Question

I have a Firebase with a users reference, which has a field user_id.
([Firebase]/[app]/users/user_id)

I insert a user & assign a user_id. However, when I read the value of user_id from userRef (a reference to users), it does not read it the first time. Subsequent reads work perfectly fine.

Using the debugger, I can see that when the user is created, a user_id is assigned. It seems like my first call refreshes the Firebase reference, so that subsequent calls are now seeing the udpated Firebase (I don’t think that is what it is – that is just how it appears).

This is my code to read the value of user_id:

var userID = 0;
userRef.on('value', function(snapshot) {
    userID = snapshot.val().user_id;
});
alert(userID);

The first time, the alert shows 0. Every time after that, it shows the correct value.

To make the problem even stranger, I also have a games reference with a game_id, created in the same way as user. But my read on game_id (in the exact same way & at the same time) works every time.

Any ideas?

Answer 1

The issue here is that .on() doesn’t (in general) trigger your callback function immediately. In particular, the first time you do a .on() at a location, Firebase has to go ask the server for the current value, wait for the response, and THEN call your callback. And it does this all asynchronously.

The way your code is currently written, “alert(userID);” is being run before your callback code (“userID = snapshot.val().user_id;”) so it always reports 0 the first time. The simple fix is to move the alert() inside your callback:

var userID = 0;
userRef.on('value', function(snapshot) {
    userID = snapshot.val().user_id;
    alert(userID);
});