error-handling

1# Error Handling & Observability
2 
3### When to Load
4 
5- **Trigger**: Try/catch patterns, retry logic, error responses, circuit breakers, structured logging
6- **Skip**: No error handling or observability involved in the current task
7 
8## Error Handling Workflow
9 
10Copy this checklist and track progress:
11 
12```
13Error Handling Progress:
14- [ ] Step 1: Define error taxonomy (categories and severity)
15- [ ] Step 2: Implement error handling by layer
16- [ ] Step 3: Set up structured logging
17- [ ] Step 4: Add retry and circuit breaker patterns
18- [ ] Step 5: Configure error tracking service
19- [ ] Step 6: Define user-facing error messages
20- [ ] Step 7: Validate against anti-patterns checklist
21```
22 
23## Error Handling Patterns by Language
24 
25### JavaScript / TypeScript
26 
27```typescript
28// Custom error hierarchy
29class AppError extends Error {
30  constructor(
31    message: string,
32    public statusCode: number = 500,
33    public code: string = "INTERNAL_ERROR",
34    public isOperational: boolean = true,
35  ) {
36    super(message);
37    this.name = this.constructor.name;
38  }
39}
40class NotFoundError extends AppError {
41  constructor(resource: string, id: string) {
42    super(`${resource} with id ${id} not found`, 404, "NOT_FOUND");
43  }
44}
45class ValidationError extends AppError {
46  constructor(public errors: Record<string, string[]>) {
47    super("Validation failed", 400, "VALIDATION_ERROR");
48  }
49}
50 
51// WRONG: Swallowing errors silently
52try {
53  await saveUser(data);
54} catch (e) {
55  // nothing here -- bug hides forever
56}
57…