How to retrieve value from field with type user or group?

When we retrieving list items with field "Person or Group" via client object model

The values that return from column "Person or Group" is "[Object Object]".

To retrieve specific field is we use the code below:

listItem.get_item ('Field name');

//My field name  is "User name" so I use like this

var  UserName=listItem.get_item('User_x0020_Name');

var itemsResult="<tr><td>"+UserName+"</td>"+"<td>"+Questionsright+"</td>"+"<td>"+Timetaken+"</td></tr>";
  $(itemsResult).appendTo("#mytable");

Result:

Function to be append is:

var  UserName=listItem.get_item('User_x0020_Name').get_lookupValue();

If field is multiple selections:

 var  UserName=listItem.get_item('User_x0020_Name')[0].get_lookupValue();

And it's will retrieve the user name