A Guide to Executing Asynchronous Function Calls in a For Loop using JavaScript

Question

A Guide to Executing Asynchronous Function Calls in a For Loop using JavaScript

Experiencing issues with asynchronous calls within a for loop in my code. The loop progresses before the async call is completed, causing unexpected behavior. As someone new to this language, I'm trying to understand callbacks and other related concepts. I've experimented with self-calling functions, promises, and timeouts, but haven't achieved the desired flow.

The goal is to ensure that the firebase call finishes before adding the profile object to the messages array.

// Retrieving chats associated with the user
Chat.allChatsByUser(uid).$loaded()
 .then(function(data) {     
   for (var i = 0; i < data.length; i++) {
   // Utilizing a self-calling function to handle async callbacks
   // Ensuring the async call is executed during each iteration of the loop
   (function(i) {
      var item = data[i];
      // Fetching profile information for the matched user
      Auth.getProfile(item.$id).$loaded()
      .then(function(profile) { 
         // Handling success case
         if (typeof profile === 'object') { 
              if(chat.keyOrder == 'true') {

              // Retrieving last chat from firebase
              // DESIRE THIS TO COMPLETE BEFORE PROCEEDING                       
              ref.child('chatting').child('messages').child(chat.chatId).on("value", function(data) {
                 profile.lastChat = data.child('lastMsg').val();
              });

             // Adding chatting user's profile to the array
             chat.messages.push(profile);    

         } else {
               // Invalid response
               return $q.reject(profile);
         }
 }, function(profile) {
   // Promise rejected
   console.log('error', error);});
 })(i);
}

Any help or guidance would be greatly appreciated.

Regards, Noel

javascript angularjs ionic-framework firebase firebase-realtime-database

Answer 1

Answer №1

It seems like your requirements are two-fold: first, you need the loop to wait until the async call is finished before continuing. While there are sophisticated methods in ES6 to achieve this, since you're not utilizing ES6, I've implemented a workaround using a while loop. Secondly, you want to make sure the firebase call completes before adding the profile object to the messages array. To accomplish this, simply place the push call inside the value event handler as suggested in the initial comment.

// Retrieves all chats for the specified profile
Chat.allChatsByUser(uid).$loaded()
    .then(function(data) {
            var i = 0;
            var inProgress = false;
            while (i < data.length) {
                // A self-invoking function for handling async callbacks
                // Ensures that each iteration of the loop runs the async call

                // Wait for the last iteration to complete
                while(inProgress);

                // The function is starting now
                inProgess = true;
                (function(i) {
                        // Increment i within this scope
                        var item = data[i++];
                        // Function to sort users and chats from newest to oldest
                        // Matched user with item.$id = uid
                        Auth.getProfile(item.$id).$loaded()
                            .then(function(profile) {
                                    // Success case
                                    if (typeof profile === 'object') {
                                        if (chat.keyOrder == 'true') {

                                            // Retrieve the last chat from firebase
                                            // WAIT FOR THIS TO FINISH BEFORE PROCEEDING                       
                                            ref.child('chatting').child('messages').child(chat.chatId).on("value", function(data) {
                                                profile.lastChat = data.child('lastMsg').val();

                                                // Wait for the event
                                                // Adds chatting user's profile to the array
                                                chat.messages.push(profile);

                                                // Allow the next iteration to continue
                                                inProgess = false;
                                            });
                                        } else {
                                            // Handle invalid response
                                            return $q.reject(profile);
                                        }
                                    },
                                    function(profile) {
                                        // Promise rejection - End script
                                        return console.log('error', error);
                                    });
                                // Closing over i argument
                            })(i);
                }

Answer 2

It seems like your requirements are two-fold: first, you need the loop to wait until the async call is finished before continuing. While there are sophisticated methods in ES6 to achieve this, since you're not utilizing ES6, I've implemented a workaround using a while loop. Secondly, you want to make sure the firebase call completes before adding the profile object to the messages array. To accomplish this, simply place the push call inside the value event handler as suggested in the initial comment.

