Though discussing and writing test code is outside the scope of this book, it is a crucial part of developing reliable software. As full-stack JavaScript applications become more mainstream over time, the need for better testing capabilities is producing a good number of testing tools in this ecosystem. In the following sections, we will first look at some of the popular testing tools that are available for testing the different parts of a MERN-based application. Then, to help you get started with writing test code for the MERN applications developed in this book, we will also discuss an example of adding a client-side test to the MERN Social application from Chapter 5, Growing the Skeleton into a Social Media Application.