<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta name="Generator" content="EditPlus®">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
<title> 事件</title>
<script type="text/javascript">
function count(){
var num1=parseInt(document.getElementById("txt1"）.value);//获取第一个输入框的值
var num2=parseInt(document.getElementById("txt2"）.value);//获取第二个输入框的值
var count1= document.getElementById("select"）.value;//获取选择框的值
var result=0;
switch(count1){
case'+':
result=num1+num2;
break;
case'-':
result=num1-num2;
break;
case'*':
result=num1*num2;
break;
case'/':
result=num1/num2;
break;
}//获取通过下拉框来选择的值来改变加减乘除的运算法则
document.getElementById("fruit").value=result;//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>
“+”在JS中也会作为连接符使用，涉及到类型转换，想实现加法可以转换下类型，如：result=parseFloat(num1)+ parseFloat(num2);就能进行运算了。
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var a = document.getElementById("txt1").value;
var b = document.getElementById("txt2").value;
var c = document.getElementById("select").value;
var a1 = Number(a);
var b1 = Number(b);
var c,d;
switch(c){
case "+": d = a1 + b1; break;
case "-": d = a1 - b1; break;
case "*": d = a1 * b1; break;
case "/": d = a1 / b1; break;
}
document.getElementById("fruit").value = d;
}
</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>
字符串要转换成数字