Extend variables outside MySQL query function in NodeJS - mysql

Question

I tried to run a function which returns a value back but am getting undefined.
function getMessageId(myId, user){
$query = "SELECT * FROM startMessage WHERE (userFrom = '"+myId+"' AND userTo = '"+user+"') OR (userFrom = '"+user+"' AND userTo = '"+ myId+"')";
connect.query($query, function(error, rows){
sql = rows[0];
console.log(sql);
return sql.id;
})
}
// running the function
msgId = getMessageId(userFrom, userTo);
console.log(msgId);
Now when I tried to console.log the sql I get the expected result like
{
id : 3,
userFrom : 3,
userTo : 1,
type : "normal",
date : "2017-06-25 06:56:34",
deleted : 0
}
But when I console.log the msgId I get undefined. I am doing this on NodeJS, please any better solution?

Answer 1

Short answer, Because its an asynchronous operation.
The outer console.log happens before the getMessageId returns.
If using callbacks, You can rewrite getMessageId as
let msgId
function getMessageId(myId, user, callback){
$query = "SELECT * FROM startMessage WHERE (userFrom = '"+myId+"' AND userTo = '"+user+"') OR (userFrom = '"+user+"' AND userTo = '"+ myId+"')";
return connect.query($query, function(error, rows){
sql = rows[0];
console.log(sql);
callback(sql.id);
})
}
function setMsgId(id) {
msgId = id;
}
And then call it as,
getMessageId(userFrom, userTo, setMsgId);
Further I would suggest you look into Promises.
Which would very well streamline the flow.
Using Promises, getMessageId should look something like
function getMessageId(myId, user){
$query = "SELECT * FROM startMessage WHERE (userFrom = '"+myId+"' AND
userTo = '"+user+"') OR (userFrom = '"+user+"' AND userTo = '"+
myId+"')";
const promise = new Promise((resolve, reject) => {
connect.query($query, function(error, rows){
sql = rows[0];
console.log(sql);
resolve(sql.id);
})
return promise.
}
Post this, You can use it as
getMessageId(myId, user).then((msgId) => console.log(msgId))

Answer 2

create a wrapper for mysql use
// service/database/mysql.js
const mysql = require('mysql');
const pool = mysql.createPool({
host : 'host',
user : 'user',
password : 'pass',
database : 'dbname'
});
const query = (sql) => {
return new Promise((resolve, reject) => {
pool.query(sql, function(error, results, fields) {
if (error) {
console.error(error.sqlMessage);
return reject(new Error(error));
}
resolve(results);
});
});
}
module.exports = { query };
then call from another script with async funcion and await
// another file, with express route example
const db = require('/service/database/mysql.js')
module.exports = async (req, res) => { // <-- using async!
let output = []; // <-- your outside variable
const sql = 'SELECT * FROM companies LIMIT 10';
await db.query(sql) // <-- using await!
.then(function(result) {
output = result; // <-- push or set outside variable value
})
.catch(e => {
console.log(e);
});
// await db.query("next query" ...
// await db.query("next query" ...
// await db.query("next query" ...
res.json(output);
}

Answer 3

This is probably NOT a proper way, and a hack, but sharing for information purpose (you may not like to use this)
Simply use an if else and call the function once inside the query (if true), and call it outside (if false)
if (id != null) {
// Get Details
const query = pool.execute('SELECT * FROM `sometable` WHERE `id` = ?', [id], function(err, row) {
if (err) {
console.log(err)
return
} else {
if (row && row.length) {
// Do Something
}
}
})
} else {
// Do Something
}