JSFiddle - React, Tailwind, and code Playground

HTML

<b>Checkbox:</b>
  <select name="myCheck[]">
  <option>select</option>
  <option value="1">a</option>
  <option value="2">b</option>
  <option value="3">c</option>
</select>
<select name="myCheck[]">
  <option>select</option>
  <option value="1">a</option>
  <option value="2">b</option>
  <option value="3">c</option>
</select>
<select name="myCheck[]">
  <option>select</option>
  <option value="1">a</option>
  <option value="2">b</option>
  <option value="3">c</option>
</select>
<input type="button" value="Click" id="btntest" />

JavaScript

function myFunction() {
var selchbox = [];
var inputfields = document.getElementsByName("myCheck");
var ar_inputflds = inputfields.length;

for (var i = 0; i < ar_inputflds; i++) {
selchbox.push(inputfields[i].value);
}
return selchbox;

}

document.getElementById('btntest').onclick = function(){
  var selchb = myFunction();     // gets the array returned by getSelectedChbox()
  alert(selchb);
  console.log(selchb);
}