<select id="sel">

  <option value="1">1</option>

  <option value="2">2</option>

  <option value="3">3</option>

</select>

​

​

const instance = new mdb.Select(document.getElementById('sel'),{

  multiple: true,

  filter: true, 

});

​

document.onclick = function(){

  console.log(instance.value)

};