Unit testing has become a primary part of good software development practice. It is a method by which individual units of source code are tested to ensure proper functioning. Each unit is theoretically the smallest testable part of an application. In a Node.js application, you might consider each module as a unit.
In unit testing, each unit is tested separately, isolating the unit under test as much as possible from other parts of the application. If a test fails, you would want it to be due to a bug in your code rather than a bug in the package that your code happens to use. A common technique is to use mock objects or mock data to isolate individual parts of the application from one another.
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