Dynamically update a dropdown list based on another dropdown list?

Question

I have a dropdown list with id='apha' with three options and their values are comma separated.

<select class="browser-default" id="alpha">
    <option value="a,b,c">One</option>
    <option value="d">Two</option>
    <option value="e,f">Three</option>
</select>

Another dropdown beta with values. Based on which option is selected in alpha those values should be there in second dropdown beta.

<select class="browser-default" id="beta">
    <option value="a">First</option>
    <option value="b">Second</option>
    <option value="c">Third</option>
    <option value="d">Fourth</option>
    <option value="e">Fifth</option>
    <option value="f">Sixth</option>
</select>

So if One is selected from alpha dropdown. Only values of aplha - a,b,c should be present in beta dropdown - First Second Third

What i have tried: Without comma separated in values

$('#alpha').on('change', function () {
  $('#alpha').html('');
  if ($('#alpha').val() == "One") {
    $('#alpha').append('<option value="a">First</option>');
    $('#alpha').append('<option value="b">Second</option>');
    $('#alpha').append('<option value="c">Third</option>');
  } 
});

Answer 1

Vanilla JS

const beta = document.getElementById('beta');
const betaOpts = [...beta.children];

document.getElementById('alpha').addEventListener(
  'change',
  (e) => {
    beta.innerHTML = betaOpts.filter(
      o => e.target.value.includes(o.value)
    ).map(o => o.outerHTML).join('')
  })

<label>Alpha</label>
<select class="browser-default" id="alpha">
  <option value="a,b,c">One</option>
  <option value="d">Two</option>
  <option value="e,f">Three</option>
</select>

<label>Beta</label>
<select class="browser-default" id="beta">
  <option value="a">First</option>
  <option value="b">Second</option>
  <option value="c">Third</option>
  <option value="d">Fourth</option>
  <option value="e">Fifth</option>
  <option value="f">Sixth</option>
</select>