如何根据选择值从数组中填充选择选项？

Question

尝试根据第一个选择元素的值从数组中填充第二个选择元素的选项。我似乎无法理解为什么它只填充第一个选择元素的数组中的项目。我知道 appendChild 导致项目继续满足需要，但我试图清除变量，但似乎创建的选项元素仍然存在。

任何帮助都会很棒，谢谢！

<select id="makeSelect" onChange="modelAppend()">
    <option value="merc">Mercedes</option>
    <option value="audi">Audi</option>
    <option value="bmw">BMW</option>
</select>

<select id="modelSelect">

</select>

<script>
var audiModels = ["TT", "R8", "A4", "A6"]; //audimodels
var mercModels = ["C230", "B28", "LTX",]; //mercmodels
var bmwModels = ["328", "355", "458i",]; //bmwmodels
var selectedMake = document.getElementById("makeSelect"); //grabs the make select
var selectedModel = document.getElementById("modelSelect"); //grabs the model select
var appendedModel = window[selectedMake.value + "Models"]; // appends "Models" to selectedMake.value and converts string into variable

function modelAppend() {
    for (var i = 0; i < appendedModel.length; i ++) { // counts items in model array
        var models = appendedModel[i]; // // sets "models" to count of model array
        var modelOptions = document.createElement("option"); //create the <option> tag
        modelOptions.textContent = models; // assigns text to option
        modelOptions.value = models; // assigns value to option
        selectedModel.appendChild(modelOptions); //appeneds option tag with text and value to "modelSelect" element
    }
}

</script>

Answer 1

这条线很可疑：

var appendedModel = window[selectedMake.value + "Models"];

您需要在值更改时获取元素，而不是在页面加载时获取。然后您还需要删除更改选项，否则如果用户多次选择，您将获得一个很长的列表。使用对象来存储数组，这使得以后访问它们变得更加容易。最好使用事件监听器而不是内联 js（尽管这不是这里的主要问题）。

试试下面的代码：

let models = {
  audiModels: ["TT", "R8", "A4", "A6"],
  mercModels: ["C230", "B28", "LTX"],
  bmwModels: ["328", "355", "458i"]
}

document.getElementById('makeSelect').addEventListener('change', e => {
  let el = e.target;
  let val = el.value + 'Models';
  let appendTo = document.getElementById('modelSelect');
  Array.from(appendTo.getElementsByTagName('option')).forEach(c => appendTo.removeChild(c));
  if (!models[val] || !Array.isArray(models[val])) {
    appendTo.style.display = 'none';
    return;
  }
  models[val].forEach(m => {
    let opt = document.createElement('option');
    opt.textContent = opt.value = m;
    appendTo.appendChild(opt);
  });
  appendTo.style.display = '';
});

<select id="makeSelect">
    <option value=""></option>
    <option value="merc">Mercedes</option>
    <option value="audi">Audi</option>
    <option value="bmw">BMW</option>
</select>

<select id="modelSelect" style="display:none">

</select>