JSFiddle - React, Tailwind, and code Playground

HTML

<select name="mySelect" id="mySelect">
    <option value="1">Item 1</option>
    <option value="2">Item 2</option>
    <option value="other">Other</option>
</select>
<input type="text" name="input-box" id="inputBox" cols="30" rows="10" class="hidden"/>

CSS

.hidden{
    display:none;
}

JavaScript

var selectEl = document.getElementById('mySelect');
var inputBox = document.getElementById('inputBox');
var listener = function(e){
if(selectEl.value == "other"){
       inputBox.className="";
}else{
       inputBox.className="hidden";
}
}


selectEl.addEventListener('change',listener);