Introduction

Cloudflare Workers run JavaScript at Cloudflare's edge, but script errors, resource limits, and deployment issues can cause failures. Workers have strict constraints: 10ms CPU time for free plans (50ms for paid), 128MB memory limit, and specific API restrictions. When a Worker fails, you may see 5xx errors, timeout messages, or unexpected behavior. Debugging requires checking logs, understanding limits, and testing locally before deployment.

Symptoms

  • Error 1101: Worker threw exception
  • Error 522/524: Worker timeout contacting origin
  • Worker script not executing or returning unexpected results
  • "Worker exceeded resource limits" error
  • Deployment fails with validation errors
  • Worker runs but modifies response incorrectly
  • Requests hang or return empty responses

Common Causes

  • JavaScript exception in Worker script (undefined variable, syntax error)
  • Worker CPU time limit exceeded (free: 10ms, paid: 50ms, unlimited: 30s)
  • Memory limit exceeded (128MB maximum)
  • Missing event handler or incorrect fetch response
  • Infinite loop or blocking synchronous code
  • Incorrect Workers KV or Durable Objects usage
  • Invalid wrangler.toml configuration
  • Subrequest timeout waiting for origin or external API

Step-by-Step Fix

  1. 1.Check Worker logs for error details:

```bash # View real-time Worker logs wrangler tail

# Or via dashboard: Workers & Pages > your-worker > Logs

# Look for exception messages and stack traces ```

  1. 1.Test Worker locally before deployment:

```bash # Run Worker locally with wrangler wrangler dev

# Test with curl curl http://localhost:8787/

# Debug with console.log (visible in wrangler dev output) ```

  1. 1.Handle exceptions properly in Worker:

```javascript // Good: Catch all errors export default { async fetch(request, env, ctx) { try { // Your Worker logic return new Response("Hello World"); } catch (error) { // Log error for debugging console.error("Worker error:", error.message);

// Return proper error response return new Response(Error: ${error.message}, { status: 500, headers: { "Content-Type": "text/plain" } }); } } }; ```

  1. 1.Fix common JavaScript errors:

```javascript // Check for common mistakes:

// Undefined variable - always declare let result = someFunction(); // Define function first

// Missing await for async operations const response = await fetch(url); // Must await

// Incorrect Response construction return new Response(body, { status: 200 }); // Status in options object

// Accessing undefined properties const value = obj?.property || "default"; // Use optional chaining ```

  1. 1.Optimize for CPU time limits:

```javascript // Reduce CPU usage:

// Use streaming instead of buffering entire body const stream = request.body; return new Response(stream);

// Avoid heavy computation in Worker // Move to Durable Objects or external service

// Cache expensive operations const cached = await caches.default.match(request); if (cached) return cached; ```

  1. 1.Handle subrequest timeouts:

```javascript // Set timeout for fetch requests async function fetchWithTimeout(url, timeoutMs = 5000) { const controller = new AbortController(); const timeout = setTimeout(() => controller.abort(), timeoutMs);

try { const response = await fetch(url, { signal: controller.signal }); clearTimeout(timeout); return response; } catch (error) { clearTimeout(timeout); if (error.name === 'AbortError') { return new Response('Request timeout', { status: 504 }); } throw error; } } ```

  1. 1.Check memory usage:

```javascript // Avoid memory-intensive operations:

// Don't load large files into memory // Use streaming instead const response = await fetch(url); return new Response(response.body); // Stream body directly

// Don't accumulate large arrays // Process in chunks or use generators

// Clear unnecessary references let largeData = null; // After use, allow GC ```

  1. 1.Validate wrangler.toml configuration:

```toml # Check wrangler.toml for errors name = "my-worker" main = "src/index.js" compatibility_date = "2024-01-01"

# For Workers with KV [[kv_namespaces]] binding = "KV" id = "your_kv_namespace_id"

# For Workers with Durable Objects [[durable_objects.bindings]] name = "MY_DO" class_name = "MyDurableObject"

# Check script path matches actual file location ```

  1. 1.Deploy and verify:

```bash # Deploy Worker wrangler deploy

# Verify deployment wrangler deployments list

# Test deployed Worker curl https://your-worker.your-subdomain.workers.dev/

# Or test on your zone curl https://yourdomain.com/ -H "Host: yourdomain.com" ```

  1. 1.Use Durable Objects for stateful operations:

```javascript // For operations needing more CPU time or state export class MyDurableObject { constructor(state, env) { this.state = state; }

async fetch(request) { // Durable Objects have 30s CPU limit (unlimited plan) // Can maintain state between requests const storage = this.state.storage; await storage.put("key", "value"); return new Response("Stored"); } } ```

Verification

After applying fixes:

  1. 1.wrangler tail shows no exceptions or errors
  2. 2.curl to Worker endpoint returns expected response
  3. 3.Worker completes within CPU time limits
  4. 4.Memory usage stays under 128MB
  5. 5.Subrequests complete without timeout
  6. 6.Deployment succeeds without validation errors