Question

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Set dropdown value based on what is selected in another dropdown using Javascript

我想要另一只手。当在select id = b上选择一个元素时，重置select on id = a，example

选择“另一个选项1”，id = a设置为“---”

为什么不工作？

HTML：

<select id="a" onchange="changedrop();">
  <option value="1">---</option>
  <option value="2">option 1</option>
  <option value="3">option 2</option>
  <option value="4">option 3</option>
</select>
<select id="b" onchange="changedrop();">>
  <option value="1">---</option>
  <option value="2">another option 1</option>
  <option value="3">another option 2</option>
</select>

JS：

function changedrop() {
  if (document.getElementById('a').value == '1')
  document.getElementById("b").value = '1';
  else if (document.getElementById('a').value == '2')
  document.getElementById("b").value = '1';
  else if (document.getElementById('a').value == '3')
  document.getElementById("b").value = '1';
  else if (document.getElementById('a').value == '4')
  document.getElementById("b").value = '1';
  else if (document.getElementById('b').value == '1')
  document.getElementById("a").value = '1';
  else if (document.getElementById('b').value == '2')
  document.getElementById("a").value = '1';
  else if (document.getElementById('b').value == '3')
  document.getElementById("a").value = '1';
};

Answer 1

问题是您需要在onchange元素上收听select事件。

＆＃13;

function changeDropA() {
  var a = document.getElementById('a');
  var b = document.getElementById("b");
  
  if (a.value == '1')
    b.value = '1';
  else if (a.value == '2')
    b.value = '1';
  else if (a.value == '3')
    b.value = '1';
  else if (a.value == '4')
    b.value = '1';
};

function changeDropB() {
  var a = document.getElementById('a');
  var b = document.getElementById("b");

  if (b.value == '1')
    a.value = '1';
  else if (b.value == '2')
    a.value = '1';
  else if (b.value == '3')
    a.value = '1';
}

document.getElementById('a').addEventListener('change', changeDropA);
document.getElementById('b').addEventListener('change', changeDropB);

＆＃13;

<select id="a">
  <option value="1">---</option>
  <option value="2">option 1</option>
  <option value="3">option 2</option>
  <option value="4">option 3</option>
</select>
<select id="b">
  <option value="1">---</option>
  <option value="2">another option 1</option>
  <option value="3">another option 2</option>
</select>

＆＃13;

根据使用Javascript在另一个下拉列表中选择的内容设置下拉值，反之亦然

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