MongoDB MonkAPI setting a variable based on find result that is available outside the db request

I'm trying to set a variable based on the result of a find through Monk API on MongoDB in a Node JS application (it's my first time of using MongoDB).

This is an example of the code I have;

var variableIWantToSet;
var collection = req.db.get('myCollection');
collection.find( { foo: 'bar' },{
  fields : { myTargetField: 1, _id: 0},
  limit : 1,
  sort : {$natural : -1}
}
, function(err, doc) {
    if (err) {
      console.log(err);
    }
    variableIWantToSet = doc[0].myTargetField;
});
console.log(variableIWantToSet);

If I console.log(doc[0].myTargetField) within the function I get the right value, but the console.log(variableIWantToSet) returns undefined.

Help appreciated. Thanks.