Google Ads Script Errors in Sentry for React Apps

Problem Statement

When integrating Google Ads (gtag.js) into older React applications (5-7+ years), you may encounter recurring Sentry errors like "Can't find variable: gmo" or "invalid or unexpected token". These occur despite the application continuing to function normally. Errors trace back to the Google Tag Manager script snippet in index.html and are characterized by:

• Non-breaking nature: Users experience no app crashes or visible issues
• External origin: Errors originate from the Google Ads JavaScript
• False positives: Errors clutter your monitoring without indicating app defects

Solutions

Approach 1: Ignore Errors in Sentry Configuration (Recommended)

Configure Sentry to exclude Google Ads-related errors using the ignoreErrors option:

import * as Sentry from '@sentry/react';

Sentry.init({
  dsn: process.env.REACT_APP_SENTRY_DSN,
  ignoreErrors: [
    "Can't find variable: gmo",
    "invalid or unexpected token"
  ],
  // Optional: Also ignore by script origin
  denyUrls: [
    /googletagmanager\.com/,
    /google-analytics\.com/
  ]
});

Why this works:

• Prevents noise from third-party scripts
• Maintains focus on application-specific errors
• Handles errors Safely without impacting user experience

WARNING

Use message-based filtering only if:

1. Errors originate exclusively from external scripts
2. You've confirmed error messages won't conflict with app errors

Approach 2: Advanced Filtering with beforeSend

For more control, use Sentry's beforeSend hook to filter programmatically:

Sentry.init({
  dsn: process.env.REACT_APP_SENTRY_DSN,
  beforeSend(event) {
    const isGoogleScriptError = event.exception?.values?.some(exception => 
      // Match error messages
      (exception.value?.includes("gmo") || 
      exception.value?.includes("invalid token")) &&
      // Verify origin in stack trace
      exception.stacktrace?.frames?.some(frame => 
        frame.filename?.match(/googletagmanager\.com|google-analytics\.com/)
      )
    );
    return isGoogleScriptError ? null : event;
  }
});

Key Explanations

Why these errors occur:

• gmo reference likely comes from Google Marketing Objects scripts
• invalid token often relates to URL parameter handling in ad tracking
• Race conditions between script loading and execution
• Google script modifications outside your control

Why not "fix" the scripts:

• Defining window.gmo = {} adds unsupported invariants
• Google's