不完善，接下来想着把运算符分开成一个一个的按钮...
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>自制计算器</title>
<style type="text/css">
input{
border: 1px solid #4169E1;
-webkit-border-radius: 12px;
-moz-border-radius: 12px;
border-radius: 12px;
padding: 10px;
}
select{
border: 1px solid #4169E1;
-webkit-border-radius: 12px;
-moz-border-radius: 12px;
border-radius: 12px;
padding: 10px;
}
input[type='button']{
padding: 10px 30px;
}
</style>
</head>
<body>
<input type="text" id="n1" placeholder="请输入第一个数" />
<input type="text" id="n2" placeholder="请输入第二个数" />
<select name="" id="fh">
<option value="+">加</option>
<option value="-">减</option>
<option value="*">乘</option>
<option value="/">除 </option>
<option value="%">取余 </option>
</select>
<input type="button" id="s" value="求值" onclick="s()"/>
</body>
<script type="text/javascript">
function s(){
var num1 = parseInt(document.getElementById('n1').value);
var num2 = parseInt(document.getElementById('n2').value);
var fh = document.getElementById('fh').value;
var sum;
switch(fh){
case '+':
sum = num1 + num2;
document.write(num1+'+'+num2+'='+sum);
break;
case '-':
sum = num1 - num2;
document.write(num1+'-'+num2+'='+sum);
break;
case '*':
sum = num1 * num2;
document.write(num1+'*'+num2+'='+sum);
break;
case '/':
sum = num1 / num2;
document.write(num1+'/'+num2+'='+sum);
break;
case '%':
sum = num1 % num2;
document.write(num1+'%'+num2+'='+sum);
break;
}
}
</script>
</html>