express.js loop execution timing issue

Question

express.js loop execution timing issue

My current code is not waiting for the loop to finish before printing {"personalchats":[]}

I need to send after the for loop has completed. How can I fix this issue?

connection.query("SELECT * FROM personalchat WHERE user1ID = ? OR user2ID = ?", [userID, userID], function(err, row, fields) {
  if (err)
    console.log(err);

  else {
    personalchats = Array();

    for (var i = 0; i < row.length; i++) {
      if (row[i].user1ID != userID)
        user2ID = row[i].user1ID;
      else
        user2ID = row[i].user2ID;

      connection.query("SELECT * FROM users WHERE userID = ?", [user2ID], function(err2, row2, fields2) {
        if (err)
          console.log(err2);

        else {


          personalchats.push({
            'success': true,
            'userID': row2[0].userID,
            'name': row2[0].name,
            'surname': row2[0].surname,
            'email': row2[0].email
          });
          console.log(personalchats);
        }


      });

    }

    res.send({
      "personalchats": personalchats
    });
  }

});

javascript express

Answer 1

Answer №1

Here is the suggested code snippet:

connection.query("SELECT * FROM privatechats WHERE user1ID = ? OR user2ID = ?", [userID, userID], function(err, row, fields) {
  if (err)
    console.log(err);

  else {
      privatechats = [];
      var userIDs = [];

      for (var i = 0; i < row.length; i++) {
        if (row[i].user1ID != userID)
          userIDs.push(row[i].user1ID);
        else
          userIDs.push(row[i].user2ID);
      }

      connection.query("SELECT * FROM users WHERE userID in (" + userIDs.join(',') + ")", function(err2, row2, fields2) {
        if (err)
          console.log(err2);

        else {

          row2.forEach(r =>
            privatechats.push({
              'success': true,
              'userID': r.userID,
              'name': r.name,
              'surname': r.surname,
              'email': r.email
            });
          })
          console.log(privatechats);
          res.send({
            "privatechats": privatechats
          });
        }
      });
    }
  }
});

Answer 2

Here is the suggested code snippet:

connection.query("SELECT * FROM privatechats WHERE user1ID = ? OR user2ID = ?", [userID, userID], function(err, row, fields) {
  if (err)
    console.log(err);

  else {
      privatechats = [];
      var userIDs = [];

      for (var i = 0; i < row.length; i++) {
        if (row[i].user1ID != userID)
          userIDs.push(row[i].user1ID);
        else
          userIDs.push(row[i].user2ID);
      }

      connection.query("SELECT * FROM users WHERE userID in (" + userIDs.join(',') + ")", function(err2, row2, fields2) {
        if (err)
          console.log(err2);

        else {

          row2.forEach(r =>
            privatechats.push({
              'success': true,
              'userID': r.userID,
              'name': r.name,
              'surname': r.surname,
              'email': r.email
            });
          })
          console.log(privatechats);
          res.send({
            "privatechats": privatechats
          });
        }
      });
    }
  }
});

Answer 3

Answer №2

You are facing synchronization issues. The database calls you make require time to finish, and it's possible that you execute the callback before receiving a response when trying to transmit that data. One solution could be utilizing 'Promise.all' or creating a function that runs after each item is added to the array in the callback, checking if all responses have been received before sending the express response.

Answer 4

You are facing synchronization issues. The database calls you make require time to finish, and it's possible that you execute the callback before receiving a response when trying to transmit that data. One solution could be utilizing 'Promise.all' or creating a function that runs after each item is added to the array in the callback, checking if all responses have been received before sending the express response.

Answer 5

Answer №3

Your code operates asynchronously. The connection.query function first retrieves results by executing your callback function function(err, row, fields) { }.

Similarly, the second query also returns results by invoking the callback function

function(err2, row2, fields2) { }

. You can access the values within this callback function and log them using console.log(personalchats). Remember to send the result within this callback function as well.

personalchats.push({
    'success': true,
    'userID': row2[0].userID,
    'name': row2[0].name,
    'surname': row2[0].surname,
    'email': row2[0].email
});
console.log(personalchats);
res.send({
    "personalchats": personalchats
});

It is crucial to note that the values of row2 will not be accessible outside the callback function scope.

Ensure you send the result after a successful connection.query operation. Here's a complete example:

connection.query("SELECT * FROM personalchat WHERE user1ID = ? OR user2ID = ?", [userID, userID], function(err, row, fields) {
    if (err)
        console.log(err);
    else {
        personalchats = Array();
        for (var i = 0; i < row.length; i++) {
            if (row[i].user1ID != userID)
                user2ID = row[i].user1ID;
            else
                user2ID = row[i].user2ID;

            connection.query("SELECT * FROM users WHERE userID = ?", [user2ID], function(err2, row2, fields2) {
                if (err)
                    console.log(err2);
                else {
                    personalchats.push({
                        'success': true,
                        'userID': row2[0].userID,
                        'name': row2[0].name,
                        'surname': row2[0].surname,
                        'email': row2[0].email
                    });
                    console.log(personalchats);
                    res.send({
                        "personalchats": personalchats
                    });
                }
            });
        }
    }
});

Answer 6

Your code operates asynchronously. The connection.query function first retrieves results by executing your callback function function(err, row, fields) { }.

Similarly, the second query also returns results by invoking the callback function

function(err2, row2, fields2) { }

. You can access the values within this callback function and log them using console.log(personalchats). Remember to send the result within this callback function as well.

personalchats.push({
    'success': true,
    'userID': row2[0].userID,
    'name': row2[0].name,
    'surname': row2[0].surname,
    'email': row2[0].email
});
console.log(personalchats);
res.send({
    "personalchats": personalchats
});

It is crucial to note that the values of row2 will not be accessible outside the callback function scope.

Ensure you send the result after a successful connection.query operation. Here's a complete example:

connection.query("SELECT * FROM personalchat WHERE user1ID = ? OR user2ID = ?", [userID, userID], function(err, row, fields) {
    if (err)
        console.log(err);
    else {
        personalchats = Array();
        for (var i = 0; i < row.length; i++) {
            if (row[i].user1ID != userID)
                user2ID = row[i].user1ID;
            else
                user2ID = row[i].user2ID;

            connection.query("SELECT * FROM users WHERE userID = ?", [user2ID], function(err2, row2, fields2) {
                if (err)
                    console.log(err2);
                else {
                    personalchats.push({
                        'success': true,
                        'userID': row2[0].userID,
                        'name': row2[0].name,
                        'surname': row2[0].surname,
                        'email': row2[0].email
                    });
                    console.log(personalchats);
                    res.send({
                        "personalchats": personalchats
                    });
                }
            });
        }
    }
});

express.js loop execution timing issue

Answer №1

Answer №2

Answer №3

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