Nodejs single callback after searching from 2 mongodb collections

As the previous answer, you can use async module to complete this task. There are many functions to control the non-blocking nature of node. Here I will recommend you to use "parallel" method. As the queries are independent of each other, it will be faster than "waterfall" method.

According to your question the code will look like this.

var async = require('async');async.parallel([    (cb) => {        Stud.findOne(            {                _id: userid            },            cb        );    },    (cb) => {        Prof.findOne(            {                _id: userid            },            cb        );    }],(err, result) => {    if (err) {        //handle error        return;    }    //result will be an array where the first element will be the result of first query and    // second element will be the query result for the second query    // so according to this .....    if (result[0]) {        //id is matched with Stud collection        //result[0] is the student doc    } else if (result[1]) {        //id is matched with Prof collection        //result[0] is the professor doc    } else {        //Neither Stud or Prof    }}

);

You can read about the async methods from the asyn documentation

You can use waterfall method of async mudule to out from this issue

async.waterfall([    function(callback) {        //your fist query method can go here        callback(null, query_result1);    },    function(first_result1, callback) {        // your second query method go here        callback(null, query_result2);    }], function (err, result) {    // final result'});

For the answer to your question, apply @abdulbarik post.

Here is other stuff about your actual code :

• Cut your request into functions
• When you are using callbacks, use them to return the error properly. Do not throw.
• You do not need to put _id key into quote

Remarks:

• Because of you are using node.js that now support ES6 (most of it), use it. Simplier to read and more efficient.

Sample about callbacks and function cut. I let you do the rest which is es6, waterfall handling.... You can look at Promise and Async/Await pattern tho.

// Check if there is a studentfunction check_student(user_id, callback) {  Stud.findOne({    _id: user_id  }, function (err, stud) {    if (err) return callback(err, false);    // stud here can worth false    return callback(false, stud);  });}

// Check if there is a proffunction check_prof(user_id, callback) {  Prof.findOne({    _id: user_id  }, function (err, prof) {    if (err) return callback(err, false);    // prof here can worth false    return callback(false, prof);  });}

// Get Stud not Prof infofunction check_user_info(user_id, callback) {  // Look if user_id match a stud  check_student(user_id, function (err, result) {    // We have an error    if (err) return callback(err, false);    // We have a student    if (result) return callback(false, result);    // Check if user_id match a prof    check_prof(user_id, function (err, result) {      // We have an error      if (err) return callback(err, false);      // We have a prof      if (result) return callback(false, result);      // No result at all      return callback(false, false);    });  });}

How you call it

check_user_info(user_id, function (err, result) {  // ...});

Example of code with promise :

        // Check if there is a student        function check_student(user_id) {          return new Promise((resolve, reject) => {            Stud.findOne({              _id: user_id            }, (err, stud) => {              if (err) return reject(err);              // prof here can worth false              return resolve(stud);            });          });        }        // Check if there is a prof        function check_prof(user_id) {          return new Promise((resolve, reject) => {            Prof.findOne({              _id: user_id            }, (err, prof) => {              if (err) return reject(err);              // prof here can worth false              return resolve(prof);            });          });        }        // Get Stud not Prof info        function check_user_info(user_id) {          return Promise.all([            check_student(user_id),            check_prof(user_id),          ]);        }

check_user_info(user_id)  .then([    stud,    prof,  ] => {    // Handle result  })  .catch((err) => {    // Handle error  });