Why does the data returned by ajax prompt object?

After running, why does the prompt result show [object Object], [object Object] instead of specific data?
This result should be like this [{title: "3333333333333"}, {title: "3333333333333"}]
Then I tried it againalert(obj.title) It prompts undefined. Why is this?

<code>$res=$xiao->field("title")->select();
$this->ajaxReturn($res);


ajax("{:U('zhuye/zhuye')}",oV1,function(str){
   var obj = eval("("+str+")");
   alert(obj);
});</code>