Unit testing our engine

Next, let's test our create engine function. Like our previous createUser request handler, the src/engines/users/create/index.js module contains two import statements, which makes it difficult to test. Therefore, just like before, we must pull these dependencies out, and import them back into src/index.js:

import createUserValidator from './validators/users/create';
...
const handlerToValidatorMap = new Map([
  [createUserHandler, createUserValidator],
]);
...
app.post('/users', injectHandlerDependencies(createUserHandler, client, handlerToEngineMap, handlerToValidatorMap, ValidationError));

Then, update the injectHandlerDependencies function to inject the validator function into the handler:

function injectHandlerDependencies(
  handler, db, handlerToEngineMap, handlerToValidatorMap, ValidationError,
) {
  const engine = handlerToEngineMap.get(handler);
const validator = handlerToValidatorMap.get(handler);
  return (req, res) => { handler(req, res, db, engine,...