// Retrieves all chats for the specified profile
Chat.allChatsByUser(uid).$loaded()
    .then(function(data) {
            var i = 0;
            var inProgress = false;
            while (i < data.length) {
                // A self-invoking function for handling async callbacks
                // Ensures that each iteration of the loop runs the async call

                // Wait for the last iteration to complete
                while(inProgress);

                // The function is starting now
                inProgess = true;
                (function(i) {
                        // Increment i within this scope
                        var item = data[i++];
                        // Function to sort users and chats from newest to oldest
                        // Matched user with item.$id = uid
                        Auth.getProfile(item.$id).$loaded()
                            .then(function(profile) {
                                    // Success case
                                    if (typeof profile === 'object') {
                                        if (chat.keyOrder == 'true') {

                                            // Retrieve the last chat from firebase
                                            // WAIT FOR THIS TO FINISH BEFORE PROCEEDING                       
                                            ref.child('chatting').child('messages').child(chat.chatId).on("value", function(data) {
                                                profile.lastChat = data.child('lastMsg').val();

                                                // Wait for the event
                                                // Adds chatting user's profile to the array
                                                chat.messages.push(profile);

                                                // Allow the next iteration to continue
                                                inProgess = false;
                                            });
                                        } else {
                                            // Handle invalid response
                                            return $q.reject(profile);
                                        }
                                    },
                                    function(profile) {
                                        // Promise rejection - End script
                                        return console.log('error', error);
                                    });
                                // Closing over i argument
                            })(i);
                }

Answer 3

Answer №2

Make sure to add chat.messages.push(profile) inside the value handler.

ref.child('chatting').child('messages').child(chat.chatId)
.on("value", function(data) {
  profile.lastChat = data.child('lastMsg').val();
  // append users profile to the chatting array
  chat.messages.push(profile);
});

Answer 4

Make sure to add chat.messages.push(profile) inside the value handler.

ref.child('chatting').child('messages').child(chat.chatId)
.on("value", function(data) {
  profile.lastChat = data.child('lastMsg').val();
  // append users profile to the chatting array
  chat.messages.push(profile);
});

Answer 5

Answer №3

If you're looking to handle this situation, I believe using the .all method is the way to go. Here's an example of how it can be implemented:

function fetchEvents(city, country) {

    return db.ref('countries').child(country).child(city).once('value').then(function(snapshot) {
        var eventPromises = [];
        snapshot.forEach(function(childSnapshot) {
            var eventId = childSnapshot.key();
            var eventPromise = db.ref('events').child(eventId).once('value').then(function(snap) {
                return snap.val();
            }, function(error) {
                console.error(error);
            });
            eventPromises.push(eventPromise);
        });
        return Promise.all(eventPromises);
    }, function (error) {
        console.error(error);
    }).then(function(data) {
        // Perform actions for each event
    });
}

Answer 6

If you're looking to handle this situation, I believe using the .all method is the way to go. Here's an example of how it can be implemented:

function fetchEvents(city, country) {

    return db.ref('countries').child(country).child(city).once('value').then(function(snapshot) {
        var eventPromises = [];
        snapshot.forEach(function(childSnapshot) {
            var eventId = childSnapshot.key();
            var eventPromise = db.ref('events').child(eventId).once('value').then(function(snap) {
                return snap.val();
            }, function(error) {
                console.error(error);
            });
            eventPromises.push(eventPromise);
        });
        return Promise.all(eventPromises);
    }, function (error) {
        console.error(error);
    }).then(function(data) {
        // Perform actions for each event
    });
}

Answer 7

Answer №4

Utilize the value event handler to add new entries to the array:

ref.child('chatting').child('messages').child(chat.chatId)
 .on("value", function(data) {
    profile.lastChat = data.child('lastMsg').val();

    // Add the chatting user's profile to the messages array
    chat.messages.push(profile); 
});

Answer 8

Utilize the value event handler to add new entries to the array:

ref.child('chatting').child('messages').child(chat.chatId)
 .on("value", function(data) {
    profile.lastChat = data.child('lastMsg').val();

    // Add the chatting user's profile to the messages array
    chat.messages.push(profile); 
});

A Guide to Executing Asynchronous Function Calls in a For Loop using JavaScript

Answer №1

Answer №2

Answer №3

Answer №4

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