API endpoint as promise chain

example.js

import domain from '../../domain';

const Handler = {
  getGroups: (request, reply) => {
    const groups = domain.groups(request, reply);

    return Promise.resolve()
      .then(groups.getGroups)
      .then(groups.transformMany)
      .then(groups.reply);
  },

  postGroup: (request, reply) => {
    const groups = domain.groups(request, reply);

    return Promise.resolve()
      .then(groups.assertGroupExist)
      .then(groups.createGroup)
      .then(groups.transformOne)
      .then(groups.replyCreated);
  },

  putUserToGroup: (request, reply) => {
    const groups = domain.groups(request, reply);
    const users = domain.users(request, reply);

    return Promise.resolve()
      .then(groups.assertGroupExist)
      .then(users.assertUserExist)
      .then(groups.addUser)
      .then(groups.replyEmpty);
  },

  removeUserFromGroup: (request, reply) => {
    const users = domain.users(request, reply);
    const groups = domain.groups(request, reply);

    return Promise.resolve()
      .then(groups.assertGroupExist)
      .then(users.assertUserExist)
      .then(groups.removeUser)
      .then(groups.replyEmpty);
  },

  getGroupUsers: (request, reply) => {
    const groups = domain.groups(request, reply);
    const groupUsers = domain.groupUsers(request, reply);

    return Promise.resolve()
      .then(groups.assertGroupExist)
      .then(groupUsers.getGroupUsers)
      .then(groupUsers.transformMany)
      .then(groupUsers.reply);
  }
};

export default Handler;