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老师批语：
实例<body>
<div class="box">
<h3>计算器</h3>
<form action="">
<table>
<tr>
<td><input type="text" id="opt1" placeholder="请输入数字"></td>
<td><select name="opt" id="">
<option value="add">+</option>
<option value="sub">-</option>
<option value="mul">*</option>
<option value="div">/</option>
</select>
</td>
<td><input type="text" id="opt2" placeholder="请输入数字"></td>
<td><input type="submit" id="submit" value="计算"></td>
</tr>
</table>
<span> 结果:</span>
<span id="res"></span>
</form>
</div>
<script>
let data1=document.getElementById("opt1");
let data2=document.getElementById("opt2");
let button=document.getElementById('submit');
let opt=document.getElementsByName('opt')[0];
let all= document.getElementById('res');
button.onclick=function ()
{
if (data1.value.length === 0)
{
alert('请输入第一个数值');
data1.focus();
return false;
}
if (isNaN(data1.value))
{
alert('第一个参数类型错误');
data1.focus();
return false;
}
if (data2.value.length === 0)
{
alert('请输入第二个数值');
data2.focus();
return false;
}
if (isNaN(data2.value))
{
alert('第二个参数类型错误');
data2.focus();
return false;
}
let res;
let flag;
switch (opt.value)
{
case 'add':
flag='+';
res=parseFloat(data1.value) + parseFloat(data2.value);
break;
case 'sub':
flag='-';
res=parseFloat(data1.value) - parseFloat(data2.value);
break;
case 'mul':
flag='*';
res=parseFloat(data1.value) * parseFloat(data2.value);
break;
case 'div':
flag='/';
if (parseFloat(data2.value)===0)
{
alert('除数不能为0');
data2.value='';
data2.focus();
return false;
}
else
{
res=parseFloat(data1.value) / parseFloat(data2.value);
res=Math.round(res*100)/100;
}
break;
}
all.innerHTML=data1.value+flag + data2.value+'='+res;
return false;
}
</script>
</body>
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