Why Does My Firebase Reference Become Undefined Outside the `once()` Function?

Why Does Firebase Lose Reference outside the once() Function?

Firebase utilizes asynchronous processing, which means retrieving data from the database is not executed sequentially. As a result, attempting to access the user list immediately after attaching a listener in the userService.getUsers function will not yield the desired output.

The code snippet provided:

.service('userService', [function() {
    this.getUsers = function() {
        var users;
        var userList;
        var ref = firebase.database().ref('/users/');
        ref.once('value').then(function(snapshot) {
            users = snapshot.val();
            for(var key in users) {
                users[key].id = key;
                // do some other stuff
            }
            console.log(users); // outputs all users
        }).then(function(){
            userList = users; 
            console.log(userList); // outputs all users
        },(function(error){
            alert('error:  ' + error);
        });
        console.log(userList); // outputs 'undefined'
    }
}]);

Demonstrates this asynchronous behavior. The console output would be:

before attaching listener
after attaching listener
got value

To resolve this issue, you have three options:

1. Use the User List in the Callback:

Move all the necessary code that operates on the user list into the callback:

ref.once('value', function(snapshot) {
        users = snapshot.val();
        for(var key in users) {
            users[key].id = key;
        }
        console.log(users); // outputs all users
    })

2. Return a Promise:

Return a promise from userService.getUsers and chain subsequent operations to it:

this.getUsers = function() {
    var ref = firebase.database().ref('/users/');
    return ref.once('value').then(function(snapshot) {
        users = snapshot.val();
        for(var key in users) {
            users[key].id = key;
            // do some other stuff
        }
        return(users);
    }).catch(function(error){
        alert('error:  ' + error);
    });
}

You can then use the promise to continue processing once the data is loaded:

userService.getUsers().then(function(userList) {
    console.log(userList);
})

3. Use Async and Await (Requires ES2017 Support):

Mark the getUsers function as async and use the await keyword to pause execution until the promise is resolved:

this.getUsers = async function() {
    var ref = firebase.database().ref('/users/');
    const snapshot = await ref.once('value');
    const users = snapshot.val();
    for(var key in users) {
        users[key].id = key;
        // do some other stuff
    }
    return users;
}

You can now use the function like:

async function getAndLogUsers() {
    const userList = await userService.getUsers();
    console.log(userList);
}

By understanding asynchronous execution in Firebase, you can effectively access data from your database and avoid potential undefined references.

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