JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="maiusculo" value="TEXTO" onkeyup="reduzir()">

<input type="text" id="minusculo" value="texto" onkeyup="aumentar()">

JavaScript

function reduzir() {
   var string;
   
   string = document.getElementById("maiusculo");
   string.value = (string.value.toLowerCase());
}

function aumentar() {
	var string;
  
  string = document.getElementById("minusculo");
  string.value = (string.value.toUpperCase());
}