JSFiddle - React, Tailwind, and code Playground

HTML

<input id='first' onkeyup='mask("first");'>
<input id='last' onkeyup='mask("last");'>

JavaScript

String.prototype.toTitleCase = function () {
    return this.replace(/\w\S*/g, function (txt) {
        return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();
    });
};

function mask(target) {
    var elem = document.getElementById(target);
    elem.value = elem.value.toTitleCase();
}