Understanding the Distinction: UI Testing Versus Unit Testing

Understanding the Distinction: UI Testing Versus Unit Testing

11 April 2024 Stephan Petzl Leave a comment Tech-Help

When developing software, especially when dealing with both a backend server and front-end web interfaces, it’s crucial to understand the distinction and proper application of UI testing and unit testing. Each type of test serves a unique purpose and is instrumental in different stages of the development lifecycle.

Unit Testing: Isolating the Building Blocks

Unit testing is a method by which individual units of source code—such as classes or methods—are tested to determine if they are fit for use. The primary goal of unit testing is to validate that each unit of the software performs as designed. This form of testing is generally conducted during the development phase and can be initiated from the very beginning, often in a Test-Driven Development (TDD) approach.

  • Objective: To ensure that each standalone unit functions correctly.
  • Scope: Narrow, focusing on small, isolated components.
  • When to Apply: During the initial development phase, ideally in conjunction with TDD practices.

UI Testing: Assessing the Integrated System

UI testing, which may also fall under the umbrella of system, functional, or acceptance testing, involves testing the integrated system to ensure it meets the required needs. This type of testing is conducted from the perspective of the end-user and verifies that the entire application functions in real-world scenarios. UI testing typically comes into play after there is complete end-to-end functionality available to test.

  • Objective: To confirm that the entire system operates as intended when used by actual users.
  • Scope: Broad, encompassing the entire application.
  • When to Apply: After unit testing, when end-to-end functionalities are ready to be evaluated.

Practical Approach to Testing in an Existing System

If you’re working with an existing system that lacks tests, you’re essentially dealing with legacy code. In such scenarios, it’s advisable to focus initially on high-level functional tests to achieve the best possible test coverage with minimal effort. Subsequently, you can invest in adding unit tests to ensure the robustness of individual components.

  1. Start with UI testing to cover broad functionalities and user interactions.
  2. Gradually introduce unit tests to refine and validate the behavior of individual units within the codebase.
  3. Continue enhancing both testing methods as the system evolves and new features are added.

For those interested in further improving their understanding of working with legacy code and testing, it is recommended to consult resources like “Working Effectively with Legacy Code” for in-depth strategies and best practices.


Both UI and unit testing are essential components of a comprehensive testing strategy. While unit testing helps maintain the reliability of individual code units, UI testing ensures that the system as a whole meets user expectations and requirements. By applying these tests judiciously at different stages of the development process, you can significantly enhance the quality and user experience of your software.

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