firebase check if child exists
// Assume we have the following data in the Database:
{
"name": {
"first": "Ada",
"last": "Lovelace"
}
}
// Test for the existence of certain keys within a DataSnapshot
var ref = firebase.database().ref("users/ada");
ref.once("value")
.then(function(snapshot) {
var a = snapshot.exists(); // true
var b = snapshot.child("name").exists(); // true
var c = snapshot.child("name/first").exists(); // true
var d = snapshot.child("name/middle").exists(); // false
});