Unit testing is a type of software testing that focuses on individual units or components of a software system. The purpose of unit testing is to validate that each unit of the software works as intended and meets the requirements. Unit testing is typically performed by developers, and it is performed early in the development process before the code is integrated and tested as a whole system. Unit tests are automated and are run each time the code is changed to ensure that new code does not break existing functionality. Unit tests are designed to validate the smallest possible unit of code, such as a function or a method, and test it in isolation from the rest of the system. This allows developers to quickly identify and fix any issues early in the development process, improving the overall quality of the software and reducing the time required for later testing.

Here are some key points about unit testing:

• Unit testing is a software development process in which the smallest testable parts of an application, called units, are individually scrutinized for proper operation.
• The main objective of unit testing is to isolate written code to test and determine if it works as intended.
• Unit testing is an important step in the development process. If done correctly, unit tests can detect early flaws in code which may be more difficult to find in later testing stages.
• Unit testing is a component of test-driven development (TDD), a pragmatic methodology that takes a meticulous approach to building a product by means of continual testing and revision.
• Unit tests are typically isolated to ensure a unit does not rely on any external code or functions.
• Unit testing allows the programmer to refactor code at a later date, and make sure the module still works correctly (i.e. Regression testing) .
• Unit testing is the cornerstone of extreme programming, which relies on frequent testing and refactoring to improve code quality.

In summary, unit testing is a crucial part of software development that allows developers to test individual units of code in isolation to ensure they work as intended. This helps to improve the overall quality of the software and reduce the time required for later testing.