JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id = 'alpha'>

JavaScript

var input = document.getElementById('alpha');

input.addEventListener('keypress',function(e) { 
	var str = String.fromCharCode(e.charCode);
    if (!(/[A-Z]|\s/i).test(str)) {
       e.preventDefault();
    }
});