Node-js to MongoDB single authentication to several database

I need some help, a programmer left several node-js scripts to me without any authentication on the code to connect it to several mongodb database and now I need to implement a authentication method, is there a way to authenticate via a single call or script to connect to several database to be used in other node-js scripts or should I really one by one code it to authenticate to each database? I've already done some research but all I see is for every database I should query I need to code or call the authentication method for node-js.

Edit: I've tried something like this but when I'm trying to do query on other script using the db I got unauthorized error when inserting.

DBCommon.js

 var connString1 = "mongodb://" + dbIP + ":" + dbPort + "/GameDB";
    MongoClient.connect(connString1,{ 'native_parser': true , "auto_reconnect": true , socketOptions: { keepAlive: 1 } }, function (err, db) {
        if (err) throw err;
        global.db = db;
        console.log("Global DB open is success. Database IP is " + dbIP + " : " + dbPort);
        Common.SetLogToFile("./DBLog", "Global DB opened on " + dbIP + " : " + dbPort);
    });

DBLogin.js

exports.DBSignUp = function (postData, callback) {
    try {
        db.collection("Member").findOne({ "id" : postData["id"] , "pw" : postData["pw"] }, function (err, row) {
            if (err) throw err;

            if (row == null) {
                //console.log("Try Insert");
                var postValue = {
                    "id" : postData["id"]
                    ,"pw" : postData["pw"]
                    ,"authToken" : ""
                    ,"regDate" : new Date()
                };

                db.collection("Member").insert(postValue, function (err, result) {
                    if (result == null) {
                        callback(ResultType.ERROR_TYPE.FAILED_REGIST_MEMBER, null);
                        return;
                    }

                    callback(1, postValue);
                });
            }
            else
                callback(ResultType.ERROR_TYPE.ALREADY_EXISTS_ID, null);
        });
    }
    catch (ex) {
     //console.log("DB exception : " + ex);
    }
}