Use created attribute with javascript in select list

im trying to access an attribute that i created in a select list.

<script language="JavaScript">
 function updateUrl()
     {
         var newUrl=document.getElementById('test').car;
         alert(newUrl);
     }
 </script>
 
 <input type="text" id="test" car="red" value="create Attribute test" size="40"/> 
 <input type="button" value="submit" onclick="updateUrl();"> 
 

it keep giving me undefined. how do i get the string red from attribute car?

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