What is Jest?
Jest is a popular Node.js unit testing framework, frequently used with React. Like React itself, Jest is used internally by Facebook as a primary development tool. Jest allows for tests to be run in parallel, greatly increasing efficiency.
Why use tests at all?
Even after you complete an app and get everything finally working, this is always the possibility of future modifications introducing new bugs. Unit testing simply makes it easier to guard against this scenario.
How do I install Jest?
Jest is included with React (and React Native). Otherwise, it can be installed using your package manager of choice
npm install --save-dev jest yarn add --dev jest