How to find a element in a form?

Hello!

I got a form with several elements. Some are select-tags and they for eg can be looking like:
<select name="telefon[0].telefonTyp" size="1">
<option value="">--</option>
<option value="1">1</option>
<option value="2">2</option>
</select>

<select name="telefon[1].telefonTyp" size="1">
<option value="">--</option>
<option value="1">1</option>
<option value="2">2</option>
</select>

Hope you can see that the only thing that differ is the names:
telefon[0].telefonTyp
telefon[1].telefonTyp

I now need to set focus on one or the other in different situations.

I then need to loop through the list of select elements and find the one to set focus to.

I have tried this to first of all find the elements with names that starts with "telefon":

function f_setFocus()
{ 
  var bFound = false;

    // for each form
    for (f=0; f < document.forms.length; f++)
    {
          // for each element in each form
          for(i=0; i < document.forms[f].length; i++)
            {
                if (document.forms[f][i].type == 'select-one')
                {
              //Kolla att det är en telefon
              alert(document.forms[f][i].name);
              var selectname = document.forms[f][i].name;
              if (selectname.startsWith('telefon'))
              {
                // set the focus to it
                document.forms[f][i].focus();
                var bFound = true;
              }
                }
                         
                // if found in this element, stop looking
          if (bFound == true)
          {
              break;
          }
            }

      // if found in this element, stop looking
      if (bFound == true)
      {
        break;
      }
    }
}

But I only get the error that the object does not support the attribute or method.

I guess it menas this method call:
selectname.startsWith('telefon'))

Do any one know how to solve this, to find out if a .name starts with a certain string?

Or do you guys see any other better solution to the whole problem?

Best regards
Fredrik