JSFiddle - React, Tailwind, and code Playground

HTML

<select name="test" id="select" onchange="selection()">
<option value="">A</option> 
<option value="B">B</option>
<option value="C">C</option>
<option value="nil">nil</option>
    </select> 
<td>test</td>                           
<td><input type="text" name="test1" id="test1" />
 <input type="button" value="submit" onclick="selection()" />

JavaScript

function selection()
{    
 var cat = document.getElementById('select').value;    
    if (cat == "B") {
        if (document.getElementById("test1").value == "") {
            alert('Mandatory');
            return false;
        }

    }
    return true;
}