Do NOT use any globals or shared, higher scoped variables for your asynchronous result in a server. Never do that. That is an anti-pattern for good reason because that can create intermittent concurrency problems because more than one request can be in process at the same time on your server so these would cause conflicting access to those variables, creating intermittent, very-hard-to-debug problems. I repeat again, NEVER.

You didn't describe an exact situation you are trying to write code for, but here's an example.

Instead, you use the result inside the context that it arrives from your asynchronous call. If you can use await, that generally makes the coding cleaner.

Here's a simple example:

const con = mysql.createConnection({
    host: "XXXXX",
    user: "XXXXX",
    password: "XXXXX",
    database: "XXX"
});

const query = util.promisify(con.query).bind(con);

http.createServer(async function(req, res) {
    if (req.path === "/query" && req.method === "GET") {
        try {
            const rows = await query('SELECT * FROM mylist');
            res.writeHead(200, { 'Content-Type': 'application/json' });
            res.end(JSON.stringify(rows));
        } catch(e) {
            console.log(e);
            res.statusCode = 500;
            res.end();
        }
    } else {
        // some other respone
        res.writeHead(200, { 'Content-Type': 'text/plain' });
        res.write("hello");
        res.end();
    }
}).listen(8080);

Things to note here:

1. Checking both path and method before handling the request.
2. Making callback function async so it can use await.
3. Making sure any promise rejection from await is caught by try/catch and an error response is sent if there's an error.
4. Sending result as JSON and setting appropriate content-type.
5. You may be using the plain http module as a learning experience, but you will very quickly find that using the simple Express framework will save you lots of programming time and make things lots easier.