使用表单中的javascript添加到多个选择列表

我想有2个空白的输入表单-类别和值，当按下一个按钮时，它们被附加/添加到2个多选择表单中，类别和值。按下按钮时，未输入数据

function doAdd() {
        //  Pick up data from the category and value input fields;
        // In my form these are named 'cat' and 'val'
        var catstr = document.getElementById("cat").value;
        var valstr = document.getElementById("val").value;

        // pick up references to the text areas;
        var cats = document.getElementById("catlist");
        var nums = document.getElementById("numlist");
        // Append text, inserting a new line character between
        // data sets.
        if (numadded > 0) {
            cats.value = cats.value + "\n";
            nums.value = nums.value + "\n";
        }

        numadded++;
        cats.value = cats.value + catstr;
        nums.value = nums.value + valstr;
}

HTML重要行

<script type="text/javascript" src="./checksubmit.js" ></script>
    <input type="text" id="val" name="val" size="10"/>
    <input type="text" id="cat" name="cat" size="30"/>
    <input type="button" onclick="doAdd();" value="Add item">
    <select multiple="multiple" id="catlist" style="width: 250px;"/>
    <select multiple="multiple" id="numlist" style="width: 250px;"/>

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我相信你想要这样的东西

function doAdd() {
    //  Pick up data from the category and value input fields;
    // In my form these are named 'cat' and 'val'
    var catstr = document.getElementById("cat").value;
    var valstr = document.getElementById("val").value;

    // pick up references to the text areas;
    var cats = document.getElementById("catlist");
    var nums = document.getElementById("numlist");

    //Create and append new options
    var catOption = new Option(catstr, valstr);
    var numOption = new Option(valstr, valstr);
    cats.appendChild(catOption);
    nums.appendChild(numOption);
}

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你想在你的选择中添加新选项吗？是的，这正是我想做的。请发布你的答案，而不是我需要的课本解决方法，但这将完成工作。格雷西亚斯！