Nodejs, MongoDB asynchronous callback problem - node.js

Question

I'm the beginner of nodejs and MongoDB.I tried to design RESTAPI but there was a problem.
Here is my router code.
app.post('/fcm',function(req,res){
var beaconid = req.body.beacon_id;
var my_token_id = req.body.user_id;
User.find({beacon_id: beaconid}, function(err, output){
if(err) return res.status(500).json({error: err});
if(!output) return res.status(404).json({error: 'user not found in User collections.'});
console.log("output user id :"+output.user_id + " beacon: " +output.beacon_id );
target_token_id = output.user_id;
res.json(output);
});
});
And this is user schema.
var userSchema = new Schema({
user_id: String,
beacon_id: String
});
The result of above function in the console is:
output user id: undefined , beacon: undefined.
But json from res function is printed properly.
This codes look like very simple but I don't know what is the problem.
Please somebody help me.

Answer 1

By using find, you are expecting 1 document or more. This means that the method <Model>.find() should logically return an array. You can make sure that is what happens by logging the output just after the query to make sure it is an array of documents.
To solve your problem, you can either access the document at index 0 of the array:
User.find({beacon_id: beaconid}, function(err, output){
if(err) return res.status(500).json({error: err});
if(!output) return res.status(404).json({error: 'user not found in User collections.'});
console.log("output user id :"+output[0].user_id + " beacon: " +output[0].beacon_id );
target_token_id = output[0].user_id;
res.json(output);
});
Or use findOne() instead, that returns only one document.
User.findOne({beacon_id: beaconid}, function(err, output){
if(err) return res.status(500).json({error: err});
if(!output) return res.status(404).json({error: 'user not found in User collections.'});
console.log("output user id :"+output.user_id + " beacon: " +output.beacon_id );
target_token_id = output.user_id;
res.json(output);
});