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_div1" style="display: none;">number 1</div>
<div id="hidden_div2" style="display: none;">number 2</div>

JavaScript

var select = document.getElementById('test'),
    nbItems = 2,
    onChange = function (event) {
        var val = this.options[this.selectedIndex].value;

        for (var i = 1; i <= nbItems; i++) {
            document.getElementById('hidden_div' + i).style.display = val == i ? 'block' : '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);
    });
}