JQuery代码小问题

Question

$(document).ready(function () {
            var ID = $("#txtID").val();
            var url = $.format("../../Controls/InfoHandler.ashx?param={0}", "getinfo");
            $.getJSON(url, function (data, status) {
                for (var i = 0; i < data.length; i++) {
                    $("#dsize").append(
                        "<div id="+data[i].InvID+">" + data[i].Size + "</div>"
                        );
                }
            });
            var $sizedivs = $("#dsize>div");//.children("div");
            var abc = $sizedivs[0];
            alert(abc.id);
        });

这段代码直接执行为什么不弹出窗口呢   在var $sizedivs = $("#dsize>div");//.children("div");处设个断点再执行就可以弹出窗口