converting WebService response to JSON

I want to convert indexed Object response to proper JSON. I’ve already tried using JSON.stringify but no success. Here is the response:

enter image description here

<html>
<head>
    <meta charset="utf-8" />
    <title></title>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
    <script>
        var catsorted;

        $(document).ready(function () {
            getCategoriesSorted();

            var recordset = catsorted;
            //console.log(recordset);
            var rset = GenerateNestedJSON(recordset);
            var arr = rset;
            console.log(arr);
        });

        function GenerateNestedJSON(recordset) {
            var array = recordset;

                async function getNestedChildren(arr, parent) {
                    var children = [];
                    for (var i = 0; i < arr.length; ++i) {
                        if (arr[i].parentid == parent) {
                            var grandChildren = await getNestedChildren(arr, arr[i].id);
                            if (grandChildren.length) {
                                arr[i].children = grandChildren;
                            }
                            children.push(arr[i]);
                        }
                    }
                    return children;
                }
                nest = getNestedChildren(array, 0);
            return nest;
        }

        function getCategoriesSorted() {

            $.ajax({

                type: 'POST',
                url: '/CategorieService.asmx/GetCategoriesSorted',
                data: "{}",
                contentType: 'application/json; charset=utf-8',
                dataType: 'json',
                async: false,
                success: function (response) {

                    catsorted = response.d;

                },
                error: function (xhr) {
                    var err = eval("(" + xhr.responseText + ")");
                    alert(err.Message);
                }
            });
        }
    </script>
</head>
<body>

</body>
</html>

getCategoriesSorted() calls a C# webservice. This webservice returns id, parentid and catname from a table. GenerateNestedJSON(recordset) nests children for each category.