Monitors document body for duplicate IDs and warns in console when that happens (potential for chrome extension)

duplicateIdsMonitoring.js

(function (window) {

    // Based on https://stackoverflow.com/a/14570614/588973
    window.observeDOM = (function () {
        var MutationObserver = window.MutationObserver || window.WebKitMutationObserver,
            eventListenerSupported = window.addEventListener;

        return function (obj, callback) {
            if (MutationObserver) {
                // define a new observer
                var obs = new MutationObserver(function (mutations) {
                    mutations.every(function(mutation) {
                        if (mutation.addedNodes.length || mutation.removedNodes.length || mutation.attributeName !== null) {
                            callback();

                            return false;
                        }

                        return true;
                    });
                });
                // have the observer observe foo for changes in children
                //noinspection JSCheckFunctionSignatures
                obs.observe(obj, {attributes: true, childList: true, subtree: true});
            }
            else if (eventListenerSupported) {
                obj.addEventListener('DOMNodeInserted', callback, false);
                obj.addEventListener('DOMNodeRemoved', callback, false);
                obj.addEventListener('DOMAttrModified', callback, false);
            }
        };
    })();

    // Based on https://stackoverflow.com/a/36129568/588973
    window.checkForDuplicateIds = function (container) {
        var elms = container.querySelectorAll('*[id]'),
            elmsById = {},
            i = 0,
            len = elms.length,
            id;
        for (i; i < len; i++) {
            if (typeof elmsById[elms[i].id] === 'undefined') {
                elmsById[elms[i].id] = [];
            }
            elmsById[elms[i].id].push(elms[i]);
        }
        for (id in elmsById) {
            if (!elmsById.hasOwnProperty(id)) continue;
            if (elmsById[id].length > 1) {
                console.warn('Multiple elements with same ID (', id, ') detected', elmsById[id]);
            }
        }
        console.info('checkForDuplicateIds finished');
    };
})(top.window);

// Init monitoring
(function (document) {
    // Give breathing room before start checking
    setTimeout(function () {

        // Do initial checking
        checkForDuplicateIds(document.body);

        // Observer body for repeated checking
        observeDOM(
            document.body,
            function () {
                checkForDuplicateIds(document.body);
            }
        );
    }, 2000);
})(document);