Unit testing is a crucial aspect of software development. It ensures that individual components of our application function as expected in isolation. In this lesson, we’ll get an overview of the unit test in NestJS.

What is a unit test?

As the name implies, unit tests focus on creating tests for the smallest units. A unit refers to the smallest testable element of a program—typically a function, method, or procedure. The primary goal of unit testing is to confirm that each unit of the software operates as intended when examined in isolation.

Let’s take an example: imagine that we’re developing a service for a financial application with NestJS. At some point, a developer decides to make a seemingly small modification to a function responsible for computing interest rates. Although the intention behind the change is to enhance the code’s efficiency, it might inadvertently lead to incorrect rounding, precision issues, or a miscalculation in the algorithm, causing inaccurate results. In the absence of thorough unit tests, this issue might escape detection during development and end up being deployed to the production environment. As a result, users experience inaccuracies in interest calculations, leading to considerable customer dissatisfaction.

By having a comprehensive suite of tests, the team could have quickly identified the problem during the development phase. The unit tests act as a safety net and detect deviations from expected behavior, preventing flawed code from reaching production.

Typically, unit tests have the following characteristics:

  • Isolation: Unit tests are designed to isolate and evaluate the behavior of a small unit of code independently from the rest of the application. This isolation helps identify issues within the unit without interference from other components.

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