--- title: "Fixing ReferenceError: sealmetrics is not defined" description: "How to keep your site working when ad blockers, privacy extensions, or DNS filters prevent the Sealmetrics tracker from loading." canonical_url: "https://docs.sealmetrics.com/troubleshooting/sealmetrics-is-not-defined" lang: "en" date_generated: "2026-09-01T17:22:38.784Z" source_hash: "8efada930304d5908502a626ac9436a4f0bd3fcf183df727cd9bec2c9ed773b4" content_type: "documentation" owner: "docs" llm_priority: "useful" source_file: "troubleshooting/sealmetrics-is-not-defined.mdx" publisher: "Sealmetrics" --- # Fixing ReferenceError: sealmetrics is not defined Canonical page: https://docs.sealmetrics.com/troubleshooting/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: ```javascript 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. --- ## The solution: stub buffer in `` (RECOMMENDED) Inject this snippet once, inline in the ``, **before any call to `sealmetrics.*`** and **before** the main tracker script: ```html ``` ### 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 `` 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 ``, etc.), wrap each tracker call with a check: ```javascript if (typeof window.sealmetrics !== "undefined" && typeof window.sealmetrics.micro === "function") { sealmetrics.micro("Add_to_cart", { product_id: {{Product ID}}, product_name: {{Product Name}}, category: {{Product Category}}, page_title: {{Page Title}} }); } ``` The `{{...}}` placeholders are GTM variable references — replace them with your own Data Layer variables. For conversions: ```javascript 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 | Aspect | Stub buffer (HEAD) | Per-call guard | |------------------------------------------------|----------------------------|--------------------------------------------| | Install effort | Once | Once per call | | Maintenance | None | High (every new tag needs the `if`) | | Events lost if the script loads late | No (queued and drained) | Yes (calls before load are lost) | | Size | ~180 bytes gzip | 0 | | Recommended | Yes | Only 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. --- ## Related - [Tracker installation](/implementation/tracker/installation) — base setup, including the pre-load queue stub for deferred loading. - [Tracker API reference](/implementation/tracker/api-reference) — full `sealmetrics()`, `conv()`, and `micro()` signatures. - [Google Tag Manager integration](/integrations/google-tag-manager) — recommended setup for GTM. - [Conversions Don't Match Your ERP, CRM, or Database](/troubleshooting/erp-crm-database-discrepancy) — diagnose substantial discrepancies between Sealmetrics and your source of truth.