JSFiddle - React, Tailwind, and code Playground

HTML

<p><textarea id="txt" cols="30" rows="5">CHRISTMAS JUST LIKE A DAY AT THE OFFICE. YOU DO ALL OF THE WORK AND THE FAT GUY IN THE SUIT GETS ALL THE CREDIT.</textarea></p>
<p><button onclick="document.getElementById('txt').value=applySentenceCase(document.getElementById('txt').value)">Capitalise sentences</button></p>

JavaScript

function applySentenceCase(str) {
    return str.replace(/.+?[\.\?\!](\s|$)/g, function (txt) {
        return txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase();
    });
}