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1 December 2023

10 Min. Read

Unit testing

What is Unit testing? A Complete Step By Step Guide

What is Unit testing? A Complete Step By Step Guide

Key Highlights

In this blog, we cover the following key highlights:

  1. Learn about how unit testing involves testing individual components of software to ensure each part functions correctly.

  2. Know how it identifies bugs early, improves code quality, and acts as code documentation.

  3. Tests are conducted in isolation from the rest of the system using techniques like stubs and mocks.

  4. Unit tests are often automated and integral to Continuous Integration and Continuous Deployment processes.

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Unit testing is a crucial step in automating tests for digital applications, serving as a guardian that verifies the performance of individual modules to ensure stability and resilience.

Unit testing is a method used in software development to check if individual parts of a program's code, known as "units," work as expected. To make it easier to understand, let’s use an analogy:

Imagine you are building a complex LEGO model, like a giant castle. Each LEGO piece in this model is like a unit in a software program. Just like you would check each LEGO piece to make sure it fits perfectly and looks right before adding it to the castle, in unit testing, each small piece of code is tested separately to ensure it works correctly.

What is Unit testing?

The primary objective of unit tests is to test that even the smallest unit of code block works perfectly and aligns with what is actually expected out of it on integration with other parts.

Software testing covers a wide range of activities, and within it, unit testing plays a specific role. Unit testing is a straightforward method that focuses on testing individual units or code blocks, one function at a time.

In the process of unit testing, QA teams examine each module in isolation, without connecting it to any external components like databases or any file systems.

Essentially, each module is treated as a self-contained program that operates independently, showcasing different behaviors under various scenarios.

Developers gain valuable insights into their code block performance through unit testing, enabling them to swiftly rectify any unexpected behavior.

What is the Importance of Unit testing?

Located at the bottom of the testing pyramid coined by Mike Cohn, unit tests are supposed to be performed extensively when testing an application. This is because testing at the unit level allows for the examination of all possible paths and edge cases within each small component of the code. Their high volume offers a comprehensive coverage of the codebase.

Mike Cohns test pyramid

Unit tests give developers more control over the quality of their code blocks before putting them together for integration testing.

  1. Catches Bugs Early: Testing each small part of the code individually helps find and fix bugs before they become bigger problems.

  2. Supports Code Refactoring: With reliable tests, you can confidently refactor or update code, knowing you haven't broken anything.

  3. Improves Code Quality: Forces you to write more modular, maintainable code which is easier to work with and understand.

  4. Saves Time and Hassle: Investing time in writing tests now saves you from much longer debugging sessions later.

  5. Facilitates CI/CD: Integral for continuous integration and deployment, making your development process faster and more efficient.

Unit tests vs Integration tests vs End-to-End tests

1. Unit Tests: The Microscopic Examination

  • 1.1. Narrow Focus:

    Think of unit tests as the magnifying glass of the software testing world. They scrutinize individual functions or modules in complete isolation, ensuring each part performs its designated task flawlessly.

  • 1.2. Agility and Recurrence:

    Due to their specific and isolated nature, unit tests can be executed rapidly, allowing for constant re-evaluation with every significant alteration in the code. This immediacy is crucial for early detection of problems.

  • 1.3. Simulating Dependencies:

    Unit tests typically utilize mocks and stubs to replicate the behavior of external components. While this helps in maintaining an isolated environment, it's important to remember that these simulations are based on presumptions of how external interactions should occur.

2. Integration Tests: The Connective Framework

  • 2.1. Verifying Interactions:

    Moving a step further, integration tests are like checking the integrity of the connections in a structure. They ensure that various components or units work harmoniously when combined.

  • 2.2. Strategic Positioning:

    These tests serve as the middle ground. More encompassing than unit tests, yet less so than end-to-end tests, they balance complexity and execution speed effectively.

  • 2.3. Defining Scope:

    A key challenge with integration tests is defining their extent. Too narrow, and they become redundant with unit tests; too broad, and they start to mirror end-to-end tests.

3. End-to-End Tests: Simulating the Real-World Experience

  • 3.1. Holistic Approach:

    End-to-end tests are akin to conducting a dress rehearsal. They simulate the user’s experience by testing the application from start to finish, including its interactions with databases, networks, and other external systems.

  • 3.2. Complex Setups:

    These tests necessitate a comprehensive setup, which makes them more elaborate and time-consuming. This complexity also means they are run less frequently.

