Developing and testing service functions

Up until now, we have always been testing code by putting it in the src/example.js file. Now, we are going to write some service functions and learn how to write actual tests for them by using Jest.

Setting up the test environment

First, we are going to set up our test environment by following these steps:

1. Install jest and mongodb-memory-server as dev dependencies:

   $ npm install --save-dev jest@29.7.0 \
     mongodb-memory-server@9.1.1

   Jest is a test runner used to define and execute unit tests. The mongodb-memory-server library allows us to spin up a fresh instance of a MongoDB database, storing data only in memory, so that we can run our tests on a fresh database instance.

2. Create a src/test/ folder to put the setup for our tests in.
3. In this folder, create a src/test/globalSetup.js file, where we will import MongoMemoryServer from the previously installed library:

   import { MongoMemoryServer } from 'mongodb-memory...