JSFiddle - React, Tailwind, and code Playground

HTML

<select id="mySelect" multiple="multiple">
      <option>Item 1</option>
      <option>Item 2</option>
      <option>Item 3</option>
      <option>Item 4</option>
      <option>Item 5</option>
</select>
<input id="myInput" type="text">

JavaScript

(function() {
    var input = document.getElementById('myInput'),
        select = document.getElementById('mySelect');
    
    select.onchange = function() {
        var i, result = [];
        for(i = 0; i < this.options.length; i++) {
            if(this.options[i].selected) {
                result.push(this.options[i].text);
            }
        }
        input.value = result.join(', ');
    };
})();