获取不到input内自己输入的值

<form method="get" action="calc.php" id="myForm">

num1:
<input id="num1" type=text name="num1" value="" />
<br/> num2:
<input id="num2" type=text name="num2" value="" />
<br/> oper:
<select name="oper" value="">
    <option value="+">+</option>
    <option value="-">-</option>
    <option value="*">*</option>
    <option value="/">/</option>
</select>
<br/>
<input type="submit" value="提交" />
<script>
    var subBtn = document.getElementById("myForm");
    var num1Val = document.getElementById("num1").value;
    var num2Val = document.getElementById("num2").value;
    subBtn.onsubmit = function () {
        alert(num1Val);
        return false;
    };
</script>

</form>

你页面加载好，num1Val的值就已经获得了，此时value值位空，你提交的时候肯定还是空。解决办法：
点击提交的时候在去获取值：

        var subBtn = document.getElementById("myForm");
     
        subBtn.onsubmit = function () {
                var num1Val = document.getElementById("num1").value;
                var num2Val = document.getElementById("num2").value;
            alert(num1Val);
            return false;
        };