How to use the mongodb-extended-json.stringify function in mongodb-extended-json

.then(allUsersDoc => {
                    if (allUsersDoc) {
                        // userIDsFlat may contain "free text" users that are not in DB
                        // We create a dict to look up which collected userIDs are really from DB
                        let userIDsFromDB = {};
                        allUsersDoc.map(usr => {
                            userIDsFromDB[usr._id] = 1;
                        });
                        const usrFile = msID + ExpImpUsers.FILENAME_POSTFIX;
                        fs.writeFileSync(usrFile, EJSON.stringify(allUsersDoc,null,2));
                        console.log('Saved: '+usrFile + ' with '+allUsersDoc.length+' users');

                        // Save mapping file old => new user ID
                        // But only with REAL DB users (skip free text users)
                        userIDsFlat.map(usrID => {
                            if (userIDsFromDB[usrID]) {
                                // default: newID === oldID
                                // This means, users are copied(!) from source DB to destination DB
                                // If newID is changed to an existing id from destination ID, this target user is used
                                let thisUser = ExpImpUsers.searchUser(allUsersDoc, usrID);
                                userIDsOuputMap[usrID] = {
                                    'newID': usrID,
                                    'hint': thisUser.username +" "+thisUser.profile.name
                                };
                            }
                        });

.then(allMinutesDoc => {
                    if (allMinutesDoc) {
                        const minFile = msID + ExpImpMinutes.FILENAME_POSTFIX;
                        fs.writeFileSync(minFile, EJSON.stringify(allMinutesDoc,null,2));
                        console.log('Saved: '+minFile + ' with '+allMinutesDoc.length+' minutes');

                        // Collect additional invited / informed users from older minutes
                        allMinutesDoc.map(min => {
                            min.visibleFor && min.visibleFor.map(userID => {      // should be identical to meeting series
                                userIDs[userID] = 1;
                            });
                            min.informedUsers && min.informedUsers.map(userID => {   // should be identical to meeting series
                                userIDs[userID] = 1;
                            });
                            min.participants && min.participants.map(part => {      // might differ from meeting series users!
                                userIDs[part.userId] = 1;
                            });
                            min.topics && min.topics.map(top => { // iterate topics
                                top.responsibles && top.responsibles.map(resp => {  // topic-responsibles
                                    userIDs[resp] = 1;

var respCallback = function(json) {
                        // Convert JSON object to MongoDB-Extended JSON
                        var e = EJSON.stringify(json);
                        var j = JSON.parse(e);
                        return res.json(j);
                    };
                    //console.log(JSON.stringify(query));

gateway.sendDeviceState = function(deviceId, state, cb) {
    var topic = util.format(config.topicFormatState, deviceId);
    var payload = { time: new Date(), data: state };

    // May not be connected. Skip publish if not.
    if(mqttClient) {
      mqttClient.publish(topic, EJSON.stringify(payload), cb);
    }
  };

.then(doc => {
                    if (doc) {
                        const attFile = msID + ExpImpFilesAttachments.FILENAME_POSTFIX;
                        fs.writeFileSync(attFile, EJSON.stringify(doc,null,2));
                        console.log('Saved: '+attFile + ' with '+doc.length+' file attachments');
                        if (doc[0]) {
                            console.log('      *** Hint *** Please manually copy all files below:');
                            console.log('      '+doc[0]._storagePath);
                        }
                        resolve({db, userIDs});
                    } else {
                        return reject ('Unknown meeting series ID: '+ msID);
                    }
                });
        });

.then(doc => {
                    if (doc) {
                        const protFile = msID + ExpImpFilesDocuments.FILENAME_POSTFIX;
                        fs.writeFileSync(protFile, EJSON.stringify(doc,null,2));
                        console.log('Saved: '+protFile + ' with '+doc.length+' protocol documents');
                        if (doc[0]) {
                            console.log('      *** Hint *** Please manually copy all files below:');
                            console.log('      '+doc[0]._storagePath.substr(0, doc[0]._storagePath.lastIndexOf('/')));
                        }
                        resolve({db, userIDs});
                    } else {
                        return reject ('Unknown meeting series ID: '+ msID);
                    }
                });
        });

.then(doc => {
                    if (doc) {
                        console.log("DB Schema Version: "+doc.version);
                        if (ExpImpSchema.MADE_FOR_SCHEMA !== doc.version) {
                            console.log("*** WARNING *** Schema mismatch!");
                            console.log("                This exporter is made for database schema version: "+ExpImpSchema.MADE_FOR_SCHEMA);
                            console.log("                But your database has schema version             : "+doc.version);
                            console.log("                Alyways migrate to the most recent DB schema before export!");
                            console.log("                Alyways use matching exporter!");
                            console.log("                Exported data may be corrupt. Continue at your own risk.");
                        }
                        const schemaFile = msID + ExpImpSchema.FILENAME_POSTFIX;
                        fs.writeFileSync(schemaFile, EJSON.stringify(doc,null,2));
                        console.log("Saved: "+schemaFile);
                        resolve(db);
                    } else {
                        return reject ("No migrations schema version found in your DB! Unable to export.");
                    }
                });
        });

.then(doc => {
                    if (doc) {
                        const topFile = msID + ExpImpTopics.FILENAME_POSTFIX;
                        fs.writeFileSync(topFile, EJSON.stringify(doc,null,2));
                        console.log('Saved: '+topFile + ' with '+doc.length+' topics');
                        resolve({db, userIDs});
                    } else {
                        return reject ('Unknown meeting series ID: '+ msID);
                    }
                });
        });

exports.toJsonString = function (doc) {
  return ESJON.stringify(doc);
};

data.on('data', d => {
      fs.appendFileSync(file, EJSON.stringify(d) + '\n');
    }).on('end', () => {
      return resolve();