  • 3.3. Potential for Inconsistencies:

    Due to their dependency on numerous components and external factors, end-to-end tests can be more unpredictable and require more upkeep.

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How Unit Tests work?

A "unit" is typically the smallest testable part of an application, like a function or a method.

Step 1: Understanding the Concept

  • Purpose: Unit tests verify the correctness of individual units of code in isolation from the rest of the application.

  • Benefits: They help catch bugs early, facilitate refactoring, and ensure that new changes don't break existing functionality.

Step 2: Setting Up the Environment

  • Testing Framework: Choose a testing framework appropriate for your programming language (e.g., JUnit for Java, PyTest for Python, etc.).

  • Installation: Install the testing framework in your development environment.

Step 3: Writing a Unit Test

  • Identify a Unit to Test: Select a specific function or method in your code.

  • Create a Test Case: Write a test function that calls the unit with various inputs.

  • Assertions: Use assertions to check if the unit's output matches the expected output.

Example (Python with PyTest):

Suppose you have a simple function to test:

def add(a, b):
    return a + b

A unit test for this function:

def test_add():
    assert add(2, 3) == 5
    assert add(-1, 1) == 0
    assert add(-1, -1) == -2

Step 4: Running the Tests

  • Execution: Use the testing framework's tools to run the tests.

  • Observing Results: The framework will report which tests passed and which failed.

Step 5: Interpreting Results and Refactoring

  • Success: All tests passing means the unit likely works as intended.

  • Failure: If a test fails, investigate the cause, correct the code, and run tests again.

  • Refactoring: With reliable tests, you can refactor code with confidence.

Step 6: Integration with Development Workflow

  • Continuous Integration (CI): Integrate unit tests into your CI pipeline to automatically run them on every commit.

  • Test-Driven Development (TDD): Optionally adopt a TDD approach where you write tests before the actual code.

Types of Unit Testing

Unit testing is broadly categorized as follows:

Manual vs. automated unit testing

  1. Manual Testing:

    This involves a developer manually writing and executing test cases without the use of any automated tools. It's more time-consuming and prone to human error but allows for nuanced understanding and is often used in exploratory testing phases.

  2. Automated Testing:

    Automated unit tests are written and executed using testing frameworks and tools. This type of testing is faster, more reliable, and can be easily integrated into continuous integration and deployment pipelines.

    Examples of frameworks include JUnit for Java, NUnit for .NET, and PyTest for Python.

Read More About: Manual Testing vs Automation Testing: A Full Comparison Guide

Role of Mocks and Stubs in Unit Tests

In unit testing, mocking and stubs play a critical role in isolating individual components of software, allowing developers to rigorously test each part in isolation. This isolation is crucial for ensuring that tests are not only reliable but also repeatable, which is a cornerstone of robust software development.

  • Mocks are simulated objects that mimic the behavior of real objects in controlled ways. They are primarily used to test the interaction between a unit and its external dependencies.

  • For instance, if a function is supposed to retrieve data from a database, a mock can be used to simulate the database and return predetermined responses. This way, the functionality of the function can be tested without the need for an actual database connection. This is particularly useful in cases where the external dependencies are unreliable, slow, or have side effects.

  • Stubs, on the other hand, are a bit simpler than mocks. They provide predefined responses to function calls, but unlike mocks, they don't usually simulate complex behaviors or record information about how they were called. They are used to "stub out" the parts of the system that are not being tested, to provide the necessary inputs for the unit under test.

  • For example, if a function requires input from a user, a stub can be used to automatically provide this input during testing.

Benefits of mocks and stubs

The use of mocking and stubs allows developers to focus on the unit under test, ensuring that it functions correctly in a controlled environment. It also significantly speeds up the testing process, as dependencies do not need to be set up and torn down. Furthermore, it makes tests less fragile, as changes in external dependencies do not necessarily require changes in the test suite. This aspect is particularly beneficial in continuous integration and delivery pipelines, where tests need to be run frequently and reliably.

Example: Mocking a database service in a user authentication test

test('authenticateUser', () => {
    expect(authenticateUser('user', 'pass')).toBeTruthy();

Unit Testing Best Practices

  • Start Early and Test Often

    Begin unit testing as soon as you start coding. Early testing helps identify issues quickly, which is more cost-effective and less time-consuming than fixing them later.

  • Example: Begin writing unit tests as soon as you implement a function. If you're writing a function to add two numbers in a calculator app, write its test concurrently.

  • Isolate the Unit

    Each unit test should focus on a single function or class. Isolation ensures that the test is only assessing the functionality of the unit in question, not the dependencies.

