一个简单易懂的做法
<!DOCTYPE html>
<html>
<head>
<title>事件</title>
<script type="text/javascript">
function count() {
//获取第一个输入框的值
var a = document.getElementById("txt1").value - 0;
//获取第二个输入框的值
var b = document.getElementById("txt2").value - 0;
//获取选择框的值
var c = document.getElementById("select");
var fruit = document.getElementById("fruit");
//获取通过下拉框来选择的值来改变加减乘除的运算法则
switch (c.value) {
case "+":
fruit.value = a + b;
break;
case "-":
fruit.value = a - b;
break;
case "*":
fruit.value = a * b;
break;
case "/":
fruit.value = a / b;
break;
}
//设置结果输入框的值
}
</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" />
</body>
</html>
