方法执行一次js

 var isFirst = true;
    $(function () {
        //一级
        $("#City").change(function () {
            var url = "/TWAddress/GetTownship/?selected=" + $("#City").val() + "";
            $.getJSON(url, function (data) {
                if (isFirst == true) {
                    $("#City option:first").remove();
                    isFirst = false;
                }