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老师批语：
以下代码实现简易计算器功能：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>计算器</title>
</head>
<body>
<div class="box">
<div class="input">
<input type="text" id="num1" name="num1" placeholder="请输入第一个数字">
<select id="cal" name="cal">
<option value="null">请选择操作</option>
<option value="add">+</option>
<option value="sub">-</option>
<option value="mul">*</option>
<option value="div">/</option>
</select>
<input type="text" id="num2" name="num2" placeholder="请输入第二个数字">
<button id="btn" name="btn">开始计算</button>
</div>
<div class="res">
<h4>计算结果：</h4>
<h3 id="result" style="color: blue"></h3>
</div>
</div>
<script>
let num1=document.getElementById('num1');
let num2=document.getElementById('num2');
let cal=document.getElementById('cal');
let btn=document.getElementsByTagName('button')[0];
let result=document.getElementById('result');
btn.onclick=function () {
let data1 = 0;
let data2 = 0;
if (num1.value.length === 0) {
alert('第一个数不能为空');
num1.focus();
return false;
} else if (isNaN(num1.value)) {
alert('第一个数必须为数字');
num1.focus();
return false;
} else if (num2.value.length === 0) {
alert('第二个数不能为空');
num2.focus();
return false;
} else if (isNaN(num2.value)) {
alert('第二个数必须为数字');
num2.focus();
return false;
} else {
data1 = parseFloat(num1.value);
data2 = parseFloat(num2.value);
}
let option = cal.value;
let temp = 0;
let flag = '';
switch (option) {
case 'null':
alert('请选择操作类型');
cal.focus();
return false;
case 'add':
flag = '+';
temp = data1 + data2;
break;
case 'sub':
flag = '-';
temp = data1 - data2;
break;
case 'mul':
flag = '*';
temp = data1 * data2;
break;
case 'div':
flag = '/';
//进行除数不能为零的判断和处理
if (data2 === 0) {
alert('除数不能为零');
num2.focus();
num2.value = '';
return false;
} else {
temp = data1 / data2;
// 四舍五入,仅保留二位小数
temp = Math.round(temp * 100) / 100;
}
break;
}
let str = data1 + ' ' +flag + ' ' + data2 + ' = ' +temp;
result.innerHTML = str;
};
</script>
</body>
</html>点击 "运行实例" 按钮查看在线实例
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