JSFiddle - React, Tailwind, and code Playground

HTML

<p>You can't put spaces at the start:</p>
<input type="text" id="noSpaces">

JavaScript

var elmt = document.getElementById('noSpaces');

elmt.addEventListener('keydown', function (event) {
    if (elmt.value.length === 0 && event.which === 32) {
        event.preventDefault();
    }
});