JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form1">
  <select name="combo1" multiple="multiple">
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="3">Three</option>
    <option value="4">Four</option>
  </select>
  <select name="combo2">
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="3">Three</option>
    <option value="4">Four</option>
  </select>
  <input type="button" name="button1" value="Check"/>
</form>

JavaScript

function combo1includes2(c1, c2) {
  // Store multi-select values in "values".
  var vs={}, opts=c1.options;
  for (var i=0; i<opts.length; i++) {
    if (opts[i].selected) {
      vs[opts[i].value] = true;
    }
  }
  return vs.hasOwnProperty(c2.value);
}
document.form1.button1.onclick = function() {
  var combo1 = document.form1.combo1
    , combo2 = document.form1.combo2;
  if (combo1includes2(combo1, combo2)) {
    alert("Ok, included.");
  } else {
    alert("Not included!");
  }
};