As before, let's implement the Login engine first. Like our other engines, we are first using a validator to validate the request object. Once the request is validated, we then use the Elasticsearch client's search method to see how many user documents match the email and digest provided. If there are non-zero documents, then a user with these credentials exists, and the engine should resolve with a token (we are using a placeholder string for now). If there are no users that match these credentials, it means that those credentials are invalid, and the engine should return with a rejected promise:
import specialEscape from 'special-escape';const specialChars = ['+', '-', '=', '&&', '||', '>', '<', '!', '(', ')', '{', '}', ...
