guardianCommentsNew.user.js

// ==UserScript==
// @name        Show New Guardian Comments
// @namespace   http://www.theguardian.com/
// @description Shows new comments
// @include     http://www.theguardian.com/*
// @version     1
// @grant       none
// ==/UserScript==

var prefix = 'gu-discussion-';
var delayTimer = 5000;

var getUUID = function () {
    return guardian.config.page.shortUrl.replace('http://gu.com/', '');
};

var setLastVisited = function () {
    window.localStorage.setItem(prefix + getUUID(), new Date());
}

var getLastVisited = function () {
    return new Date(window.localStorage.getItem(prefix + getUUID())) || new Date();
};

var isFirstVisit = function () {
    return !(window.localStorage.getItem(prefix + getUUID()));
}

var originalLastVisit = getLastVisited();

var highlightNewComments = function(newSince) {
    function isNewer(d1, d2) {
        if (d1.getTime() > d2.getTime()){
           return true;
        }
        return false;
    }

    var count = 0;
    var comments = document.querySelectorAll('.discussion__main-comments li.d-comment');

    for (var i=0, l=comments.length; i<l; i++) {
        var c = comments[i];
        var timestamp = new Date(c.getAttribute('data-comment-timestamp'));
        // if timestamp is newer than last viewed
        if ( isNewer(timestamp, new Date(newSince)) ) {
            c.style.backgroundColor = '#EEF6F9';
            c.style.borderLeft = '10px solid #E2EEF3';
            count++;
        }
    }

    var commentCount = document.querySelector('.js-discussion-comment-count');
    var commentCountHTML = commentCount.innerHTML;
    commentCount.innerHTML = commentCountHTML + '<br />' + count + ' new';
};

var reloadOnPagination = function () {
    var pagination = document.querySelectorAll('.pagination__list a');
    for (var i=0, l=pagination.length; i<l; i++) {
        pagination[i].addEventListener('click', function () {
            window.setTimeout(function(){
                highlightNewComments(originalLastVisit);
            }, delayTimer);
        });
    }
};

window.setTimeout(function(){
    if (!isFirstVisit()) {
        highlightNewComments(originalLastVisit);
        reloadOnPagination();
    }
    setLastVisited();
}, delayTimer);

Todo list:

• don't run on first load (otherwise it highlights everything)
• prune the localStorage data occasionally (otherwise voracious readers will have tons of entries)
• show a counter (next to total count?) of new stuff in a thread
• make highlighting a bit prettier