问题在那，实现不了。实现四则运算

<!DOCTYPE html>

<html>

<head>

</head>

<body>

<option value="/">/</option><input type="number" value="0" id="num" onchange="sum()">=

function sum(change){

var num1 = document.getElementById("num1").value;

var num = document.getElementById("num").value;

var sum = document.getElementById("sum")

if(x=="*"){

sum.innerHTML=num1*num}

if(x=="-"){

sum.innerHTML=num1-num }

if(x=="/"){

sum.innerHTML=num1/num }

if(x=="+"){

sum.innerHTML=num1+num

}

</script>

</body>

</html>

x为定义。。改为下面这样，不单是x的问题。。

    <input type="number" value="0" id="num1" onchange="change()">
    <select id="select" onchange="change(this.value)">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
        </select>
        <input type="number" value="0" id="num" onchange="change()">=
        <label id="sum">0</label>
        <script type="text/javascript">
            function change() {
                var num1 = document.getElementById("num1").value;
                var num = document.getElementById("num").value;
                var x = document.getElementById("select").value

                if (x == "*") {
                    sum.innerHTML = num1 * num
                }

                if (x == "-") {
                    sum.innerHTML = num1 - num
                }

                if (x == "/") {
                    sum.innerHTML = num1 / num
                }

                if (x == "+") {
                    sum.innerHTML = num1 + num
                }
            }
        </script>