javascript - 根据其他列表选定值更新下拉列表中的选定值

Question

我试图弄清楚如何使用 Vanilla JS 在几个下拉列表中动态选择值。我有 3 个具有相同值的下拉列表。它们中的每一个都应该同时选择不同的值（不允许重复）。因此，一旦用户更改了一个列表中的另一个值，另一个列表的值应该相应地更新。基本上，在这种情况下，我需要交换他们的号码。但我无法完成任务，因为我对 JS 和整体编程都很陌生。我试过以下：

// defining initial values
var startPoint = document.querySelectorAll("select");

// function to recalculate value of each list on change
function calculateNewValues() {
  var changes = new Array();
  // getting updated values from lists as an array
  var updatedValues = document.querySelectorAll("select");
  // looping through array
  for (let i = 0; i < updatedValues.length; i++) {
    // if in updated array value of current index isn't equal to value
    // of index in initial array
    if (updatedValues[i].value != startPoint[i].value) {
      // creating variable changes for tracking
      changes[i] = updatedValues[i].value;
    }
    // if var changes has been defined (i.e. value of any list was updated)
    if (changes.length > 0) {
      // finding index of initial array with same value
      var key = startPoint.findIndex(changes[i]);
      // setting value of found index to previously stored value in updated list
      updatedValues[key].value = startPoint[i].value;
    }
  }

  updatedValues.forEach(element => {
    console.log(element.value);
  });
}
// event listeners for change of every dropdown list
const lists = document.querySelectorAll("select");
for (k = 0; k < lists.length; k++) {
  lists[k].addEventListener("change", calculateNewValues, false);
}

<p>
  <select name="list1" id="list1">
    <option value="1" id="list1option1" selected>1</option>
    <option value="2" id="list1option2">2</option>
    <option value="3" id="list1option3">3</option>
  </select>
</p>
<p>
  <select name="list2" id="list2">
    <option value="1" id="list2option1">1</option>
    <option value="2" id="list2option2" selected>2</option>
    <option value="3" id="list2option3">3</option>
  </select>
</p>
<p>
  <select name="list3" id="list3">
    <option value="1" id="list3option1">1</option>
    <option value="2" id="list3option2">2</option>
    <option value="3" id="list3option3" selected>3</option>
  </select>
</p>

当涉及到时，我可能会犯一些错误，if (changes.length > 0) { 但我不明白如何才能使这部分变得更好。

提前非常感谢。

标签： javascriptdropdown

Answer 1

您已分配startPoint给 querySelectorAll。每当您编写相同的 querySelectorAll 时，它将包含完全相同的对象。所以在你的情况下startPoint，updatedValues总是一样的。

1. 我已定义startPoint为一个值数组并将其值设置为setStartingPointValues().
2. 添加target为正在更新的当前列表。
3. 更新if (target.id == updatedValues[i].id && updatedValues[i].value != startPoint[i])并检索target的旧值。
4. var key = startPoint.findIndex(x => x == target.value);将找到与新值相似的值的选择。如果我们找到 key != -1，则使用 oldValue 更新该选择。
5. setStartingPointValues()在更改事件结束时调用。

// defining initial values
var startPoint = [];

// store values for all select.
function setStartingPointValues() {
  startPoint = [];
  document.querySelectorAll("select").forEach(x => startPoint.push(x.value));
}

setStartingPointValues();

// function to recalculate value of each list on change
function calculateNewValues() {
  let target = this;
  let oldValue = 0;
  // getting updated values from lists as an array
  var updatedValues = document.querySelectorAll("select");
  // looping through array
  for (let i = 0; i < updatedValues.length; i++) {
    // if in updated array value of current index isn't equal to value    
    // of index in initial array    
    if (target.id == updatedValues[i].id && updatedValues[i].value != startPoint[i]) {
      // creating variable changes for tracking
      // changes[i] = updatedValues[i].value;
      oldValue = startPoint[i];
    }
    // if var changes has been defined (i.e. value of any list was updated)

    // finding index of initial array with same value
    var key = startPoint.findIndex(x => x == target.value);
    // setting value of found index to previously stored value in updated list
    if (key !== -1)
      updatedValues[key].value = oldValue;

  }

  setStartingPointValues();
}
// event listeners for change of every dropdown list
const lists = document.querySelectorAll("select");
for (k = 0; k < lists.length; k++) {
  lists[k].addEventListener("change", calculateNewValues, false);
}

<p>
  <select name="list1" id="list1">
    <option value="1" id="list1option1" selected>1</option>
    <option value="2" id="list1option2">2</option>
    <option value="3" id="list1option3">3</option>
  </select>
</p>
<p>
  <select name="list2" id="list2">
    <option value="1" id="list2option1">1</option>
    <option value="2" id="list2option2" selected>2</option>
    <option value="3" id="list2option3">3</option>
  </select>
</p>
<p>
  <select name="list3" id="list3">
    <option value="1" id="list3option1">1</option>
    <option value="2" id="list3option2">2</option>
    <option value="3" id="list3option3" selected>3</option>
  </select>
</p>