高手进，text光标定位问题

以下是测试代码，为什么点击textarea的时候正常，而text却提示“参数无效”呢？求大家帮看看，能不能改成对两者都有效的函数<HTML><HEAD><TITLE>定位</TITLE>
<META content="text/html; charset=gb2312" http-equiv=Content-Type>
<SCRIPT language=JScript>
<!--
 function savePos(textBox){
if(document.selection){
            var range = document.selection.createRange();
            if(range.parentElement().id == textBox.id){
                var range_all = document.body.createTextRange();
                range_all.moveToElementText(textBox);
                for (start=0; range_all.compareEndPoints("StartToStart", range) < 0; start++)
                    range_all.moveStart('character', 1);
for (var i = 0; i <= start; i ++){
if (textBox.value.charAt(i) == '\n')
start++;
}
                 var range_all = document.body.createTextRange();
                 range_all.moveToElementText(textBox);
                 for (end = 0; range_all.compareEndPoints('StartToEnd', range) < 0; end ++)
                     range_all.moveStart('character', 1);
                     for (var i = 0; i <= end; i ++){
                         if (textBox.value.charAt(i) == '\n')
                             end ++;
                     }
                }
            }
    }
//-->
</script>
  
<META content="MSHTML 5.00.2920.0" name=GENERATOR></HEAD>
<BODY oncontextmenu=window.event.returnValue=false>
<form name="myform" action="">
<textarea name="area" cols="35" rows="15"
 onmouseup="savePos(this)"
 onKeydown="savePos(this)" 
                              onKeyup="savePos(this)" 
                              onmousedown="savePos(this)"
                              onfocus="savePos(this)"></textarea>
    <input type="text" name="textfield" size="35" onmouseup="savePos(this)"
onKeydown="savePos(this)" 
                              onKeyup="savePos(this)" 
                              onmousedown="savePos(this)"
                              onfocus="savePos(this)">   
</form>
</BODY>
</HTML>