The idea of dependency injection is to make every dependency a parameter of the function.

At the moment, our createUser function relies on entities outside of its parameters; this includes the create function and the ValidationError class. If we were to use dependency injection, we'd modify our createUser function to have the following structure:

function createUser(req, res, db, create, ValidationError) {
  create(req)
    .then(onFulfilled, onRejected)
    .catch(...)
}

Then, we would be able to inject the following dependencies from our tests:

...
import ValidationError from '../../../validators/errors/validation-error';
import createUser from '.';

const generateCreateStubs = {
  success: () => stub().resolves({ _id: 'foo'})
}

describe('create', function () {
  describe('When called with valid request object', function (done) {
    ...
    createUser(req, res, db, generateCreateStubs.success(), ValidationError)
      .then((result) => {
        // Assertions here
      })
  })
})