Unit testing the request handler

First, we'll move the src/handlers/users/create.js module into its own directory. Then, we will correct the file paths specified in the import statements to point to the correct file. Lastly, we will create an index.unit.test.js file next to our module to house the unit tests.

Let's take a look at the createUser function inside our request handler module. It has the following structure:

import create from '../../../engines/users/create';
function createUser(req, res, db) {
  create(req, db)
 .then(onFulfilled, onRejected)
 .catch(...)
}

First, it will call the create function that was imported from src/engines/users/create/index.js. Based on the result, it will invoke either the onFulfilled or onRejected callbacks inside the thenblock.

Although our createUser function depends on the create function, when writing a unit test, our test should test only the relevant unit, not its dependencies. Therefore, if the result of our tests relies on thecreate function, we...