JSFiddle - React, Tailwind, and code Playground

HTML

<input id='upcase' />

JavaScript

document.getElementById('upcase').onkeypress = function(e) {
   e = e || window.event;
   var char = String.fromCharCode(e.which || e.keyCode);
   var upCase = char.toUpperCase();
   e.which = e.keyCode = upCase.charCodeAt(0);   
   //return false;
};