Unit Testing vs. Functional Testing: In-Depth Comparison

Oliver Moradov
What is Unit Testing?What is Functional Testing?
Unit testing involves isolating the smallest testable parts of an application into units that you can verify and validate independently. It is a key step in the software development process.
Unit testing helps isolate specific code and determine if the unit works as intended. The goal is to facilitate early detection of code flaws that may be difficult to find and fix in later testing stages.
Functional testing helps verify that each application feature works as intended. It involves comparing each function to the relevant requirement to determine whether the output is consistent with end-user expectations. 
You can implement functional testing by providing sample inputs. Next, you capture the resulting outputs and verify that the actual outputs are the same as the expected outputs.
Functional tests employ a black-box testing approach that checks functionality without prior knowledge of the internal workings of the software, such as programming languages or software design. 

In this article:

Unit Testing vs. Functional Testing: How Do They Work?

Unit Testing

Unit testing helps evaluate and fix small units during the development phase. You can run unit tests automatically or manually. Ideally, you should run these tests frequently to verify your code-in-progress is working as intended.

A unit test includes three stages: 

  • Planning—preparing and reviewing the unit test.
  • Cases and scripting—creating the relevant test cases and scripts.
  • The unit test—running the test.

It can help you implement a test-driven development (TDD) approach that requires you to first write failing unit tests, write code, and then refactor the application until the test eventually passes. The result is usually a predictable and explicit code base. Here are key best practices to help you set this up:

  • Test each case independently in an isolated environment to ensure there are no dependencies in the code. 
  • Code criteria to verify all test cases and use a testing framework to report failed tests. 
  • Do not create a test for each line of code because it takes up too much time. Instead, create tests focused on the code affecting the behavior of your software.

A unit test should only include components that are vital to the performance of the tested unit. This approach enables you to modify your source code without any immediate concerns about how these changes may affect the functionality of other units or the entire program. 

After testing all relevant units and verifying they are working efficiently and as intended, you can start evaluating larger components by using integration testing. 

Related content: Read our guide to unit testing examples

Functional Testing

Functional testing involves verifying that an application can correctly execute a specific task. While non-functional testing checks the application’s overall performance, scalability, security, compatibility, and reliability, functional testing verifies the execution. 

Instead of checking how processing occurs, functional testing checks the processing’s results. It may involve simulating actual system use, but the test does not make system structure assumptions.

You test individual features and functions by feeding input into the software and examining the output. Here are key steps typically involved in functional testing:

  • Identify functions your software needs to perform.
  • Create input data according to the functions’ specifications.
  • Determine the desired output according to the specifications.
  • Execute your test case.
  • Compare the actual output with the expected output.

Unit Testing vs. Functional Testing

What Is the Purpose of Each Testing Type?

Unit tests isolate and test individual code units to verify they function as intended. You use unit testing to verify that specific system behaviors produce the intended results. The purpose is to create a robust codebase with minimal cost and provide documentation for high-level testing such as functional testing and integration testing.

Functional tests help verify that the output produces the expected user requirements. You use functional testing to check the functionalities of the entire system and ensure different components work together as intended. Its purpose is to check an entire application, including its hardware, networking infrastructure, front-end UI, and the back-end database.

Unit tests point to a specific issue that requires fixing. Since functional testing checks the entire application, it mainly indicates a general issue without pointing out a specific problem.

How Do They Improve Software Quality?

Unit testing can help you capture code and fix it later and protects against regression. It ensures you identify code issues almost immediately, minimizing re-work and re-test efforts later. 

Functional testing helps ensure the entire application works as intended. It usually involves thorough tests validating critical application functionality, like user logins, payment gateway, signups, and critical user workflows. 

Here are common functional testing types that help improve software quality:

  • Integration testing—verifies all integration components work together. 
  • Smoke testing—validates critical build features to save time and reduce regressions.
  • System testing—checks the entire application in the context of real user scenarios. 
  • End-to-end tests—help increase test coverage and minimize risks associated with integrating new code into a system or application.

How Do Testing Techniques Differ?

Unit testing is a white box testing technique with authorized access to source code. It checks the application’s internal workings, sensitizing all execution paths and data structures in the tested unit.

Functional testing is a black-box technique that checks the software’s functionality without sifting through the internal code structure. It tests the system against user or business requirements, comparing the resulting output with the expected output.

Related content: Read our guide to unit testing best practices (coming soon)

How Do They Impact Test Coverage?

Test coverage is a key software testing metric that helps measure the quality of a test. It may serve different purposes for each type of test. High code coverage generally provides confidence that the project is properly maintained and developed, but it does not necessarily improve the code quality.

Poorly written unit tests with high test coverage cannot ensure improved code quality. Test coverage can help establish traceability between test cases and requirements in functional testing. Ideally, your functional test coverage should indicate specific features that satisfy the acceptance criteria and those still in progress. 

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