GA4 Cookie Changes Monitor

ga4_cookie_monitor.js

/**
 * Monitor the GA4 _ga cookie via CookieStore.
 * @param {function} onChange - callback(oldValue, newValue) triggered when _ga changes
 */
function monitorGACookie(onChange) {
    if (!("cookieStore"in window))
        return;

    const COOKIE_NAME = "_ga";
    const STORAGE_KEY = "tracked_ga_cookie";

    // Check if analytics storage consent is granted
    function analyticsAllowed() {
        try {
            const consent = window.google_tag_data?.ics?.entries?.analytics_storage;
            return !!(consent?.update || consent?.default || consent?.implicit);
        } catch {
            return false;
        }
    }
    // Get current cookie value
    async function getCurrentGA() {
        const cookie = await cookieStore.get(COOKIE_NAME);
        return cookie ? cookie.value : null;
    }

    // Initialize tracking
    (async () => {
        if (!analyticsAllowed())
            return;

        const currentValue = await getCurrentGA();
        const storedValue = localStorage.getItem(STORAGE_KEY);

        if (storedValue !== currentValue) {
            onChange(storedValue, currentValue);
            localStorage.setItem(STORAGE_KEY, currentValue || "");
        }
    }
    )();

    // Listen for future changes
    cookieStore.addEventListener("change", async (event) => {
        if (!analyticsAllowed())
            return;

        for (const cookie of event.changed) {
            if (cookie.name === COOKIE_NAME) {
                const oldValue = localStorage.getItem(STORAGE_KEY);
                const newValue = cookie.value;
                if (oldValue !== newValue) {
                    onChange(oldValue, newValue);
                    localStorage.setItem(STORAGE_KEY, newValue);
                }
            }
        }

        for (const cookie of event.deleted) {
            if (cookie.name === COOKIE_NAME) {
                const oldValue = localStorage.getItem(STORAGE_KEY);
                onChange(oldValue, null);
                localStorage.setItem(STORAGE_KEY, "");
            }
        }
    }
    );
}