How to make a sync database query with Mysql and node.js?

Question

I'm trying to make a SELECT query in my database using mysql nodejs module. The problem is that connection.query function is an asyncronous function so I put it inside a promise like this:

function execQuery(query, fields){
    const connection = mysql.createConnection({
        host: process.env.DB_HOST,
        port: process.env.DB_PORT,
        user: process.env.DB_USER,
        password: process.env.DB_PASSWORD,
        database: process.env.DB_DATABASE,
    });

    return new Promise((resolve, reject) => {
        connection.query(query, fields, (err, results) => {
            connection.end();
            if (err){
                return reject(err);
            }else{
                return resolve(results);
            }
        }
    ) 

This function is called by another function:

function getPasswordFromUser(username){
    query = 'SELECT password FROM User WHERE username=?'
    parameters = [username]

    execQuery(query, parameters)
        .then(result => {
            if (result !== undefined && result.length === 1){
                console.log('here1')
                return result
            }
        })
        .catch(err => console.log(err))

    console.log('here2')
}

the problem is that despite I return a promise and use then , in my console I receive:

here2
here1

So my query keeps running in asynchronous mode and I can't get the result.

What is the best way to do this using promise?

Answer 1

Simply you are not returning the promise, and that's it.

do: return execQuery(query, parameters)

And you use it like a simple promise.

getPasswordFromUser('userid')
.then((result)=>{
  ..do stuff..
});

or if you are using async-await use a async function and await the getPasswordFromUser function. For example for an express route.

app.post('/getPassword', async (req,res) =>{
    const userID = req.body.userID;
    const password = await getPasswordFromUser(userID);
    res.send(password);
});

Note: The route is a joke, I hope you are not storing password in plain text, and if you want to match the password(like login) you should do something like: select count(*) from user where username=username and password=hash(password); or maybe you are matching the password using bcrypt or something.

Answer 2

You're seeing here2 first because it's not inside the then . The program continues to execute after it starts the execQuery function.

You can return a promise:

function getPasswordFromUser(username){
    query = 'SELECT password FROM User WHERE username=?'
    parameters = [username]

    return execQuery(query, parameters)
        .then(result => {
            if (result !== undefined && result.length === 1){
                console.log('here1')
                return result
            }
        })
        .catch(err => console.log(err))
}

Then add .then(result => { ... }) to the function call to get the result.