resolver.js

// resolver code
employee: async (root, { where: { email, slackId } }, { dataSources }) => {
  if (!email && !slackId) {
    throw new Error('Email or SlackId are required.');
  }

  if (email) {
    return getEmployeeFromEmail(email, dataSources);
  }
  if (slackId) {
    return getEmployeeFromSlackId(slackId, dataSources);
  }
  return null;
},
// ...