redditDraftSaver.js

// ==UserScript==
// @name         Reply Saver
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Saves unposted top-level replies to localStorage, and restores them on nav back to the page.
// @author       carcigenicate
// @match        https://old.reddit.com/r/*/comments/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    console.time('Reply Saver Userscript');

    const CACHE_TIME_HOURS = 72;
    const PARENT_KEY = 'COMMENT_SAVER_CACHE';

    function isExpired(timestamp) {
        const saveDate = new Date(timestamp);
        const now = new Date();
        return (now - saveDate) >= (CACHE_TIME_HOURS * 3600000);
    }

    function handleExpiredEntries() {
        const rawEntries = localStorage.getItem(PARENT_KEY);
        const parsed = JSON.parse(rawEntries) ?? {};
        for (const [key, {timestamp}] of Object.entries(parsed)) {
            if (isExpired(timestamp)) {
                delete parsed[key];
            }
        }
        localStorage.setItem(PARENT_KEY, JSON.stringify(parsed));
    }

    if (localStorage.getItem(PARENT_KEY) === null) {
        localStorage.setItem(PARENT_KEY, "{}");
    }

    handleExpiredEntries();

    const commentBox = document.querySelector('.usertext-edit textarea');

    const pathPieces = document.location.href.split('/');
    const commentIndex = pathPieces.indexOf('comments');

    const subreddit = pathPieces[commentIndex - 1];
    const postId = pathPieces[commentIndex + 1];

    const cacheKey = `${subreddit}:${postId}`;
    const rawCache = localStorage.getItem(PARENT_KEY);
    const cache = JSON.parse(rawCache);
    const existingEntry = cache[cacheKey];

    let populatedComment = '';
    if (existingEntry) {
        commentBox.value = existingEntry.comment;
        populatedComment = existingEntry.comment;
    }

    addEventListener('beforeunload', () => {
        const currentComment = commentBox.value;
        if (currentComment || populatedComment) {
            // Refetching to avoid multiple tabs having their own "cached cache" in memory causing write issue.
            const unloadRawCache = localStorage.getItem(PARENT_KEY);
            const unloadCache = JSON.parse(unloadRawCache);
            unloadCache[cacheKey] = { comment: currentComment, timestamp: new Date() };
            localStorage.setItem(PARENT_KEY, JSON.stringify(unloadCache));
        }
    });

    console.timeEnd('Reply Saver Userscript');
})();