Node.js mysql in function returns undefined - node.js

Question

Here's my code
function getprefix(guildid) {
con.connect(function(err) {
con.query('SELECT * from ' + configcontent.guilds_table_name + ' WHERE guild_id=' + String(guildid), function(err, result) {
if (err) {
throw err;
} else {
return result[0].prefix;
}
});
});
}
I wanted to make this function to return one value from one column (from first result), but it returns undefined. When i tried to write it on screen (using console.log(result[0].prefix);) it works.
Please help. Sorry for bad description or english. Its my first question on StackOverflow

Answer 1

Your function is asynchronous, so it immediately returns result, now it returns undefined. Value from DB returned later, and passed to callback. You need to use promises(preferred)/callbacks to return async results from your function, something like this:
function getprefix(guildid) {
return new Promise((resolve, reject) => {
con.connect(function (err) {
con.query('SELECT * from ' + configcontent.guilds_table_name + ' WHERE guild_id=' + String(guildid), function (err, result) {
if (err) {
// throw err;
// instead of throwing we passing error to result promise
reject(err)
} else {
// return result[0].prefix;
// instead of return we pass result to promise
resolve(result[0].prefix)
}
});
});
}
}
so usage will look like:
getprefix(guildid)
.then(prefix => console.log(prefix))
.catch(e => console.err('Unable to get prefix', e))
it would be great to check the different ways to handle async programming in JS with callbacks/promises/async awaits in details.

Answer 2

You can't return value from a callback.
Before you make your query, connect to the db. Don't use the callback function for that, cuz making a query supposed to be made with con.query
function getprefix(guildid) {
con.connect();
con.query('SELECT * from ' + configcontent.guilds_table_name + ' WHERE guild_id=' + String(guildid), function(err, result) {
if (err) {
throw err;
} else {
// do stuff with 'results'
}
});
EDIT
You can use async/await like this to get access to the return value, but you have to wrap your code in an IIFE (Immediately Invoked Function Expression) or use top level await, but it's only available in the latest version of Node.
Note, that I only used IIFE, because I don't have any other function or logic where I could call my query. If you call it in another function, make it async.
// Top level action
(async () => {
const output = await makeQuery();
console.log(output);
})();
// Query
function makeQuery() {
return new Promise((resolve, reject) =>
db.query("SELECT * FROM users", (err, res) => {
if (err) reject(err);
else resolve(res);
})
);
}