tried the solution https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call but I could not make it work

Question

When loading data directly from geojson file and using leaflet layer control have no issue. Below code works perfectly fine:

var currentSchools = new L.GeoJSON.AJAX('data/schoolWGS84.geojson');
        currentSchools.addTo(mymap);
        
        var futureSchools = new L.GeoJSON.AJAX('data/futureschoolWGS84.geojson');
        futureSchools.addTo(mymap);
        
        var baseMaps = {
            "OpenStreetMap": osm,
            "Stadia OSM": stadia
            };

        var overlays = {
            "Curent Schools": currentSchools,
            "Future Schools": futureSchools
            
        };
        
        var layerControl = L.control.layers(baseMaps, overlays).addTo(mymap);

However, when one of the geojson layer is added using jQuery.ajax, the layer is added successfully on the map but could not get the layer onto layer control: throws error at L.control.layer: Uncaught TypeError: Cannot read properties of undefined (reading 'setZIndex') The code is as below:

var currentSchools;
$.ajax({url:'LoadDataFromSQLServer.php', success:function(response){
            currentSchools = L.geoJSON(JSON.parse(response));             
            currentSchools.addTo(mymap);
}});

I tried using callback function as stated in the thread here. The result from the callback function can only be used within the function. I need to pass the result to external variable (external to callback function e.g. rrr). One of the many codes I tried is as below:

> function currentSchools() {
>         var result;
> 
>         $.ajax({
>             url: 'LoadDataFromSQLServer.php',
>             success: function(response) {
>                 myCallback(response);
>             }
>         });
> 
>         return result;
>     }
> 
>     function myCallback(response) {
>         L.geoJSON(JSON.parse(response)).addTo(mymap);
>     }        
>     var rrr = currentSchools();
>     alert(rrr);

I am new to this, so any help to make my code work would be appreciated.