How to Fix Sentry Errors “Variable Not Found: gmo” and “Invalid or Unexpected Token” in Your React App with Google Ads

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If you’ve recently added a Google Ads snippet to your React app and started seeing Sentry errors like “variable not found: gmo” and “invalid or unexpected token”, you’re probably scratching your head wondering what went wrong. It’s like inviting a new friend over and suddenly your house alarms start blaring! These errors popped up in Sentry for a 7-8-year-old React app built with Create React App, as reported on Stack Overflow on February 11, 2025, right after Google Ads went live. Don’t worry—I’m here to break down why this happens, what these errors mean, and how to fix the Sentry errors gmo unexpected token Google Ads React issue step-by-step. Think of this as your troubleshooting buddy guiding you back to a quiet, error-free app. Let’s dive in!

What Are These Sentry Errors?

Picture your React app humming along, and then you plug in Google Ads to boost some search traffic. Next thing you know, Sentry—your trusty error watchdog—starts barking about two issues:

  • “Variable not found: gmo”: This means something in your app is looking for a variable called gmo, but it’s nowhere to be found—like calling a friend who didn’t show up.
  • “Invalid or unexpected token”: This is a syntax hiccup, where the JavaScript parser trips over something it doesn’t understand, like a random comma in a sentence.

These errors started at a steady rate after adding the Google Ads snippet to index.html, according to the Stack Overflow post. The good news? They’re not crashing your app—users keep clicking away, no error pages needed. The bad news? They’re cluttering Sentry and might hint at deeper issues.

The Google Ads Snippet in Question

Here’s what’s likely in your index.html (with a fake ID for safety):

<!-- For Google Ads to be run on Search (not on the website) -->
<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=AW-123456789"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag() { dataLayer.push(arguments); }
  gtag('js', new Date());
  gtag('config', 'AW-123456789');
</script>
<!-- End -->

This is standard Google Ads code, loading gtag.js and setting up tracking. So why’s it triggering Sentry?

Why These Errors Happen with Google Ads

Let’s play detective. These errors tie back to how Google Ads interacts with your old-school React app.

“Variable not found: gmo”

  • What’s gmo?: Some guess it’s “Google Marketing Object” (per Stack Overflow, February 11, 2025), but Google’s official docs (developers.google.com/tag-platform/gtagjs) don’t mention it. It might be an internal or third-party script expecting gmo to exist.
  • Why Now?: Adding gtag.js might load extra scripts (like ad trackers) that assume gmo is defined. In a 7-8-year-old app, this could clash with outdated globals or missing polyfills.
  • Sentry Clue: The error’s non-critical—your app keeps running, suggesting it’s a side effect, not a core break.

“Invalid or unexpected token”

  • Syntax Trouble: This points to a parsing error, maybe from gtag.js injecting code that React’s build (via Webpack/Babel) chokes on—like malformed JSON or a stray character.
  • Query Params: The Google Ads snippet might add URL parameters (e.g., gclid), and if your app doesn’t handle them, it could misfire somewhere downstream.

The Connection

Both errors started post-Google Ads launch (DevGem.io, February 12, 2025), and Sentry traces them to scripts tied to index.html. It’s not breaking your app, but it’s noisy enough to bug you in Sentry.

Step 1: Check for the gmo Variable

First, let’s hunt for this mysterious gmo.

Search Your Code

  • Open your project in VS Code (or whatever you use).
  • Search globally for gmo (Ctrl+Shift+F or Cmd+Shift+F).
  • Look in:
  • index.html
  • src/**/*.js (all JavaScript files)
  • Any third-party scripts or plugins.

If You Find It

  • Undefined?: If it’s used but not defined (e.g., console.log(gmo)), add a fallback:
  const gmo = window.gmo || {};
  • Old Code?: If it’s in legacy code, it might predate Google Ads—comment it out and test.

