azusagasaku Error Handling Patterns
Consistent, robust error handling patterns for production applications.
When to Activate
- Designing error types or exception hierarchies for a new module or service
- Adding retry logic or circuit breakers for unreliable external dependencies
- Reviewing API endpoints for missing error handling
- Implementing user-facing error messages and feedback
- Debugging cascading failures or silent error swallowing
Core Principles
- Fail fast and loudly — surface errors at the boundary where they occur; don't bury them
- Typed errors over string messages — errors are first-class values with structure
- User messages ≠ developer messages — show friendly text to users, log full context server-side
- Never swallow errors silently — every
catchblock must either handle, re-throw, or log - Errors are part of your API contract — document every error code a client may receive
TypeScript / JavaScript
Typed Error Classes
// Define an error hierarchy for your domain
export class AppError extends Error {
constructor(
message: string,
public readonly code: string,
public readonly statusCode: number = 500,
public readonly details?: unknown,
) {
super(message)
this.name = this.constructor.name
// Maintain correct prototype chain in transpiled ES5 JavaScript.
// Required for `instanceof` checks (e.g., `error instanceof NotFoundError`)
// to work correctly when extending the built-in Error class.
Object.setPrototypeOf(this, new.target.prototype)
}
}
export class NotFoundError extends AppError {
constructor(resource: string, id: string) {
super(`${resource} not found: ${id}`, 'NOT_FOUND', 404)
}
}
export class ValidationError extends AppError {
constructor(message: string, details: { field: string; message: string }[]) {
super(message, 'VALIDATION_ERROR', 422, details)
}
}
export class UnauthorizedError extends AppError {
constructor(reason = 'Authentication required') {
super(reason, 'UNAUTHORIZED', 401)
}
}
export class RateLimitError extends AppError {
constructor(public readonly retryAfterMs: number) {
super('Rate limit exceeded', 'RATE_LIMITED', 429)
}
}
Result Pattern (no-throw style)
For operations where failure is expected and common (parsing, external calls):
type Result<T, E = AppError> =
| { ok: true; value: T }
| { ok: false; error: E }
function ok<T>(value: T): Result<T> {
return { ok: true, value }
}
function err<E>(error: E): Result<never, E> {
return { ok: false, error }
}
// Usage
async function fetchUser(id: string): Promise<Result<User>> {
try {
const user = await db.users.findUnique({ where: { id } })
if (!user) return err(new NotFoundError('User', id))
return ok(user)
} catch (e) {
return err(new AppError('Database error', 'DB_ERROR'))
}
}
const result = await fetchUser('abc-123')
if (!result.ok) {
// TypeScript knows result.error here
logger.error('Failed to fetch user', { error: result.error })
return
}
// TypeScript knows result.value here
console.log(result.value.email)
API Error Handler (Next.js / Express)
import { NextRequest, NextResponse } from 'next/server'
function handleApiError(error: unknown): NextResponse {
// Known application error
if (error instanceof AppError) {
return NextResponse.json(
{
error: {
code: error.code,
message: error.message,
...(error.details ? { details: error.details } : {}),
},
},
{ status: error.statusCode },
)
}
// Zod validation error
if (error instanceof z.ZodError) {
return NextResponse.json(
{
error: {
code: 'VALIDATION_ERROR',
message: 'Request validation failed',
details: error.issues.map(i => ({
field: i.path.join('.'),
message: i.message,
})),
},
},
{ status: 422 },
)
}
// Unexpected error — log details, return generic message
console.error('Unexpected error:', error)
return NextResponse.json(
{ error: { code: 'INTERNAL_ERROR', message: 'An unexpected error occurred' } },
{ status: 500 },
)
}
export async function POST(req: NextRequest) {
try {
// ... handler logic
} catch (error) {
return handleApiError(error)
}
}
React Error Boundary
import { Component, ErrorInfo, ReactNode } from 'react'
interface Props {
fallback: ReactNode
onError?: (error: Error, info: ErrorInfo) => void
children: ReactNode
}
interface State {
hasError: boolean
error: Error | null
}
export class ErrorBoundary extends Component<Props, State> {
state: State = { hasError: false, error: null }
static getDerivedStateFromError(error: Error): State {
return { hasError: true, error }
}
componentDidCatch(error: Error, info: ErrorInfo) {
this.props.onError?.(error, info)
console.error('Unhandled React error:', error, info)
}
render() {
if (this.state.hasError) return this.props.fallback
return this.props.children
}
}
// Usage
<ErrorBoundary fallback={<p>Something went wrong. Please refresh.</p>}>
<MyComponent />
</ErrorBoundary>
Python
Custom Exception Hierarchy
class AppError(Exception):
"""Base application error."""
