Xtykutl

I want fetch data from firebase after that I want to execute another function. Second function have to wait until first one is complete .

this.oAngularFireDatabase.database.ref('Users').orderByKey()

            .on('value', snapshot => {

              if (snapshot.hasChildren()) {

                snapshot.forEach(innerSnap => {

                  if (innerSnap.hasChild(user.uid)) {

                    //User role key

                    this.loggedInUserUserRoleKey = innerSnap.key;

                    //User id

                    this.loggedInUserId = user.uid;



                    //User name

                    this.loggedInUserName = innerSnap.child(user.uid).child("user_name").val();



                    if (innerSnap.child(user.uid).hasChild("user_image")) {

                      //User Image

                      this.loggedInUserImage = innerSnap.child(user.uid).child("user_image").val();

                    }

                    return false;

                  }

                })

              }

            }) 

I can't call then function after on it gives me an error.
In my above code, I want call another function after all data are fetch from firebase.

The Firebase on() method can fire multiple times: once when it initially loads the data, and again whenever the data changes. Since a promise (the thing you call then() on) can only resolve once, on() can't return a promise.

There are two options here:

1. 
   

   You want to only load the data once.

   
   
   

   If this is the case, you should use Firebase's once() method, which does return a promise.

   
   
   

   this.oAngularFireDatabase.database.ref('Users').orderByKey()
   
       .once('value').then(snapshot => {
   
         if (snapshot.hasChildren()) {
   
           snapshot.forEach(innerSnap => {
   
             if (innerSnap.hasChild(user.uid)) {
   
               //User role key
   
               this.loggedInUserUserRoleKey = innerSnap.key;
   
               //User id
   
               this.loggedInUserId = user.uid;
   
   
   
               //User name
   
               this.loggedInUserName = innerSnap.child(user.uid).child("user_name").val();
   
   
   
               if (innerSnap.child(user.uid).hasChild("user_image")) {
   
                 //User Image
   
                 this.loggedInUserImage = innerSnap.child(user.uid).child("user_image").val();
   
               }
   
               return false;
   
             }
   
           })
   
         }
   
       }).then(value => {
   
           // TODO: perform subsequent action on boolean value
   
       })
   
   

   
   


2. 
   

   You want to listen for changes on the data too.

   
   
   

   If this is the case, you should put the subsequent action you want to take into the on() callback:

   
   
   

   this.oAngularFireDatabase.database.ref('Users').orderByKey()
   
       .on('value', snapshot => {
   
         if (snapshot.hasChildren()) {
   
           snapshot.forEach(innerSnap => {
   
             if (innerSnap.hasChild(user.uid)) {
   
               //User role key
   
               this.loggedInUserUserRoleKey = innerSnap.key;
   
               //User id
   
               this.loggedInUserId = user.uid;
   
   
   
               //User name
   
               this.loggedInUserName = innerSnap.child(user.uid).child("user_name").val();
   
   
   
               if (innerSnap.child(user.uid).hasChild("user_image")) {
   
                 //User Image
   
                 this.loggedInUserImage = innerSnap.child(user.uid).child("user_image").val();
   
               }
   
             }
   
           })
   
           // TODO: perform subsequent action on data
   
         }
   
       }) 
   
   

   
   

Note that both of these operations look pretty expensive for what they're trying to accomplish: scanning a JSON tree for a specific value is an anti-pattern in Firebase, and typically means you should modify/augment your JSON to allow a direct lookup or query.

For example, I suspect you now have a structure like /Users/$randomkey/$uid: { ..user data... }. For better performance, consider storing the user data directly under their UID: /Users/$uid: { ..user data... }. This removes the need for a query, and allows you to directly load the data for a user from this.oAngularFireDatabase.database.ref('Users').child(user.uid).