async await calling function node.js

Question

I have this function, that queries my database, and then calls getNumOfSessionForTrainerClientList function:

var select = "select * from USERS WHERE ASSIGNED_TRAINER = ?"
mysqlconn.connect(function(err) {
    if (err) {
        console.error('Database connection failed: ' + err.stack);
        return;
    }

    mysqlconn.query(select, [req.session.username], async function(err, rows) {
        if (err) {
            console.log(err);
        } else {
            
            let numOfSessionsLeft = {
                numberOfSessionsLeftIs: 0
            }
            for (var i = 0; i < rows.length; i++) {
                getNumOfSessionForTrainerClientList(req, res, rows[i], numOfSessionsLeft)
                rows[i]["NUM_OF_SESSIONS_PER_MONTH"] = (parseInt(rows[i]["NUM_OF_SESSIONS_PER_MONTH"]) - numOfSessionsLeft.numberOfSessionsLeftIs)
                console.log(numOfSessionsLeft)
            }
          

        }
        mysqlconn.end();

    })
})

and then inside the function getNumOfSessionForTrainerClientList , I have this:

async function getNumOfSessionForTrainerClientList(req, res, rows, numOfSessionsLeft) {

var getNumOfSessionForTrainerClientList = "select * from SCHEDULE WHERE CLIENT_USERNAME = ? AND ASSIGNED_TRAINER = ?"
mysqlconn.connect(async function(err) {
    if (err) {
        console.error('Database connection failed: ' + err.stack);
        return;
    }
        mysqlconn.query(getNumOfSessionForTrainerClientList, [rows["USERNAME"], req.session.username], async function(err, sessionData) {
            if (err) {
                console.log(err);
            } else {
                numOfSessionsLeft.numberOfSessionsLeftIs = 1;
                console.log(numOfSessionsLeft.numberOfSessionsLeftIs)

            }

        })

})

}

however, what is happening is that this line: rows[i]["NUM_OF_SESSIONS_PER_MONTH"] = (parseInt(rows[i]["NUM_OF_SESSIONS_PER_MONTH"]) - numOfSessionsLeft.numberOfSessionsLeftIs) is actually assigning 0 to rows[i]["NUM_OF_SESSIONS_PER_MONTH"] because that variable assignment is happening before the function call for getNumOfSessionForTrainerClientList finishes. So it is happening out of sync. I am not sure how to fix this, i have run into issues with async await before, but nothing like this. would appreciate the help.

Answer 1

For loop is synchronous in nature and hence doesn't wait for your method getNumOfSessionForTrainerClientList to return to continue the iteration.

The right way to do is:

1. Put your code inside an async block and use await() while calling getNumOfSessionForTrainerClientList call.
2. Second, your function getNumOfSessionForTrainerClientList should return a promise.

Sample:

async function functionA() {
    var select = "select * from USERS WHERE ASSIGNED_TRAINER = ?"
    mysqlconn.connect(function(err) {
        if (err) {
            console.error('Database connection failed: ' + err.stack);
            return;
        }
    
        mysqlconn.query(select, [req.session.username], async function(err, rows) {
            if (err) {
                console.log(err);
            } else {
                
                let numOfSessionsLeft = {
                    numberOfSessionsLeftIs: 0
                }
                for (var i = 0; i < rows.length; i++) {
                    await getNumOfSessionForTrainerClientList(req, res, rows[i], numOfSessionsLeft)
                    rows[i]["NUM_OF_SESSIONS_PER_MONTH"] = (parseInt(rows[i]["NUM_OF_SESSIONS_PER_MONTH"]) - numOfSessionsLeft.numberOfSessionsLeftIs)
                    console.log(numOfSessionsLeft)
                }
            }
            mysqlconn.end();
        })
    })
}

And your function getNumOfSessionForTrainerClientList should look like:

async function getNumOfSessionForTrainerClientList(req, res, rows, numOfSessionsLeft) {

    return new Promise((resolve, reject) => {

        var getNumOfSessionForTrainerClientList = "select * from SCHEDULE WHERE CLIENT_USERNAME = ? AND ASSIGNED_TRAINER = ?"
        mysqlconn.connect(async function(err) {
            if (err) {
                console.error('Database connection failed: ' + err.stack);
                return reject(err);
            }
                mysqlconn.query(getNumOfSessionForTrainerClientList, [rows["USERNAME"], req.session.username], async function(err, sessionData) {
                    if (err) {
                        console.log(err);
                        return reject(err);
                    } else {
                        numOfSessionsLeft.numberOfSessionsLeftIs = 1;
                        console.log(numOfSessionsLeft.numberOfSessionsLeftIs)
                        return resolve();

                    }
                })

        })
    })

}

And then call functionA() with whatever params you need to.

Answer 2

You call the function that includes async codes therefore getNumOfSessionForTrainerClientList doesn't depend the codes above (starts with(rows[i]["NUM_OF and console) what you should do is

1. Make the function async by returning prom

2. execute it with await keyword

   async function trainerClientList(){ return new Promise((resolve,reject)=>{ var getNumOfSessionForTrainerClientList = "select * from SCHEDULE WHERE CLIENT_USERNAME =? AND ASSIGNED_TRAINER =?" mysqlconn.connect(async function(err) { if (err) { console.error('Database connection failed: ' + err.stack); resolve() } mysqlconn.query(getNumOfSessionForTrainerClientList, [rows["USERNAME"], req.session.username], async function(err, sessionData) { if (err) { console.log(err); } else { numOfSessionsLeft.numberOfSessionsLeftIs = 1; console.log(numOfSessionsLeft.numberOfSessionsLeftIs) } }) }) resolve() })

   }

   for (var i = 0; i < rows.length; i++) { await trainerClientList() rows[i]["NUM_OF_SESSIONS_PER_MONTH"] = (parseInt(rows[i]["NUM_OF_SESSIONS_PER_MONTH"]) - numOfSessionsLeft.numberOfSessionsLeftIs) console.log(numOfSessionsLeft) }