JSFiddle - React, Tailwind, and code Playground

HTML

<form id="selectId"> 

<select id="event-list" name="cat" multiple="multiple" style="height:150px;width:350px;"> 
    <option value="1" selected="selected">- Sub categories -</option> 
    <option value="2">Website development</option> 
    <option value="50">Тестирование ПО</option> 
</select> 
</form>

JavaScript

var form = document.querySelector('#selectId'), 
    select = form.querySelectorAll('select option'), 
    arr; 

form.addEventListener('change', function(e) {
	arr = [];
  select.forEach(function(el) {
  	if(el.selected) {
    	arr.push(el.value);
    }
  });
  alert(arr);
});