利用函数实现简单的计算机

var num1 = prompt('输入第一个数字');
var operator = prompt('输入运算符');
var num2 = prompt('输入第二个数字');
function getnum(num1,num2) {
return[num1 + num2,num1 - num2, num1 * num2,num1 / num2];
}
var str = getnum(num1,num2);
if (operator == '+') {
alert(str[0]);
}else if(operator == '-') {
alert(str[1]);
}else if(operator == '*') {
alert(str[2]);
}else if(operator == '/') {
alert(str[3]);
}