JavaScript 实现计算器退格键删除效果

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style type="text/css">
.myinput{
width:70px; height:30px;
}
.tf{
width:220px; height:30px;
margin-bottom:5px; font-size:26px;
font-family:Tahoma, Geneva, sans-serif;
color:#fff; border:2px solid #999;
background:#000; text-align:right;
}
</style>
<script language="javascript" type="text/javascript">
window.onload = function()
{
 var tf = $("tf");
 for( var i=0;i<11;i++ ){
 $("btn"+i).onclick = function(){
  if(this.value == "." && tf.value == "") return false;
  if(this.value == "." && tf.value.indexOf(".") != -1) return false;
  if(tf.value == "0"){
  if(this.value == "."){
   tf.value += this.value;
  }
  }else{
  tf.value += this.value; 
  }
 }
 }
 $("back").onclick = function(){
 tf.value = tf.value.replace(/.$/,'')
 }
}
function $(id){return document.getElementById(id);}
</script>
</head>
<body>
<input class="tf" name="textfield" type="text" id="tf" size="30" />
<br />
<input class="myinput" type="submit" name="button" id="btn0" value="0" />
<input class="myinput" type="submit" name="button" id="btn1" value="1" />
<input class="myinput" type="submit" name="button" id="btn2" value="2" />
<br />
<input class="myinput" type="submit" name="button" id="btn3" value="3" />
<input class="myinput" type="submit" name="button" id="btn4" value="4" />
<input class="myinput" type="submit" name="button" id="btn5" value="5" />
<br />
<input class="myinput" type="submit" name="button" id="btn6" value="6" />
<input class="myinput" type="submit" name="button" id="btn7" value="7" />
<input class="myinput" type="submit" name="button" id="btn8" value="8" />
<br />
<input class="myinput" type="submit" name="button" id="btn9" value="9" />
<input class="myinput" type="submit" name="button" id="btn10" value="." />
<input class="myinput" type="submit" name="button" id="back" value="退格" />
</body>
</html>