Fix Beacon API Error in Svelte

What Is the Beacon API Error?

Few errors are as confusing as Beacon API in Svelte. Here's what's actually going on and how to fix it.

Why It Happens

This happens when navigator.sendBeacon() fails, typically because of payload size limits or Content Security Policy restrictions.

The Fix

const payload = JSON.stringify(analyticsData);

if (new Blob([payload]).size > 64000) {
  // Too large for Beacon — use fetch keepalive
  fetch('/analytics', {
    method: 'POST', body: payload, keepalive: true,
    headers: { 'Content-Type': 'application/json' }
  });
} else {
  navigator.sendBeacon('/analytics', payload);
}

Common Mistakes to Avoid

Many developers make the mistake of silently catching this error without logging it, which makes debugging much harder later. Another common pitfall is applying a fix that works locally but fails in production due to environment differences. Always verify your fix works in a staging environment before deploying. Additionally, ensure your error handling doesn't mask the original error — preserve the stack trace and error message for future debugging sessions.

Prevention

With [Bugsly](https://bugsly.dev), you can monitor for this error in production and get alerted with full error context.

Key Takeaways

• Always handle this error gracefully with proper error handling
• Check your environment configuration
• Test thoroughly before deploying to production