    • This test isolates the add method of the Calculator class.

def test_addition():
    result = Calculator.add(1, 2)
    assert result == 3
  • Use Mocks and Stubs

    Dependencies like databases or network services should be replaced with mocks or stubs. This approach isolates the unit from external factors and ensures that tests are not affected by outside changes.

    Example: If your function depends on a database, use a mock database to simulate database interactions.

from unittest.mock import MagicMock
database = MagicMock()
database.query.return_value = {"id": 1, "name": "Test"}

  • Write Clear, Descriptive Test Cases Each test case should be named clearly and describe what it tests. This practice makes it easier to understand what’s being tested and why a test might have failed.

    Example: Instead of naming a test something vague like testProcess, name it testProcess_ShouldReturnValidResult_WhenInputIsValid.

  • Test for Both Success and Failure Cases

    Ensure that your tests cover both scenarios where the unit should succeed and where it should fail. Testing for failure is as important as testing for success to catch unexpected behaviors.

def test_add_valid_numbers():
    assert Calculator.add(2, 2) == 4

def test_add_invalid_numbers():
    with pytest.raises(TypeError):
        Calculator.add("two", 2)
  • Keep Tests Independent

    Each test should be able to run independently of the others. Tests that depend on the outcome of other tests can be difficult to maintain and diagnose when failures occur.

    Example: Ensure that test_user_creation doesn't depend on test_user_login. Each test should be able to run and pass/fail on its own.

  • Automate the Testing Process

    Automated tests can be run quickly and frequently. This automation is crucial for continuous integration and deployment environments.

    Example: Use continuous integration tools like Jenkins or Travis CI to automatically run your tests every time you push new code to your version control system.

  • Practice Test-Driven Development (TDD)

    In TDD, you write the test before the actual code. This approach ensures that testing is an integral part of the development process rather than an afterthought.

    • First, write a failing test:

    • Then, implement the feature to pass the test.

def test_multiply():
    assert Calculator.multiply(2, 3) == 6
  • Ensure Code Coverage

    Strive for high code coverage, but remember that 100% coverage is not always practical or necessary. Prioritize critical and complex parts of the code.

    Example: Use tools like Istanbul for JavaScript or for Python to measure the extent of your code covered by unit tests.

  • Refactor Tests When Necessary

    Just like your code, tests should be refactored and improved over time. Keeping your tests clean and up to date is essential for maintainability.

  • Example: If you've refactored a method to take additional parameters, make sure to update all associated tests to reflect this change.

  • Use Assertive and Behavior-Driven Frameworks

    These frameworks make tests more readable and help express the intent of the test more clearly.

def test_empty_list():
    assert not sort_list([])
  • Review and Update Tests Regularly

    Regularly review your tests to ensure they are still relevant and effective, especially after major code changes.

  • Document the Tests and Results

    Proper documentation helps in understanding the purpose and scope of each test and the reasons for any failures.

Unit testing Advantages

  • Enhanced Debugging Efficiency: Unit tests significantly narrow down the potential locations of bugs in the codebase. For developers, this means spending less time in the debugging process, as they can quickly identify which unit of code is failing.

  • Support for Agile and Continuous Integration Practices: Unit tests are integral to agile methodologies and continuous integration systems. They allow developers to frequently integrate code and verify its correctness automatically, facilitating a more dynamic and iterative development process.

  • Better Design Decisions: Writing unit tests often forces developers to think critically about the design of their code. This leads to better software design choices, such as using interfaces and dependency injections, which in turn make the code more modular and adaptable to change.

  • Facilitates Test-Driven Development (TDD): For proponents of TDD, unit testing is not just a testing approach but a software development philosophy. It encourages developers to think about the requirements and design before writing the code, leading to more thought-out, robust, and reliable software solutions.

  • Risk Mitigation in Code Refactoring: When modifying legacy code or undertaking significant refactoring, unit tests serve as a safety net, reducing the risk associated with changes. This is particularly valuable in complex systems where the impact of changes can be difficult to predict.

  • Performance Optimization: Unit tests can be used to benchmark performance, allowing developers to identify performance regressions and optimize code effectively.

  • Reduced Costs Over the Software Lifecycle: While unit testing requires an upfront investment, it saves costs in the long term by reducing the frequency and severity of bugs, which are more expensive to fix in later stages of development or after deployment.

  • Greater Confidence in Code Stability: With a comprehensive suite of unit tests, developers can be more confident about the stability and reliability of their code, leading to less stress and higher job satisfaction.

