JSFiddle - React, Tailwind, and code Playground

HTML

<select id="test" name="form_select">
   <option value="0">None</option>
   <option value ="1">Number 1</option>
    <option value ="2">Number 2</option>
</select>

<div id="hidden_div" style="display: none;">number 1</div>
<div id="hidden_div2" style="display: none;">number 2</div>

JavaScript

var select = document.getElementById('test'),
onChange = function(event) {
    var div1 = 'hidden_div';
    var div2 = 'hidden_div2';
    
    var index1 = this.options[this.selectedIndex].value == 1;
    var index2 = this.options[this.selectedIndex].value == 2;
      
    if(index1 || index2){
        document.getElementById(div1).style.display = index1 ? 'block' : 'none';
        document.getElementById(div2).style.display = index2 ? 'block' : 'none';
    }
    else{
        document.getElementById(div1).style.display = 'none';
        document.getElementById(div2).style.display = 'none';
    }
};

// attach event handler
if (window.addEventListener) {
    select.addEventListener('change', onChange, false);
} else {
    // of course, IE < 9 needs special treatment
    select.attachEvent('onchange', function() {
        onChange.apply(select, arguments);
    });
}