Fix CORS Blocked Error in Svelte

What Is the CORS Blocked Error?

The dreaded CORS Blocked in Svelte can halt your progress if you don't know where to look. Let's fix it step by step.

Why It Happens

This occurs when the server doesn't include the correct Access-Control-Allow-Origin headers, blocking cross-origin requests from the browser.

The Fix

// Add CORS middleware on the server
app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', 'http://localhost:3000');
  res.header('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE');
  res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  if (req.method === 'OPTIONS') return res.sendStatus(200);
  next();
});

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