If You Don’t Find It

  • It’s likely from gtag.js or a downstream script. A DevGem.io post (February 12, 2025) suggests initializing it manually:
  <script>
    window.gmo = window.gmo || {}; // Add this line
    window.dataLayer = window.dataLayer || [];
    function gtag() { dataLayer.push(arguments); }
    gtag('js', new Date());
    gtag('config', 'AW-123456789');
  </script>
  • Add that line, run npm start, and check Sentry. Fixed? Great!

Step 2: Handle Search Parameters

The “unexpected token” might link to URL params like gclid (Google Click ID) from Ads. Your app, built with minimal query param use, might not handle them well.

Add useSearchParams

In App.js (or your main component):

import { useEffect } from "react";
import { useSearchParams } from "react-router-dom"; // If using React Router

function App() {
  const [searchParams] = useSearchParams();

  useEffect(() => {
    if (searchParams.get("gclid")) {
      console.log("Ad click detected:", searchParams.get("gclid"));
      // Optional: localStorage.setItem("gclid", searchParams.get("gclid"));
    }
  }, [searchParams]);

  return <div>Your app here</div>;
}

export default App;
  • Install: If you don’t have React Router, add it:
  npm install react-router-dom
  • Why: This catches gclid params cleanly, preventing syntax errors from unhandled URL junk.

Step 3: Update Your Google Ads Snippet

The stock snippet might be fine, but let’s tweak it for safety.

Modified Snippet

<script async src="https://www.googletagmanager.com/gtag/js?id=AW-123456789"></script>
<script>
  window.gmo = window.gmo || {}; // Safety net for gmo
  window.dataLayer = window.dataLayer || [];
  function gtag() {
    try {
      dataLayer.push(arguments); // Wrap in try-catch
    } catch (e) {
      console.error("gtag error:", e);
    }
  }
  gtag('js', new Date());
  gtag('config', 'AW-123456789');
</script>
  • Try-Catch: Catches any gtag hiccups, keeping Sentry quieter.
  • window.gmo: Predefines it, dodging the “not found” error.

Run npm start and watch Sentry. Progress?

Step 4: Filter Sentry Noise

If the errors persist but don’t break anything, let’s tell Sentry to chill.

Ignore in Sentry

  • Go to Sentry Project Settings > Inbound Filters.
  • Add:
  • “Filter by message”: variable not found: gmo
  • “Filter by message”: invalid or unexpected token
  • Save and wait—new instances should drop off.

Why It Works

These are non-critical (no fallback pages in Sentry Replays, per Stack Overflow, February 11, 2025). Ignoring them clears the noise while you dig deeper.

Step 5: Debug Like a Pro

Still seeing errors? Time to sleuth.

Check Sentry Details

  • Open an error in Sentry.
  • Look at:
  • Stack Trace: Points to index.html or a script from googletagmanager.com?
  • User Agent: Mostly mobile? Could be ad-related browser quirks.
  • URL: Any gclid params?

Test Locally

  • Add the snippet to your dev setup.
  • Open Chrome DevTools (F12) > Console.
  • Look for gmo errors or syntax issues when the page loads.

Example Finding

If you see Uncaught ReferenceError: gmo is not defined in the console, Step 3’s window.gmo fix should nail it.

Why This Happens

Google Ads’ gtag.js is a black box—it loads extra scripts for tracking that might expect gmo or inject messy code. A 7-8-year-old Create React App might lack modern error handling (e.g., no useSearchParams), making it fragile against this. The DevGem.io post (February 12, 2025) suggests it’s a common clash with legacy apps post-Ads integration.

Prevent It Next Time

  • Update React: Move from Create React App to Vite or Next.js for better modern support.
  • Test Ads Early: Add the snippet in dev first, watch Sentry.
  • Sentry Rules: Filter out low-impact errors upfront.

Wrapping Up: Silence Those Sentry Alerts!

The Sentry errors “variable not found: gmo” and “invalid or unexpected token” from your Google Ads snippet are annoying but fixable. Define gmo, handle URL params, tweak your snippet, filter Sentry, and debug smart—you’ll quiet those alerts fast. Your React app’s old-school roots don’t have to trip you up. So, tweak that index.html, fire up your dev server, and enjoy an error-free Sentry dashboard.

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