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.

CodePudding user response：

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 callinggetNumOfSessionForTrainerClientList 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  ) {
                    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.

CodePudding user response：

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) }