def __init__(self, message: str, code: str, status_code: int = 500):
super().__init__(message)
self.code = code
self.status_code = status_code
class NotFoundError(AppError):
def __init__(self, resource: str, id: str):
super().__init__(f"{resource} not found: {id}", "NOT_FOUND", 404)
class ValidationError(AppError):
def __init__(self, message: str, details: list[dict] | None = None):
super().__init__(message, "VALIDATION_ERROR", 422)
self.details = details or []
FastAPI Global Exception Handler
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
app = FastAPI()
@app.exception_handler(AppError)
async def app_error_handler(request: Request, exc: AppError) -> JSONResponse:
return JSONResponse(
status_code=exc.status_code,
content={"error": {"code": exc.code, "message": str(exc)}},
)
@app.exception_handler(Exception)
async def generic_error_handler(request: Request, exc: Exception) -> JSONResponse:
# Log full details, return generic message
logger.exception("Unexpected error", exc_info=exc)
return JSONResponse(
status_code=500,
content={"error": {"code": "INTERNAL_ERROR", "message": "An unexpected error occurred"}},
)
Go
Sentinel Errors and Error Wrapping
package domain
import "errors"
// Sentinel errors for type-checking
var (
ErrNotFound = errors.New("not found")
ErrUnauthorized = errors.New("unauthorized")
ErrConflict = errors.New("conflict")
)
// Wrap errors with context — never lose the original
func (r *UserRepository) FindByID(ctx context.Context, id string) (*User, error) {
user, err := r.db.QueryRow(ctx, "SELECT * FROM users WHERE id = $1", id)
if errors.Is(err, sql.ErrNoRows) {
return nil, fmt.Errorf("user %s: %w", id, ErrNotFound)
}
if err != nil {
return nil, fmt.Errorf("querying user %s: %w", id, err)
}
return user, nil
}
// At the handler level, unwrap to determine response
func (h *Handler) GetUser(w http.ResponseWriter, r *http.Request) {
user, err := h.service.GetUser(r.Context(), chi.URLParam(r, "id"))
if err != nil {
switch {
case errors.Is(err, domain.ErrNotFound):
writeError(w, http.StatusNotFound, "not_found", err.Error())
case errors.Is(err, domain.ErrUnauthorized):
writeError(w, http.StatusForbidden, "forbidden", "Access denied")
default:
slog.Error("unexpected error", "err", err)
writeError(w, http.StatusInternalServerError, "internal_error", "An unexpected error occurred")
}
return
}
writeJSON(w, http.StatusOK, user)
}
Retry with Exponential Backoff
interface RetryOptions {
maxAttempts?: number
baseDelayMs?: number
maxDelayMs?: number
retryIf?: (error: unknown) => boolean
}
async function withRetry<T>(
fn: () => Promise<T>,
options: RetryOptions = {},
): Promise<T> {
const {
maxAttempts = 3,
baseDelayMs = 500,
maxDelayMs = 10_000,
retryIf = () => true,
} = options
let lastError: unknown
for (let attempt = 1; attempt <= maxAttempts; attempt++) {
try {
return await fn()
} catch (error) {
lastError = error
if (attempt === maxAttempts || !retryIf(error)) throw error
const jitter = Math.random() * baseDelayMs
const delay = Math.min(baseDelayMs * 2 ** (attempt - 1) + jitter, maxDelayMs)
await new Promise(resolve => setTimeout(resolve, delay))
}
}
throw lastError
}
// Usage: retry transient network errors, not 4xx
const data = await withRetry(() => fetch('/api/data').then(r => r.json()), {
maxAttempts: 3,
retryIf: (error) => !(error instanceof AppError && error.statusCode < 500),
})
User-Facing Error Messages
Map error codes to human-readable messages. Keep technical details out of user-visible text.
const USER_ERROR_MESSAGES: Record<string, string> = {
NOT_FOUND: 'The requested item could not be found.',
UNAUTHORIZED: 'Please sign in to continue.',
FORBIDDEN: "You don't have permission to do that.",
VALIDATION_ERROR: 'Please check your input and try again.',
RATE_LIMITED: 'Too many requests. Please wait a moment and try again.',
INTERNAL_ERROR: 'Something went wrong on our end. Please try again later.',
}
export function getUserMessage(code: string): string {
return USER_ERROR_MESSAGES[code] ?? USER_ERROR_MESSAGES.INTERNAL_ERROR
}
Error Handling Checklist
Before merging any code that touches error handling:
- Every
catchblock handles, re-throws, or logs — no silent swallowing - API errors follow the standard envelope
{ error: { code, message } } - User-facing messages contain no stack traces or internal details
- Full error context is logged server-side
- Custom error classes extend a base
AppErrorwith acodefield - Async functions surface errors to callers — no fire-and-forget without fallback
- Retry logic only retries retriable errors (not 4xx client errors)
- React components are wrapped in
ErrorBoundaryfor rendering errors