JSFiddle - React, Tailwind, and code Playground

HTML

<select name="box" id="box" multiple>
<option value="a">A</option>
<option value="b">B</option>
<option value="c">C</option>
<option value="d">D</option>
</select> 

<button onclick="bla()">Click to display selected items</button>
<script>
    function bla() {

 var sel = document.getElementById("box");
 var sResult = "";

 for (var i = 0; i < sel.options.length; i++) {
     
     if (sel.options[i].selected){

	sResult += sel.options[i].value + ','


     }

     
 }
    if (sResult.length > 1)  sResult = sResult.substring(0,sResult.length-1);
alert(sResult)

}
</script>