# Gotchas & Best Practices

## Common Errors

### 1000: "Snippet execution failed"

Runtime error or syntax error. Wrap code in try/catch:

```javascript
try {
  return await fetch(request)
} catch (error) {
  return new Response(`Error: ${error.message}`, { status: 500 })
}
```

### 1100: "Exceeded execution limit"

Code takes >5ms CPU. Simplify logic or move to Workers.

### 1201: "Multiple origin fetches"

Call `fetch(request)` exactly once:

```javascript
// ❌ Multiple origin fetches
const r1 = await fetch(request)
const r2 = await fetch(request)
// ✅ Single fetch, reuse response
const response = await fetch(request)
```

### 1202: "Subrequest limit exceeded"

Pro: 2 subrequests, Business/Enterprise: 5. Reduce fetch calls.

### "Cannot set property on immutable object"

Clone before modifying:

```javascript
const modifiedRequest = new Request(request)
modifiedRequest.headers.set('X-Custom', 'value')
```

### "caches is not defined"

Cache API NOT available in Snippets. Use Workers.

### "Module not found"

Snippets don't support `import`. Use inline code or Workers.

## Best Practices

### Performance

- Keep code <10KB (32KB limit)
- Optimize for 5ms CPU
- Clone only when modifying
- Minimize subrequests

### Security

- Validate all inputs
- Use Web Crypto API for hashing
- Sanitize headers before origin
- Don't log secrets

### Debugging

```javascript
newResponse.headers.set('X-Debug-Country', request.cf.country)
```

```bash
curl -H "X-Test: true" https://example.com -v
```

## Available APIs

**✅ Available:** `fetch()`, `Request`, `Response`, `Headers`, `URL`, `crypto.subtle`, `crypto.randomUUID()`, `atob()`/`btoa()`, `JSON`

**❌ NOT Available:** `caches`, `KV`, `D1`, `R2`, `Durable Objects`, `WebSocket`, `HTMLRewriter`, `import`, Node.js APIs

## Limits

| Resource              | Limit   |
| --------------------- | ------- |
| Snippet size          | 32KB    |
| Execution time        | 5ms CPU |
| Subrequests (Pro/Biz) | 2/5     |
| Snippets/zone         | 20      |

## Performance Benchmarks

| Operation   | Time    |
| ----------- | ------- |
| Header set  | <0.1ms  |
| URL parsing | <0.2ms  |
| fetch()     | 1-3ms   |
| SHA-256     | 0.5-1ms |

**Migrate to Workers when:** >5ms needed, >5 subrequests, need storage (KV/D1/R2), need npm packages, >32KB code
