var cars = $('#mypage').data('carsdata');

if (cars == null) {
    var clientId = $("input[name='ClientId']").val();
    worksheet.ajax.execute('GET','/Cars/GetData/', { clientId: clientId }, function (res) {
        if (res == "") {
            return;
        }

        cars = res.Owners.filter(function (e) {
            if (e.Email) {
                return true;
            }
            return false;
        }).map(function (e) {
            return { key: e.Id, label: e.Email };
        });

        console.log("1" +cars.length); // cars lenght is 3
 });


console.log("2" +cars.length); // cars is null

console.log("1" +cars.length);

console.log("2" +cars.length);