Get the number of concurrent users in Firebase

Add a listener to .info/connected and add/remove childs using push()/remove().

Example:

javascript
onValue(ref(db, '.info/connected'), snapshot => {
    
    if (snapshot.val()) {

        let key = push(ref(db, 'concurrent'), {connected: true})

        onDisconnect(key).remove()

    }
}) 

When the page loads, the listener will push a new object into the concurrent branch.

JSON
{
    "concurrent": {
        "AsZjsk" : {
            "connected": true
        }
    }
}

When the user closes the website or is inactive, onDisconnect() function will remove the branch automatically.

The total number of online users is the number of childs on the concurrent branch.

Example:

javascript
onValue(ref(db, 'concurrent'), snapshot => {

    if(snapshot.val()){

        let concurrent = snapshot.val()

        let total = Oject.keys(concurrent).length

        console.log(`There are ${total} concurrent (online) users!`)

    }

})