, paths, and data.      


An open-source Java code analysis and reporting tool similar to JMockit. Supports various types of code coverage.

PHP Unit  

Designed forr PHP developers that offers built-in assertions. 

These tools and frameworks offer developers various methods for conducting testing and provide support for different languages and technologies. The choice of a particular tool depends on the project's specific needs and the development team's preferences. 

Bonus: what is TDD  

Development through testing (TDD), or TFD, is a "tests first, then code to those tests" approach. An iterative development method where developers write test cases before writing production code. Test cases are created for each feature before its code appears. The goal is to change the development process itself, and as adherents of this method claim, it allows for minimizing the number of bugs in the final product.  

The tests in TDD contain the conditions that the production code must meet. Each test case describes and then validates what the production code must do. The production code is then written and validated by these test cases. If the test fails, developers refactor the code until they achieve the result, i.e., compliance with the requirements in the test case. 

The three stages of TDD are:   

  • Creating test cases that test some functionality.  
  • Correcting the code if the tests fall. Make the adjustments needed to pass the test. 
  • Code refactoring. If the code passes all the tests, developers optimize their code, removing everything redundant and conflicting. 

Mastering development through testing is rather difficult and can be done only by experienced developers. But if the team already masters TDD, it will be a valuable tool.  

Benefits of Unit Testing by QA Professionals  

Unit testing offers several specific benefits specific to QA professionals: 


 Early detection of defects  

Unit tests are performed very early in development, before the code becomes part of a larger system. This identifies defects and bugs early on, making them cheaper and less time-consuming to fix.  


Specialization in testing

QA specialists can become experts in writing unit tests for specific modules or components. This allows them to better understand the functionality and potential problems in those parts of the code. 


Increased confidence 

Unit tests provide confidence in the stability and reliability of the code. QA professionals can rely on the results of Unit tests when performing broader testing.  


Optimize testing time  

By automating unit tests, QA specialists can save time previously spent on manual testing of the same functions.  

In the next part of this article, we will look at the basic steps that will help QA professionals implement Unit testing practices in their work. 

Implementation Steps

Preparing for Unit Testing  

Before you start creating Unit tests, you need to perform a number of preparatory steps.  

Choosing a framework for Unit-testing  

One of the first steps in developing Unit tests is choosing the right framework for Unit testing. Depending on the programming language and technology, many frameworks are available for writing and executing Unit tests. Some popular frameworks include JUnit for Java, NUnit for .NET, Pytest for Python, and many others. The choice of framework depends on the specific needs of the project.    

Choice of the test environment 

To execute unit tests, you must choose a test environment, data set and settings required to execute the tests correctly. The test environment should be identical or as close as possible to the real working environment of the application. 

Writing test methods  

Creating test methods is a key step in the Unit testing process. Test methods are codes that test the functionality of specific parts of the code. These methods should be independent and not interact with each other. Each test method should have a clearly defined expected result, The test is considered successful if it meets the expectations. Otherwise, the test is considered unsuccessful. 

Conducting Unit Testing  

The unit testing phase takes place after writing the test methods and creating the test environment. Unit tests are run at this stage, and a check is performed to see that the real results correspond to the expected results specified in the test methods.   

Running the tests  

Unit tests can be run either manually or automatically. Many modern integrated development environments (IDEs) and project management systems (e.g., Maven or Gradle) provide tools to automate the execution of unit tests. This allows you to execute the tests quickly and efficiently and to integrate them into your continuous integration (CI/CD) process.  

Analyzing results  

The results of unit tests can be presented in the form of reports or logs. QA specialists analyze these results to determine if the tests were successful or if there are bugs that need attention.  

Code Correction  

If unit tests reveal bugs or defects, a report is generated for developers. They then make adjustments to the code to correct the issues. This process can happen iteratively until all tests have been successfully passed.  

Additional recommendations on how to perform unit testing 

To effectively implement unit testing in the work of QA specialists, it is worth considering the following recommendations:  

  • Training: Mastering the basics and methodology of unit testing is an important step. Conducting specialized courses and training increases the efficiency of unit testing implementation.  
  • Collaboration: Developers and QA specialists can collaborate to create better unit tests and ensure code reliability.  

Who and how to do unit tests on a project is decided at the initial stage, but the main thing is that it is effective. 


In conclusion, unit testing significantly benefits QA professionals by allowing them to improve the testing process, detect defects early, and increase code reliability and stability. This practice also optimizes testing time and increases confidence in software quality.

To summarize, unit testing is a tool that helps QA professionals do their job more efficiently and improve the quality of software products. It is recommended to implement unit testing as part of the testing process and continuously improve the skills of working with it. 

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How do you start unit testing?

How do you start unit testing?

To start unit testing, need to: 
1. Identify the units of your code.  
2. Write unit tests for each unit of code.  
3. Run and update your unit tests regularly.  

What are the methods of unit testing?

There are two main methods of unit testing: 
- White-box testing, also known as glass-box testing, is a unit testing method that uses the internal structure of the code to design test cases. Mainly used in the automation of unit testing. 
- Black-box testing, also known as opaque-box testing, is a unit testing method that tests the functionality of a unit without knowing its internal structure. Mainly used in the manual execution of unit testing. 

How do you unit test data?

To unit test data, you need to: 
1. Identify the different types of data that your code uses. 
2. Create test data for each type of data. Test data should represent the real-world data your code will encounter.   
3. Use the test data to verify the behavior of your code. 

How to do a unit test if you are a developer?

Developers typically do unit testing by: 
1. Writing unit tests for each unit of code as they develop it. 
2. Running their unit tests regularly ensures their code works as expected. 
3. Fixing any defects that are found by the unit tests. 
4. Update unit tests when making changes and code. 

How do you do software unit testing?

Testers can perform unit testing to help ensure code quality. 
Here's how it's done: 
1. Identify the units of code that need to be tested. This can be done by working with developers to understand the architecture of the code. 
2. Write unit tests for each unit of code. Unit tests should be designed to test the functionality of the code. 
3. Run unit tests regularly. Unit tests should be run as often as possible, for example, after every code change. 
4. Analyze the results of unit tests. If any unit test fails, the code must be examined and fixed. 
5. Update unit tests when making changes and code.