One of the four major requirements for an application to be considered production-ready is to pass test procedures, whether they are manual or automated.

Even though we've been manually testing our application at each step of the development process and making sure the endpoints work as intended, it's unfortunately not enough to assert that every component is defect free, or in other words, that they perform as expected and handle errors properly.

To ensure that all is functioning as intended, we can use an automated testing method called Unit Testing, which allows us to test each component in isolation against different scenarios and pinpoint any unforeseen defect that might cause the application to malfunction.

Setting up Jest

Let's install the jest package as a development dependency:

$ npm install --save-dev jest

Let's add this test script to the package.json file:

{  "scripts": {    "test": "jest"  },}

And let's create a new directory named tests/unit.

$ mkdir -p tests/unit

Testing the loaders

Testing the environment loader