Javascript 表单提交时触发的onchange事件

我希望用户能够记录他们执行的操作。我按类别对动作进行分组，然后使用两个选择菜单和JS，用户只会看到他们选择的类别中的动作。还有一个数量输入，根据选择的操作生成

我的问题是，当我提交表单时，会出现以下错误：

未捕获的TypeError:无法将属性“onchange”设置为null

选择框和JS实现的功能将一直工作到表单提交为止

index.js

document.addEventListener("DOMContentLoaded", () => {

  // First select 
  let cat_select = document.getElementById("id_post_cat");
  cat_select.onchange = () => handleCatChange(cat_select.value);

  // Second select
  let desc_select = document.getElementById("id_post_action");
  desc_select.onchange = () => handleDescChange(desc_select.value);
});

const handleCatChange = (cat) => {
  let quantity_div = document.getElementById("quantity_div");
  quantity_div.innerHTML = "";

  // Fetching the actions in the category selected and populating second select
  fetch(`/action/${cat}`)
    .then((response) => response.json())
    .then((data) => {
      let desc_select = document.getElementById("id_post_action");
      let optionHTML = "<option>---------</option>";

      data.actions.forEach((action) => {
        optionHTML +=
          '<option value"' + action.id + '">' + action.desc + "</option>";
      });
      desc_select.innerHTML = optionHTML;
    })
    .catch((err) => console.log(err));
};

const handleDescChange = (desc) => {
  let quantity_div = document.getElementById("quantity_div");
  quantity_div.innerHTML = "";

  let time_actions = [
    "Public transport instead of driving",
    "Walk/cycle instead of drive",
  ];
  let quant_actions = [
    "Unplug unused electrical items",
    "Repurpose a waste object",
    "Use a reusable bag",
    "Buy an unpackaged item",
    "Buy a locally produced item",
    "Buy a second hand item",
    "Buy an object in bulk",
    "Use a refillable bottle/to-go mug",
    "Drink a tap beer instead of bottled beer",
  ];

  if (time_actions.includes(desc)) {
    formAdder("Distance* (km)");
  } else if (quant_actions.includes(desc)) {
    formAdder("Quantity*");
  }
};
const formAdder = (label_content) => {
  let quantity_div = document.getElementById("quantity_div");
  // Label
  let label = document.createElement("label");
  label.innerHTML = `${label_content}`;
  label.setAttribute("for", "id_post_quantity");
  label.classList += "requiredField";

  // Input
  let input = document.createElement("input");
  input.setAttribute("id", "id_post_quantity");
  input.setAttribute("name", "post_quantity");
  input.setAttribute("required", "");
  input.classList += "form-control";
  quantity_div.append(label, input);
};

record.html

<form action="{% url 'record' %}" method="post">
    {% csrf_token %}
    {{form.post_cat|as_crispy_field}}
    {{form.post_action|as_crispy_field}}
    <div class="form-group" id="quantity_div">
    </div>
    <button class="btn btn-success" type="submit">Post</button>
</form>

{%csrf_令牌%}
{form.post|cat|as|crispy|u field}
{form.post|u action|as_crispy_field}
邮递

---

如果指针指向正确的方向，我们将不胜感激。谢谢。

似乎在加载的每个页面上都调用了onchange。这意味着，当重定向到索引页面（其中不存在选择框）时，getElementById返回空值等

我通过添加条件来检查元素是否出现在页面上，从而对此进行了更改：

document.addEventListener("DOMContentLoaded", () => {
  // First select
  let cat_select = document.getElementById("id_post_cat");
  if (!!cat_select) {
    cat_select.onchange = () => handleCatChange(cat_select.value);
  }

  // Second select
  let desc_select = document.getElementById("id_post_action");
  if (!!desc_select) {
    desc_select.onchange = () => handleDescChange(desc_select.value);
  }
});

---

有没有更优雅/正确的方法来解决相同的问题？

您知道哪个“onchange”会触发错误吗？听起来您正在设置它的元素不存在。在设置元素的onchange事件之前，我会先记录该元素，看看哪个是空的。谢谢您的评论。这是触发错误的第一个选择。在提交表单之前，两个onchange函数工作正常。由于某种原因提交表单时，会再次调用第一个onchange，并出现第一个select为null的错误

document.addEventListener("DOMContentLoaded", () => {
  // First select
  let cat_select = document.getElementById("id_post_cat");
  if (!!cat_select) {
    cat_select.onchange = () => handleCatChange(cat_select.value);
  }

  // Second select
  let desc_select = document.getElementById("id_post_action");
  if (!!desc_select) {
    desc_select.onchange = () => handleDescChange(desc_select.value);
  }
});