JSFiddle - React, Tailwind, and code Playground

HTML

<input id="a" type="text">

JavaScript

var input = document.getElementById("a");
input.addEventListener("input", ()=>{
	//input.value = input.value.toUpperCase();
  input.value = input.value.replace(/[^A-Z]/gi);
  //input.value = input.value.toUpperCase().replace(/[^A-Z]/gi, '');
});