form表单提交前进行ajax验证

function checkDate() {          
    var result = true;         //定义返回标识
     $.ajax({
             async: false,    //设置为同步
             type: "post",
             url: "/Mall/Channel/CheckChannelDate",
             data: { startDate: $("#PayStartTime").val() },
             success: function (data) {
                      if (data != ‘ok‘) {
                          alert(data);
                          result = false
                          }                                    
                     }
                });
                return result; //在ajax方法外层返回true or false
        }