What is Unit Testing?

What is Unit Testing? – A useful introduction

A unit test is the first level of testing in the SDLC (Software Development Lifecycle). They are code-level automated tests written and run by software developers, designed to validate sections of the application referred to as units.

A unit is the smallest testable part of any software that can run independently. How a unit is defined depends on the type of program. For instance, the component or unit could comprise a few lines of code, a class, a single function, or a procedure, and sometimes unit testing could mean testing an entire feature.

The Importance of Unit Testing

The primary objective for all types of testing is to identify bugs, but unlike other forms, unit testing provides immediate feedback. It also ensures that the developed code works, as well as enabling faster bug resolution. It reaches beyond finding problems with the code, as these four examples show:

Quick Identification and Resolution of Bugs

If a developer encounters a bug during unit testing, it can be processed right away. This is because the method helps familiarise the developer with the code. If a future update breaks another part of the system, the unit test can help address those areas. In short, it help developers detect problems quickly so they can address and fix them immediately.

Aids Design Improvements

This testing method help in reviewing the software under development. Developers examine each of the defined units for errors, fix the flaws, analyze the result, which is then used to improve the program’s code. Unit testing also helps in refactoring the code. It offers the development teams an opportunity to understand the code base, verify the developed code’s accuracy, determine its reuse possibilities, and to make faster code changes. Refactoring helps developers reconstruct the code to make it more isolated and testable.

Enhances the Debugging Process

Examining the code to ensure everything works also enables a reduction in overall debugging time. When a software bug is discovered, unit tests lead to faster debugging, fixing, and code improvisation, reducing the probability of repeated bug occurrences.

Helpful in Creating Code Documentation

A developer conducting unit tests needs in-depth knowledge about the code to be able to write the tests. Writing unit tests helps developers to understand the code better. Developers also find that unit tests serve as useful documentation because they carry details on how to use a library or framework. The documentation works as a guide, providing an idea of the program’s implementation and overall working.

What is Unit Testing?

How is Unit Testing done?

Before starting to conduct unit tests, the first step is to identify the testable components. Testing every line of code is not possible, and poorly designed untestable code complicates the overall process.

Unit testing is conducted using the AAA (Arrange, Act, Assert) method. Testing starts by initializing (or arranging) a small piece of the application it wants to test. Then an action (act) is performed, usually by appointing the process, and finally, a conclusion on the behavior is asserted. The test passes if the output matches the expected behavior but failing indicates a problem.

Unit testing can be a very time-consuming process if performed manually, so it is usually automated. When automated, developers write test code that assesses the source code, and the automated code notifies developers about the failed test cases. The test then ensures that the program works as expected by repeatedly inputting the same values, then comparing the output. The desired outcome is that the same result is returned each time.

Characteristics of Unit Tests

When writing unit tests, ensure that each is independent of the other:

  • Each of the defined unit tests must be independent.
  • Dependencies can cause problems and may lead to incorrect consequences, so it’s better to avoid dependencies such as environment settings, database dependencies, etc.
  • Any other test sequence must not impact unit tests. A single test must not be dependent upon running any other tests before it.
  • If tests are dependent, it may become difficult to identify the problematic area.

Ensure Tests are Deterministic

Variations in the test result indicate an unsuccessful test, such as if the test passes sometimes but fails at others. The variations could be down to several factors but determining the cause of failure is essential because the test becomes irrelevant if the cause of the bug remains unknown.

Avoid Using Random Inputs

Because the test is designed to use random parameters, uncertainty will be introduced. Runs like these are difficult to reproduce because the data will, by default, change each time.

Write Tests that are Easy to Maintain

Testing is a measure of quality. Apart from determining any flaws in the code, unit tests must also focus on the application’s overall implementation and code complexity and how easy it is to change and maintain.


The purpose of unit testing is to assess code. As such, the results may not be evident or usable to the business or end-users. For testers, it lets us isolate and resolve any problems in the code, which ultimately results in improved software quality. As the first level, it helps identify bugs in the cycle early and encourages clean code writing, but the method has no bearing on whether the built integrated system will work as expected.

Priya Rani

Priya Rani

An Enthusiastic QA Expert who loves to share knowledge and experience through blogging.