if(document.getElementById("target_obj_id")){

} else {

}

1.
 var target_obj = jQuery('#target_obj_id');
  if (target_obj.length > 0) { //如果大于0 标识 id 为target_obj_id的对象存在，否则不存在

  } else {

  }

2、
 if (target_obj[0]) {

  } else {

 }

3. $.isEmptyObject({}) // true
  $.isEmptyObject({ foo: "bar" }) // false

//判断并遍历每个对象
if($("#t_specialsearch select").length <= 0){
          alert("对象为空");
}else{
    //console.log($("#t_specialsearch select")[0]);
    $("#t_specialsearch select").each(function(){
      //alert($(this));
      console.log($(this));
    });

    for(var i in $("#t_specialsearch select")){
      console.log($("#t_specialsearch select")[i]);
    }
}