{
    "_id" : ObjectId("5a8ea03d2601be24b086ccd4"),
    "userId" : 20,
    "text" : "Hi",
    "__v" : 0
}

{
        "_id" : ObjectId("5a8ea03d2601be24b086ccd4"),
        "userId" : 20,
        "voice" : "d2601be24bd22601be24b",
        "__v" : 0
    }

Message
  .find({userId: '20'}, {_id: 0, text: ''})
  .exec((err, obj) => {
    if (err) {
      console.log(err);
    }

    for (const val of Object.values(obj)) {
      console.log(val.text);
    }
  });

Hi    // for text
undefined  // for voice