Browser Extension window.onload workaround

window.onDOMscriptReady.js

(() => {
    let subscribers, observer;

    // natively declared <script> elements in html can't have onload= attribute
    // due to the default extension CSP that forbids inline code (and we don't want to relax it),
    // so we're using MutationObserver to add onload event listener to the script element to be loaded
    window.onDOMscriptReady = (srcSuffix, timeout = 1000) => 
    {
        if (!subscribers) 
        {
            subscribers = new Map();
            observer = new MutationObserver(observe);
            observer.observe(document.head, { childList: true });
        }
        return new Promise((resolve, reject) => 
        {
            const listeners = subscribers.get(srcSuffix);
            if (listeners)
                listeners.push(resolve);
            else
                subscribers.set(srcSuffix, [resolve]);

            // a resolved Promise won't reject anymore
            setTimeout(() => {
                emptyAfterCleanup(srcSuffix);
                reject(new Error('Timeout'));
            }, timeout);
        });
    };

    return;

    function observe(mutations) {
        for (const { addedNodes } of mutations) {
            for (const n of addedNodes) {
                if (n.src && getSubscribersForSrc(n.src)) {
                    n.addEventListener('load', notifySubscribers);
                }
            }
        }
    }

    function getSubscribersForSrc(src) {
        for (const [suffix, listeners] of subscribers.entries()) {
            if (src.endsWith(suffix)) {
                return { suffix, listeners };
            }
        }
    }

    function notifySubscribers(event) {
        this.removeEventListener('load', notifySubscribers);
        for (let data; (data = getSubscribersForSrc(this.src));) {
            data.listeners.forEach(fn => fn(event));
            if (emptyAfterCleanup(data.suffix)) {
                return;
            }
        }
    }

    function emptyAfterCleanup(suffix) {
        if (!subscribers) {
            return true;
        }
        subscribers.delete(suffix);
        if (!subscribers.size) {
            observer.disconnect();
            observer = null;
            subscribers = null;
            return true;
        }
    }
})();