Create a new Retrieve Salt engine at src/engines/auth/salt/retrieve/index.js. In it, we need to use the Elasticsearch client's search method to find the user's document by email, extract the digest from the document, and then extract the salt from the digest:

const NO_RESULTS_ERROR_MESSAGE = 'no-results';

function retrieveSalt(req, db, getSalt) {
  if (!req.query.email) {
    return Promise.reject(new Error('Email not specified'));
  }
  return db.search({
    index: process.env.ELASTICSEARCH_INDEX,
    type: 'user',
    body: {
      query: {
        match: {
          email: req.query.email,
        },
      },
    },
    _sourceInclude: 'digest',
  }).then((res) => {
    const user = res.hits.hits[0];
    return user
      ? user._source.digest
      : Promise.reject(new Error(NO_RESULTS_ERROR_MESSAGE));
  }).then(getSalt);
}

export default retrieveSalt;

This function requires the getSalt method from the bcrypt library, which would be injected by the handler function. Next, create a file at src/handlers/auth/get-salt/index.js to house the handler function, which simply passes the request on to the engine and generates standard responses based on the result of the engine:

function retrieveSalt(req, res, db, engine, _validator, getSalt) {
  return engine(req, db, getSalt).then((result) => {
    res.status(200);
    res.set('Content-Type', 'text/plain');
    return res.send(result);
  }, (err) => {
    if (err.message === 'Email not specified') {
      res.status(400);
      res.set('Content-Type', 'application/json');
      return res.json({ message: 'The email field must be specified' });
    }
    throw err;
  }).catch(() => {
    res.status(500);
    res.set('Content-Type', 'application/json');
    return res.json({ message: 'Internal Server Error' });
  });
}

export default retrieveSalt;

Lastly, in src/index.js, import the engine and handler and use it to create a new endpoint:

import { getSalt } from 'bcryptjs';
import retrieveSaltHandler from './handlers/auth/salt/retrieve';
import retrieveSaltEngine from './engines/auth/salt/retrieve';
const handlerToEngineMap = new Map([
  [retrieveSaltHandler, retrieveSaltEngine],
  ...
]);
app.get('/salt', injectHandlerDependencies(retrieveSaltHandler, client, handlerToEngineMap, handlerToValidatorMap, getSalt));

Since we are now using the bcryptjs package in our implementation code, and not just our test code, we should move it from devDependencies to dependencies:

$ yarn remove bcryptjs
$ yarn add bcryptjs

Lastly, we should also modify the injectHandlerDependencies function to pass through the getSalt dependency:

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

export default injectHandlerDependencies;

Now, when we run the E2E tests, they should all pass.