Writing our first unit test

Let's write unit tests for the generateValidationErrorMessage function. But first, let's convert our src/validators/errors/messages.js file into its own directory so that we can group the implementation and test code together in the same directory:

$ cd src/validators/errors
$ mkdir messages
$ mv messages.js messages/index.js
$ touch messages/index.unit.test.js

Next, in index.unit.test.js, import the assert library and our index.js file:

import assert from 'assert';
import generateValidationErrorMessage from '.';

Now, we are ready to write our tests.

Describing the expected behavior

When we installed the mocha npm package, it provided us with the mocha command to execute our tests. When we run mocha, it will inject several functions, including describe and it, as global variables into the test environment. The describe function allows us to group relevant test cases together, and the it function defines the actual test case.

Inside index.unit.tests.js, let's define our...