JSFiddle - React, Tailwind, and code Playground

HTML

<p>
    Input: <code id="input">fOO BaR_baZ</code>
</p>

<p>
    Output: <code id="output"></code>
</p>

CSS

code { background-color:#eee; }

JavaScript

function foo(str) {
    return $.map(str.split(/\s|_/), function(word) {
        return word.charAt(0).toUpperCase() + word.slice(1).toLowerCase();
    }).join("");
}

$("#output").text( foo( $("#input").text() ) );