js简易计算器，怎么在此代码基础上实现退格功能哇
<!DOCTYPE html>
<html lang="en">
<head> <meta charset="UTF-8">
<title>6-4</title>
<script>
function count(sign) {
var result = 0;
var num1 = parseFloat(document.myform.txtNum1.value);
var num2 = parseFloat(document.myform.txtNum2.value);
switch(sign){
case "+":
result=num1+num2;break;
case "-":
result=num1-num2;break;
case "*":
result=num1*num2;break;
case "/":
result=num1/num2;break; }
document.myform.txtresult.value=result; }
</script>
</head>
<body>
<form action="" method="post" name="myform">
<table border="0">
<tr>
<td></td>
<td colspan="3"><H3>简易计算器</H3>
</td>
</tr>
<tr>
<td>第一个数</td>
<td colspan="3"><INPUT name = "txtNum1" type="text" id = "txtNum1" size="25">
</td>
</tr>
<tr>
<td>第二个数</td>
<td colspan="3"><INPUT name = "txtNum2" type="text" id = "txtNum2" size="25">
</td>
</tr>
<tr>
<td>
<INPUT name="addButton" type="button" id="addButton" value="+" onclick="count('+')"> </td>
<td>
<INPUT name="subButton" type="button" id="subButton" value="-" onclick="count('-')"> </td>
<td>
<INPUT name="mulButton" type="button" id="mulButton" value="×" onclick="count('*')"> </td>
<td>
<INPUT name="divButton" type="button" id="divButton" value="÷" onclick="count('/')"> </td>
</tr>
<tr>
<td>计算结果</td>
<td colspan="3"><INPUT name = "txtresult" type="text" id = "txtresult" size="25">
</td>
</tr>
</table>
</form>
</body>
</html>
题主要的代码如下
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>6-4</title>
<script>
function count(sign) {
focusInput = null;
var result = 0;
var num1 = parseFloat(document.myform.txtNum1.value);
var num2 = parseFloat(document.myform.txtNum2.value);
switch (sign) {
case "+":
result = num1 + num2; break;
case "-":
result = num1 - num2; break;
case "*":
result = num1 * num2; break;
case "/":
result = num1 / num2; break;
}
document.myform.txtresult.value = result;
}
var focusInput
function del() {
if (focusInput) {
focusInput.value = focusInput.value.substring(0, focusInput.value.length - 1);
focusInput.focus();
}
}
</script>
</head>
<body>
<form action="" method="post" name="myform">
<table border="0">
<tr>
<td></td>
<td colspan="3">
<H3>简易计算器</H3>
</td>
</tr>
<tr>
<td>第一个数</td>
<td colspan="3">
<INPUT name="txtNum1" type="text" id="txtNum1" size="25" onfocus="focusInput=this">
</td>
</tr>
<tr>
<td>第二个数</td>
<td colspan="3">
<INPUT name="txtNum2" type="text" id="txtNum2" size="25" onfocus="focusInput=this">
</td>
</tr>
<tr>
<td>
<INPUT name="addButton" type="button" id="addButton" value="+" onclick="count('+')">
</td>
<td>
<INPUT name="subButton" type="button" id="subButton" value="-" onclick="count('-')">
</td>
<td>
<INPUT name="mulButton" type="button" id="mulButton" value="×" onclick="count('*')">
</td>
<td>
<INPUT name="divButton" type="button" id="divButton" value="÷" onclick="count('/')">
</td>
<td>
<INPUT name="divButton" type="button" id="divButton" value="←" onclick="del()">
</td>
</tr>
<tr>
<td>计算结果</td>
<td colspan="3">
<INPUT name="txtresult" type="text" id="txtresult" size="25" onfocus="focusInput=null">
</td>
</tr>
</table>
</form>
</body>
</html>
有帮助麻烦点下【采纳该答案】，谢谢~~有其他问题可以继续交流~
<!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>
<title> 实现计算器 </title>
<script type="text/javascript">
function calc(){
//获取控件的值
var num1 = form1.num1.value;
var num2 = form1.num2.value;
var opr = form1.opr.value;
if(num1==""){
alert("请输入第一个参数的值");
//给控件设置焦点
form1.num1.focus();
return;
}
//判断值是否为合法的number值:isNaN
if(isNaN(num1)){
alert("参数一含有非法字符，请输入0~9之间的数字");
form1.num1.value="";//清空文本框的内容
form1.num1.focus();
return;
}
if(num2==""){
alert("请输入第二个参数的值");
//给控件设置焦点
form1.num2.focus();
return;
}
if(isNaN(num2)){
alert("参数二含有非法字符，请输入0~9之间的数字");
form1.num2.value="";//清空文本框的内容
form1.num2.focus();
return;
}
/*
把字符串转换为整形：parseInt
把字符串转换为浮点型：parseFloat
*/
if(opr=="+"){
form1.result.value=parseFloat(num1)+parseFloat(num2);
}else if(opr=="-"){
form1.result.value=parseFloat(num1)-parseFloat(num2);
}else if(opr=="*"){
form1.result.value=parseFloat(num1)*parseFloat(num2);
}else if(opr=="/"){
form1.result.value=parseFloat(num1)/parseFloat(num2);
}
}
</script>
</head>
<body>
<h1>实现计算器</h1>
<form action="" method="post" name="form1">
<input type="text" name="num1">
<select name="opr">
<option value="+">加</option>
<option value="-">减</option>
<option value="*">乘</option>
<option value="/">除</option>
</select>
<input type="text" name="num2">
<input type="button" value="=" onclick="calc();">
<input type="text" name="result">
</form>
</body>
</html>