What Is Unit Testing?

Before we get started setting up our environment and writing any code, let's define exactly what unit testing is, why it's worth doing, and how to get started in incorporating it in our projects.

At a high-level, unit testing refers to the practice of testing certain functions and areas – or units – of our code. This gives us the ability to verify that our functions work as expected. That is to say that for any function and given a set of inputs, we can determine if the function is returning the proper values and will gracefully handle failures during the course of execution should invalid input be provided.

Ultimately, this helps us to identify failures in our algorithms and/or logic to help improve the quality of the code that composes a certain function. As you begin to write more and more tests, you end up creating a suite of tests that you can run at any time during development to continually verify the quality of your work.

A second advantage to approaching development from a unit testing perspective is that you'll likely be writing code that is easy to test. Since unit testing requires that your code be easily testable, it means that your code must support this particular type of evaluation. As such, you're more likely to have a higher number of smaller, more focused functions that provide a single operation on a set of data rather than large functions performing a number of different operations.

A third advantage for writing solid unit tests and well-tested code is that you can prevent future changes from breaking functionality. Since you're testing your code as you introduce your functionality, you're going to begin developing a suite of test cases that can be run each time you work on your logic. When a failure happens, you know that you have something to address.

In computer programming, unit testing is a software testing method by which individual units of source code, sets of one or more computer program modules together with associated control data, usage procedures, and operating procedures are tested to determine if they are fit for use. Intuitively, one can view a unit as the smallest testable part of an application. In procedural programming, a unit could be an entire module, but it is more commonly an individual function or procedure. In object-oriented programming, a unit is often an entire interface, such as a class, but could be an individual method. Unit tests are short code fragments created by programmers or occasionally by white box testers during the development process.

Ideally, each test case is independent from the others. Substitutes such as method stubs, mock objects,fakes, and test harnesses can be used to assist testing a module in isolation. Unit tests are typically written and run by software developers to ensure that code meets its design and behaves as intended.