JSFiddle - React, Tailwind, and code Playground

HTML

<table>
        <tr>
        <td><textarea class="amount" id="fields_2386" placeholder="Weekly $"></textarea></td>
            <td><input class="amount" id="fields_2378" type="text" placeholder="Fortnightly $" readonly></td>
        </tr>
    </table>

JavaScript

$("#fields_2386").keyup(function(){
 var input1 = parseInt($("#fields_2386").val());
 if(!isNaN(input1)){
     $("#fields_2378").val(input1 * 2);
 }
 else{
     $("#fields_2378").val("");
     alert("Please enter a valid number");
 }
});