How can I return the count of documents returned by a query?
I have a routing file, which have the following code:

```
router.post('/facebookLogin', function(req, res, next){
    var User=require('../models/user');
    var a=User.facebookUserExist(req.body.id, req.body.email);
    console.log(a);
    res.end();
});

```

And here is the content of the User model file:

```
var User=function(data){
    this.data=data;
}
User.prototype.data={};
User.prototype.facebookUserExist=function(id, email){
    var output;
    db.collection('users').find({
        $or:[
            {
                facebookID:id
            },
            {
                email:email
            }
        ]
    }).count(function(err, numOfDocs){
        output=numOfDocs;
    });
    return output;
}
module.exports=new User;

```

I set the value of the output variable in the count method callback, but the function still return undefined.