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Fixing ReferenceError: sealmetrics is not defined

A guide for the customer's IT / development team. It resolves the ReferenceError: sealmetrics is not defined that appears when a blocker prevents the tracker from loading.

The problem​

When a visitor has an ad blocker, privacy extension, corporate firewall, or DNS filter active (uBlock Origin, AdGuard, Brave Shields, NextDNS, Pi-hole, etc.), the browser will not download t.sealmetrics.com/t.js. The window.sealmetrics variable is never created.

If your code (for example, a GTM tag) calls the API directly:

sealmetrics.micro("Add_to_cart", { ... });

the browser throws ReferenceError: sealmetrics is not defined. That error halts execution of the JavaScript block where the call lives. If that block also contains UI, form, or navigation logic, the page breaks for the user.

This is not a tracker bug. It is the normal consequence of invoking any third-party library (GA4, Meta Pixel, Hotjar, SealMetrics…) without protecting yourself against the "script blocked" case.

Inject this snippet once, inline in the <head>, before any call to sealmetrics.* and before the main tracker script:

<script>
!function(w){w.sealmetrics=w.sealmetrics||function(){(w.sealmetrics.q=w.sealmetrics.q||[]).push(['pv',arguments])};w.sealmetrics.q=w.sealmetrics.q||[];w.sealmetrics.conv=w.sealmetrics.conv||function(){w.sealmetrics.q.push(['cv',arguments])};w.sealmetrics.micro=w.sealmetrics.micro||function(){w.sealmetrics.q.push(['mc',arguments])}}(window);
</script>
<script src="https://t.sealmetrics.com/t.js?id=YOUR_ACCOUNT_ID" defer></script>

What it does​

  • Creates window.sealmetrics, sealmetrics.conv, and sealmetrics.micro as empty functions that enqueue calls into sealmetrics.q.
  • If the real script loads: it drains the queue in FIFO order and sends every pending event.
  • If the real script is blocked: the calls are absorbed silently. Zero console errors. Zero broken pages.

Characteristics​

  • Size: ~180 bytes gzipped. No Core Web Vitals impact.
  • Idempotent: pasting it twice does nothing. The || guards take care of that.
  • No dependencies: ES5, compatible with every browser.
  • Industry-standard pattern, same as fbq (Meta Pixel), gtag (Google), _paq (Matomo).

Installing it from GTM​

If you do not have access to the site HTML and everything is managed from GTM:

  1. Create a new Custom HTML tag.
  2. Paste the stub snippet (the <script>!function(w){...}(window);</script> block).
  3. Trigger: Initialization - All Pages (gtm.init). This is the earliest GTM event.
  4. Priority: maximum (for example, 1000). This guarantees the stub runs before any other tag that calls sealmetrics.*.
  5. Publish the container.

From that point on, every other tag can call sealmetrics.micro(...) or sealmetrics.conv(...) directly, without wrapping each call in a typeof guard.

Alternative solution: per-call guard​

If for any reason you cannot inject the stub (restrictive policies, no access to <head>, etc.), wrap each tracker call with a check:

if (typeof window.sealmetrics !== "undefined" && typeof window.sealmetrics.micro === "function") {
  sealmetrics.micro("Add_to_cart", {
    module_title:     google_tag_manager["rm"]["208828905"](16),
    click_text:       google_tag_manager["rm"]["208828905"](17),
    page_title:       google_tag_manager["rm"]["208828905"](18),
    page_subcategory: google_tag_manager["rm"]["208828905"](19),
    cookies:          google_tag_manager["rm"]["208828905"](20),
    type_appointment: google_tag_manager["rm"]["208828905"](21)
  });
}

For conversions:

if (typeof window.sealmetrics !== "undefined" && typeof window.sealmetrics.conv === "function") {
  sealmetrics.conv("purchase", 89.99, { currency: "EUR" });
}

When to use this option​

  • Only when you cannot inject the stub.
  • Assume the if has to be repeated in every GTM tag that calls the tracker. With many events, the maintenance cost grows fast.

Summary table​

AspectStub buffer (HEAD)Per-call guard
Install effortOnceOnce per call
MaintenanceNoneHigh (every new tag needs the if)
Events lost if the script loads lateNo (queued and drained)Yes (calls before load are lost)
Size~180 bytes gzip0
RecommendedYesOnly as a fallback

What NOT to do​

  • Do not disable the tracker. The error does not come from the tracker — it comes from invoking it without protection. Disabling it fixes nothing structurally and leaves you without measurement.
  • Do not wrap every call in try/catch. It works, but it hides other legitimate errors in the same block. The typeof guard is cleaner.
  • Do not load the tracker with async and assume it is available immediately. Even when it loads, there is a window of time during which window.sealmetrics does not exist yet. The stub solves this without effort.
  • Do not paste the stub inside an iframe (for example, a Shopify Web Pixel). The stub must live on the host page so the queued calls are drained by the tracker loaded in the same context.

Verification​

After installing the stub:

  1. Open the site in a browser with uBlock Origin (or any blocker) active and filtering sealmetrics.com.
  2. Open the browser console.
  3. Navigate and fire events (clicks, conversions, etc.).
  4. ReferenceError: sealmetrics is not defined must not appear.
  5. The page must continue working normally (forms, navigation, UI).

Then disable the blocker and repeat: the calls must reach the SealMetrics backend exactly as before.