JSFiddle - React, Tailwind, and code Playground

HTML

<select name="b_company" id="b_company" onchange="showHideInput(this)">
    <option value="0" selected="selected">Individual</option>
    <option value="1">Company</option>
</select>

<div class="control-group" id="vat" style="display:none;">
    <label class="control-label" for="vatnumber">V.A.T. Number</label>
    <div class="controls">
        <input id="s_vatnumber" type="text" name="s_vatnumber" value="" />
    </div>
</div>

JavaScript

function showHideInput(sel) {
    var value = sel.value;  
    if(value==0)
        document.getElementById('vat').style.display = 'none';
    if(value==1)
        document.getElementById('vat').style.display = 'block';
    

};