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老师批语：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
</style>
</head>
<body>
<div class="box">
<form>
<table>
<tr>
<td><input type="text" name="num1" placeholder="请输入数1"></td>
<td><select name="option" id="">
<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>
</td>
<td><input type="text" name="num2" placeholder="请输入数2"></td>
<td><button type="button">计算</button></td>
</tr>
<tr>
<td><h3>结果</h3></td>
<td><h3 id="result"></h3></td>
</tr>
</table>
</form>
</div>
<script>
//获取对象
let num1 = document.getElementsByName('num1')[0];
let num2 = document.getElementsByName('num2')[0];
let opt = document.getElementsByName('option')[0];
let btn =document.getElementsByTagName('button')[0];
let result = document.getElementById('result');
btn.onclick = function(){
let a1 = num1.value;
let a2 = num2.value;
//检测操作数的正确
if(num1.value.length ===0){
alert('不能为空');
num1.focus();
return false;
}else if(isNaN(num1.value)){
alert('必须为数字');
num1.focus();
return false;
}else if(num2.value.length ===0){
alert('不能为空');
num2.focus();
return false;
}else if(isNaN(num2.value)){
alert('必须为数字');
num2.focus();
return false;
}
//处理操作符
let option = opt.value;
let flag = '';
let res ;
switch(option){
case 'null':
alert('请选择操作类型');
opt.focus();
return false
case 'add':
flag = '+';
res = a1 + a2;
break;
case 'sub':
flag = '-';
res = a1-a2;
break;
case 'mul':
flag = '*';
res = a1*a2;
break;
case 'div':
flag = '/';
if(a2 === 0){
alert('除数不能为0');
num2.focus();
return false;
}else{
res =a1/a2;
res = Math.round(res * 100)/100;
}
break;
}
let str ='<span style="color:red">';
str +=a1 + flag + a2 + '=' +res;
str +='</span>';
result.innerHTML = str;
}
</script>
</body>
</html>点击 "运行实例" 按钮查看在线实例
个人总结：
1.<input type="text" name="num1" placeholder="请输入数1"> ~~let num1;
想要获取输入的值 必须是 num1.value，而不是直接用num。
2.注意switch的用法
switch(){
case 0: //注意这里是冒号
break;
}
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