javascript object returning null

Question

if i log the snapshot.val i see there id data. but when i log a it displays null.

            var a = null;
            var b = null;
            firebase.database().ref('userInfo/' + senederId).on('value', snapshot => {
                console.log(snapshot.val()); // returning data
                a = {
                    usersInfo: snapshot.val()
                }        
            });
            console.log(a); // its null in here