Unit testing Disadvantages

  • Limited Scope of Testing: Unit tests only cover the smallest parts of the application, often missing out on issues that arise during integration or in the broader system. This limited scope can give developers a false sense of security regarding the overall health of the application.

  • Complexity in Testing Certain Units: Some parts of the application, such as those involving intricate dependencies, third-party services, or complex logic, can be challenging to unit test effectively. This complexity can lead to either inadequate testing or a significant increase in the effort required to write comprehensive tests.

  • Potential for Redundant Tests: In the pursuit of thorough unit testing, developers may end up writing tests that do not contribute significantly to software quality or that overlap with other tests. This redundancy can waste valuable time and resources.

  • Overhead in Test Maintenance: As the application evolves, so must the unit tests. This creates an ongoing maintenance burden, as updating tests for every change in the code can be time-consuming and might slow down the development process.

  • Initial Productivity Dip: For teams new to unit testing, there's often an initial decrease in productivity as developers acclimate to writing tests. This learning curve can be steep, particularly in complex projects, leading to delays in the early stages of development.

  • Risk of Over-Mocking: In unit testing, mocking dependencies is common, but overuse of mocks can lead to tests that pass in isolation but fail when the system is integrated. This over-mocking can mask issues that only appear in a real-world environment.

  • Integration Blind Spots: Unit tests do not guarantee that the pieces of the application will work together seamlessly. Developers might overlook integration issues, which can lead to significant problems in later stages of development.

  • Resource Intensiveness for Large Codebases: In large-scale projects, the sheer volume of unit tests can become a resource-intensive aspect, both in terms of computational resources to run the tests and human resources to manage them.

Tools to Perform Unit Testing

The main goal of unit testing is to ensure that each unit of the software performs as designed. This practice leads to more reliable code, easier maintenance, and a reduction in bugs. Here are some key tools commonly used for unit testing:

  1. JUnit: A popular framework used primarily for Java applications. It is an open-source tool that offers a simple platform to write and run repeatable tests.

  2. NUnit: Similar to JUnit but designed for the .NET framework. It's widely used for its rich attribute set and user-friendly approach to test cases and suites.

  3. Mockito: A Java-based mocking framework, useful for effectively writing tests for classes with external dependencies. It allows programmers to create and configure mock objects.

  4. Mocha: A feature-rich JavaScript test framework running on Node.js, making asynchronous testing simple and fun. It's often used with Chai, an assertion library.

  5. pytest: A robust Python testing tool that supports simple unit tests to complex functional testing. It's known for its simple syntax.

  6. QUnit: A powerful, easy-to-use JavaScript unit testing framework. It runs on any JavaScript environment and is used for both client-side and server-side JavaScript testing.

Know more - Most Popular Unit Testing Tools

Each tool offers unique features suited to specific programming languages and needs. The choice of tool depends on the project requirements, the development environment, and the team's familiarity with the tool. Employing unit testing with these tools ensures a more stable and error-free codebase, facilitating a smoother development process.


Unit tests are an essential tool in a developer's arsenal, offering a robust way to ensure code reliability and maintainability. By writing and running tests regularly, you can significantly reduce bugs and improve the quality of your software. Remember, a well-written unit test is simple, fast, and independent.

Unit testing is not just about finding errors, but also about ensuring code quality and functionality throughout the development process.

Explore this handy reference guide to quickly compare the three types of testing and discover the individual benefits of each.

Keep in mind that relying solely on unit tests won't solve all your testing challenges. While they are effective for testing individual functions, comprehensive integration testing is essential to assess the functionality of all your services working together and ensure everything operates as intended.

Frequently Asked Questions (FAQs)

1. What is Unit testing and its types?

Unit testing is a software testing method where individual components or units of a program are tested in isolation. Types include white-box (testing internal logic) and black-box (testing external behavior) unit testing.

2. Why automate unit testing?

Automating unit testing enhances software development by swiftly and repeatedly testing code changes. It ensures early detection of bugs, accelerates the development process, and facilitates seamless integration into continuous integration pipelines. Automated unit testing improves code reliability and streamlines the overall development workflow, fostering more efficient and error-free software production.

3. What are the types of unit testing?

Unit testing types include white-box testing, examining internal logic, and black-box testing, assessing external behavior. Other types involve integration testing, validating interactions between units, and mocking, simulating dependencies. These approaches collectively ensure comprehensive coverage and robust code quality in software development.
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