javascript 四则运算

<script type ="text/javascript">

var num1 = prompt("请输入第一个数" ",");

var num2= prompt("请输入第二个数" ",");

var method= prompt("请输入运算符" ",");

functiona(){

swich(method){

case " * ":

alert(num1 * num2);

case " / ":

alert(num1 / num2);

case " + ":

alert(num1 + num2);

case " - ":

alert(num1 - num2);

}

}

<script/>

<input name = "btn" type = "butten" value ="计算机" onclick "a()"/>