node js - Node mysql - Result is undefined

Question

app.js

MyDatabase = require("./mysql");
...
var availableRooms = mydb.checkRoomStatus();
console.log("Rooms " + availableRooms);
//undefined.

mysql.js

MyDatabase.prototype.checkRoomStatus = function() {
this.con.query('SELECT * FROM rooms WHERE status = "open" LIMIT 1',function(err,rows){
  if(err) throw err;

  console.log('Data received from Db:\n');
  console.log(rows); // results are coming here.
  return rows;
});

}

The first console.log is outputting "undefined" for availableRooms variable.
I think i should use a callback function for this kind of request.
But i dont know how to use it. When i search in internet, i dont find anyone using separate file for mysql to get callback results.

can you give the link to the exact node-mysql package that you are using? there are many, so, to avoid any confusion before people start answering. — Parthapratim Neog
– Parthapratim Neog, Commented Mar 31, 2016 at 6:25
@ParthapratimNeog I don't think this will help, as you can see the OP try to return result from callback.. — Ziki
– Ziki, Commented Mar 31, 2016 at 6:28
Possible duplicate of How do I return the response from an asynchronous call? — Ziki
– Ziki, Commented Mar 31, 2016 at 6:31
npm install mysql - This is how i installed mysql package for node. Which one is stable? @Parthapratim Neog I think your solution is what i was searching for. I will try it out and let you know. — Mahesh
– Mahesh, Commented Mar 31, 2016 at 6:42

Parthapratim Neog · Accepted Answer · 2016-03-31 06:51:41Z

3

You need to return callback(rows) to return your rows and you need to catch it in your callback. I hope it's clear. Try this out, and see if it works.

change your mysql code to

MyDatabase.prototype.checkRoomStatus = function(callback) {
this.con.query('SELECT * FROM rooms WHERE status = "open" LIMIT 1',function(err,rows){
  if(err) throw err;

  console.log('Data received from Db:\n');
  console.log(rows); // results are coming here.
  return callback(rows);
});

Now in app.js

MyDatabase = require("./mysql");
...
mydb.checkRoomStatus(function(res){
    var availableRooms = res;
    console.log("Rooms " + availableRooms);
});

edited Mar 31, 2016 at 6:51

answered Mar 31, 2016 at 6:33

Parthapratim Neog

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1 Comment

Vishwanth Iron Heart Over a year ago

I'm trying to do the same thing, but in mysql.js, it says MyDatabase is not defined, where exactly is it created? and where did mydb object come?

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