<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script type="text/javascript">
function count() {
//获取第一个输入框的值
var num1 = parseInt(document.getElementById("txt1").value);
//获取第二个输入框的值
var num2 = parseInt(document.getElementById("txt2").value);
//获取选择框的值
var num3 = document.getElementById("select").value;
//获取通过下拉框来选择的值来改变加减乘除的运算法则
//设置结果输入框的值
var sum = "";
switch (num3) {
case "+":
sum = num1 + num2;
break;
case "-":
sum = num1 - num2;
break;
case "*":
sum = num1 * num2;
break;
case "/":
sum = num1 / num2;
break;
}
document.getElementById("fruit").value = sum;
}
</script>
</head>
<body>
<input type='text' id='txt1' />
<select id='select'>
<option value='+'>+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type='text' id='txt2' />
<input type='button' value=' = ' onclick="count()" />
<!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' onclick="count()" />
</body>
</html>
居然时把 document.getElementById("fruit").value = sum; 的Id写成ld的原因。