Mongoose query fed into array

That last line responds with '[]'

This is because User.find() is an async call. So when you step out of the loop when i = 8, you will hit console.log(authors) before the data has returned.

Another Issue

It seems that you are making 8 separate calls to the database when you only need to make 1 call. If you have an array called collected that you are looping through. and it looks like this.

var collected = [
  { author : "23423423"},
  { author : "23423424"},
  { author : "23423425"},
  { author : "23423426"},
];

You can gather all of the twitter ids using the .map function.

var twitterIds = collected.map(function(a){return a.author});

Now twitterIds == ["23423423", "23423424", "23423425", "23423426"].

You can then use $in operator to select the documents where the value of twitter.id equals any value in the specified array.

You can then do this User.find({ "twitter.id" : {$in : twitterIds}});

It will return a document containing all of the matching documents which you then can manipulate. Now you have reduced to 1 call instead of 8.

Refactor with Two Options

I would refactor the code in either of the two ways:

1) Return the Query

function search(twitterIds){
   var query = User.find({ "twitter.id" : twitterIds});
   return query;
}

Then you can use it as so:

var searchQuery = search(twitterIds);
searchQuery.exec(function(err,users){
  if(err) return console.log(err);
  //do something with users..
  users.forEach(function(user){
  console.log(user.twitter.username);// do something here
  });
}

2) Return the Promise

function search(twitterIds){
   var promise = User.find({ "twitter.id" : twitterIds}).exec();
   return promise;
}

Then you can use it as so:

var promise = search(twitterIds);
promise.then(function(users){
  //do something...
}).error(function(err){
   console.log(errr);
});

Before using the async, library I would start here to see if this would get the job done.