const users = [
  {
    id: '123',
    name: 'John Smith',
    posts: [
      {title: 'Some amazing title', content: 'Here is some amazing content'},
      {title: 'My favorite title', content: 'My favorite content'},
      {title: 'A not-so-good title', content: 'The not-so-good content'},
    ]
  },
  {
    id: '456',
    name: 'Mary Michaels',
    posts: [
      {title: 'Some amazing title', content: 'Here is some amazing content'},
      {title: 'My favorite title', content: 'My favorite content'},
      {title: 'A not-so-good title', content: 'The not-so-good content'},
    ]
  },
]
function getUserPosts(id, cb) {
 const user = users.find(el => el.id === id);
  if (cb) {
    if (user) {
      return cb(null, user);
    }
    return cb('User Not Found', null);
  }
  return new Promise(function(resolve, reject){
    if (user) {
      resolve(user);
    } else {
      reject('User not found');
    }
  });
}

/* The above code is collapsed to simulate an API you might use to get user posts for a
 * particular user from a database.
 
 * The API can take a callback as a second argument: getUserPosts(<id>, <callback>); 
 * The callback function first argument is any error and second argument is the user.
 * For example: 
    getUserPosts('123', function(err, user) {
      if (err) { 
        console.log(err) 
      } else {
        console.log(user);
      }
    });
 
 * getUserPosts also returns a promise, for example: getUserPosts.then().catch();
 
 * The ID's that will generate a user are the of type string and they are '123' and '456'.
 * All other IDs will return an error.
 */

getUserPosts('123', function(err, user) {
  if (err) {
    console.log(err);
  } else {
    console.log(user);
  }
});

getUserPosts('129', function(err, user) {
  if (err) {
    console.log(err);
  } else {
    console.log(user);
  }
});

getUserPosts('456')
  .then(user => console.log(user))
  .catch(err => console.log(err));