JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="txt1" />
<input type="text" id="txt2"/>
<input type="text" id="txt3"/>

JavaScript

$('#txt2').keyup(function(){

var var1=document.getElementById('txt1').value;
var var2=document.getElementById('txt2').value;
alert(var2);
  document.getElementById('txt3').value=result;

});