jQuery对象未定义但它可以打印得很好，我无法访问其属性

Question

将注意力转移到第二个console.log，即我周围的console.logs工作正常并打印last_pos，其中有两个名为“top”和“top”的int属性，我不断得到“错误：last_pos未定义”剩下”。无论我是否尝试点符号或括号表示，我都会收到错误。这样的矛盾怎么会发生？

  var last_dropped_id = '';
  var last_pos = '';
  $( ".drop-area" ).droppable({

   tolerance: "touch",
drop: function( event, ui ) {

  var valMy = "center";
  var valAt = "top center";
  if(last_dropped_id != undefined){
    var last_pos = $(last_dropped_id).position();

    console.log(last_pos);
    console.log(last_pos['left']);
    console.log("WTF");
    console.log(last_pos);
    //valMy+= "+" + last_pos;
    // valAt+= "+" + last_pos.top;

  }
  var $this = $(this);

  ui.draggable.position({
    my: valMy,
    at: valAt,
    of: $this,
    using: function(pos) {
      $(this).animate(pos, 400, "linear");

   },

  });


    last_dropped_id = '#' + ui.draggable.attr('id');

  console.log(last_dropped_id);
}



});