how to get the push id value in firebase web

Firebase Database

I want to get the value(111122223333) of the push id in AllocEmp. im using the code:

    var fb = 
    firebase.database().ref().child('UserComplaints').child('AllocEmp');
    fb.once('value', snap => {
        alert(snap.val);
    });

this code returns null. please help me