nodeJS - Cannot set property of undefined - node.js

Question

i know this question may be asked about 1000 times here but i cant find the error in my script.
I am trying to iterate through 2 arrays to get a name from an city_id and an organisation_id in a each() loop. i would like to write there values to the "mother"-object to get all informations in one place. Here is the code i've written so far:
let express = require('express');
let router = express.Router();
let request = require('request');
let connection = require('../lib/mysql');
router.get('/', function(req, res) {
if(req.session.loggedin === true){
getList((err, finlist) => {
console.log(finlist)
});
} else {
const cssPath = '/stylesheets/style.css'
res.render('login', { cssPath, title: 'Login' });
}
});
function getList(callback) {
var result = [];
connection.query('SELECT * FROM lists ', (err, rows) => {
if(err) throw err;
var r=0;
rows.forEach(function(item) {
result[r] = item;
getCities((err, cty) => {
result[r].city = cty[item.city_id].name;
getOrganisations((err, org) => {
result[r].organisation = org[item.organisation_id].name;
});
callback(result);
});
r++;
});
});
}
function getCities(callBack) {
var result=[];
connection.query('SELECT * FROM cities ', (err, rows) => {
if (err) throw err;
rows.forEach(function (cty) {
result[cty.id] = cty;
});
if (err) {
callBack(err, null);
} else {
callBack(null, result);
}
});
}
function getOrganisations(callBack) {
var result=[];
connection.query('SELECT * FROM organisations ', (err, rows) => {
if(err) throw err;
rows.forEach(function(org) {
result[org.id] = org;
});
if (err) {
callBack(err, null);
} else {
callBack(null, result);
}
});
};
module.exports = router;
I always get the error
TypeError: Cannot set properties of undefined (setting 'city')
at /opt/alarmprocessor/routes/settings.js:53:32
. . .
which is the line result[r].city = cty[item.city_id].name;
King regards for helping me out :)
Tried to set it as an array, as an Object, made console outputs everywhere... seems all to be fine.. Maybe i am too new to NodeJS so it hasnt been clicked in my Head ;D

Answer 1

This error occur because result[r] doesn't exist line 53. It's declared but doesn't "exist" -> it's undefined. If you perform a mere console.log(result[r]); line 52 you will get an undefined, and you can't set properties (like city) to an undefined value.
The quick fix would be to use optional chaining like this:
result[r]?.city = cty[item.city_id].name;
it won't fix your code, it will only stop crashing by ignoring the assignment.