some_ref1.once('value', function(snapshot){
   // x is going to be a list
   var x = snapshot.val();
   // looping through x
   for (var i = 0; i < x.length; i++){
       // do something and get another list y

       // loop through y
       for (var j=0; j<y.length; j++){
           // go to firebase again
           some_ref2 = firebase.database().ref("some_other_location");
           some_other_location.once('value', function(snapshot)){
               var final_output = snapshot.val();
               console.log(final_output);
           }
       }
   }
})