Suggestion on content-script TODO: PERFORMANCE ISSUE

I've tested the code and seems to be work fine, please see if you can use it in your next release. Thanks.
        function attachEvent() {

            const cacheRule = [
                'textarea',
                'iframe',
                '[contentEditable]',
                '[role="textbox"]',
                '[aria-multiline="true"]'
            ].map(rule => (rule + `:not([${SAVE_TARGET}])`) ).join(',');

            attacheEventTimer = null;
            const editables = document.querySelectorAll(cacheRule); 
            console.log(window.location.href + " attachEvent " +
                        "editables.length:" + editables.length);
            for (const ta of editables) {
                const rn = Math.random();
                const isTEXTAREA = ta.tagName === 'TEXTAREA';
                // Mark ta with SAVE_TARGET so that script will not
                // another keyup event listener to it.
                ta.setAttribute(SAVE_TARGET, true);
                ta.dataset.tcId = isTEXTAREA ? rn : `w-${rn}`;
                ta.addEventListener('keyup', me.saveToStorage);
            }
        }

        // TODO: PERFORMANCE ISSUE
        //    Some textarea might not appear when document finished loading,
        //    but appear when user does something, code here is used to
        //    check every two seconds.
        // browser.runtime.sendMessage({behavior: 'get_options'}).
        // then(setting =>
        //   window.setInterval(attachEvent, setting.intervalToSave));
        browser.runtime.sendMessage({behavior: 'get_options'}).
        then(setting => {
             const timeout = setting.intervalToSave * 2;
             console.log(window.location.href +
                         " attacheEventTimer(" + timeout + ")");
             attacheEventTimer = setTimeout(attachEvent, timeout);
             obseveDOM(document.body, _ => {
                 // Use a timer so that if there is rapid change of the DOM
                 // the script will not call attachEvent repeatedly
                 if (!attacheEventTimer) {
                     attacheEventTimer = setTimeout(attachEvent, timeout);
                 }
             });
        });
    },

let attacheEventTimer;

// https://stackoverflow.com/questions/3219758/detect-changes-in-the-dom
// https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
const MutationObserver = window.MutationObserver ||
        window.WebKitMutationObserver;

function obseveDOM(obj, callback) {
    if (obj && obj.nodeType === 1) {
        // https://developer.mozilla.org/en-US/docs/Web/API/Element
        // Can only obser nodeType === Node.ELEMENT_NODE (1)
        const observer = new MutationObserver(callback);
        // start observing obj for changes in children
        observer.observe(obj, { childList: true, subtree: true });
        return observer; // can call objserver.disconnect();
    }
}
wildskyf / TextareaCache

Suggestion on content-script TODO: PERFORMANCE ISSUE #112
