搞了半天。
<!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 txt1Val = document.getElementById("txt1").value;
//获取第二个输入框的值
var txt2Val = document.getElementById("txt2").value;
//获取选择框的值
var opera = document.getElementById("select");
var index = opera.selectedIndex;
var operaVal = opera.options[index].value;
var result;
//获取通过下拉框来选择的值来改变加减乘除的运算法则
if(operaVal == "+"){
result = (txt1Val-0) + (txt2Val-0);
}else if(operaVal == "-"){
result = txt1Val - txt2Val;
}else if(operaVal == "*"){
result = txt1Val * txt2Val;
}else{
result = txt1Val / txt2Val;
}
console.log(txt1Val+"`````````"+txt2Val+"`````````"+result)
//设置结果输入框的值
document.getElementById("fruit").value = result
}
</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>