"mark all as read" button for old.reddit.com userscript

gistfile1.js

// ==UserScript==
// @name         "mark all as read" button for old.reddit.com
// @description  Adds a "mark all as read" button to https://old.reddit.com/message/unread/
// @match        https://old.reddit.com/message/unread/*
// @author       relthyg
// ==/UserScript==
(function() {
    function getModhash() {
        // Prefer the global reddit object, fall back to the hidden input
        return (typeof reddit !== 'undefined' && reddit.modhash)
            || document.querySelector('input[name="uh"]')?.value
            || null;
    }

    function markAllAsRead() {
        var modhash = getModhash();

        if (!modhash) {
            console.log('"mark all as read": could not find modhash, aborting.');
            return;
        }

        fetch(location.origin + "/api/read_all_messages", {
            method: "POST",
            headers: { "Content-Type": "application/x-www-form-urlencoded" },
            body: "uh=" + encodeURIComponent(modhash) + "&api_type=json"
        })
        .then(function(res) {
            if (res.ok) {
                location.reload();
            } else {
                console.log('"mark all as read" failed. Status:', res.status);
            }
        })
        .catch(function(err) {
            console.log('"mark all as read" error:', err);
        });
    }

    function addButton() {
        var firstUnread = document.querySelector('.message.new');

        if (!firstUnread || document.getElementById('mark-all-read-btn')) {
            return;
        }

        var btn = document.createElement('button');
        btn.id = 'mark-all-read-btn';
        btn.textContent = 'mark all as read';
        btn.style.cssText = 'margin:15px 0px 0px 13px;';
        btn.onclick = markAllAsRead;
        firstUnread.parentNode.insertBefore(btn, firstUnread);
    }

    if (document.readyState === 'loading') {
        document.addEventListener('DOMContentLoaded', addButton);
    } else {
        addButton();
    }
})();