error-handling

name	error-handling
description	Use when implementing custom error classes, error middleware, structured logging, retry logic, or graceful shutdown patterns in backend applications.
disable-model-invocation	true

Error Handling Patterns

Overview

Backend error handling patterns for building robust and debuggable services.

Error Types

Custom Error Classes

// Base application error
class AppError extends Error {
  constructor(
    message: string,
    public code: string,
    public statusCode: number = 500,
    public isOperational: boolean = true
  ) {
    super(message);
    this.name = this.constructor.name;
    Error.captureStackTrace(this, this.constructor);
  }
}

// Specific error types
class ValidationError extends AppError {
  constructor(message: string, public fields?: Record<string, string>) {
    super(message, 'VALIDATION_ERROR', 400);
  }
}

class NotFoundError extends AppError {
  constructor(resource: string, id?: string) {
    super(
      id ? `${resource} with id ${id} not found` : `${resource} not found`,
      'NOT_FOUND',
      404
    );
  }
}

class UnauthorizedError extends AppError {
  constructor(message: string = 'Unauthorized') {
    super(message, 'UNAUTHORIZED', 401);
  }
}

class ForbiddenError extends AppError {
  constructor(message: string = 'Forbidden') {
    super(message, 'FORBIDDEN', 403);
  }
}

class ConflictError extends AppError {
  constructor(message: string) {
    super(message, 'CONFLICT', 409);
  }
}

class RateLimitError extends AppError {
  constructor(public retryAfter: number) {
    super('Too many requests', 'RATE_LIMIT', 429);
  }
}

Error Response Format

Standard Format

interface ErrorResponse {
  error: {
    code: string;
    message: string;
    details?: unknown;
    stack?: string;  // Only in development
  };
  meta: {
    requestId: string;
    timestamp: string;
  };
}

function formatError(error: AppError, requestId: string): ErrorResponse {
  const response: ErrorResponse = {
    error: {
      code: error.code,
      message: error.message,
    },
    meta: {
      requestId,
      timestamp: new Date().toISOString(),
    },
  };

  if (error instanceof ValidationError && error.fields) {
    response.error.details = error.fields;
  }

  if (process.env.NODE_ENV === 'development') {
    response.error.stack = error.stack;
  }

   response;
}

Example Responses

// Validation error
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid input data",
    "details": {
      "email": "Invalid email format",
      "password": "Must be at least 8 characters"
    }
  },
  "meta": {
    "requestId": "req_abc123",
    "timestamp": "2024-01-15T10:30:00Z"
  }
}

// Not found error
{
  "error": {
    "code": "NOT_FOUND",
    "message": "User with id 123 not found"
  },
  "meta": {
    "requestId" 
     
  




   
     
     
  
   
     
     
  

Error Handling Middleware

Express Middleware

import { Request, Response, NextFunction } from 'express';

// Async handler wrapper
function asyncHandler(fn: Function) {
  return (req: Request, res: Response, next: NextFunction) => {
    Promise.resolve(fn(req, res, next)).catch(next);
  };
}

// Error handler middleware
function errorHandler(
  error: Error,
  req: Request,
  res: Response,
  next: NextFunction
) {
  const requestId = req.headers['x-request-id'] as string || generateId();

  // Log error
  logger.error('Request failed', {
    requestId,
    error: error.message,
    stack: error.stack,
    path: req.path,
    method: req.,
  });

  
   (error  ) {
     res.(error.).(
      (error, requestId)
    );
  }

  
   internalError =  (
    process.. === 
      ? 
      : error.,
    ,
    ,
      
  );

  res.().((internalError, requestId));
}


app.(, ( (req, res) => {
   user =  userService.(req..);
   (!user)   (, req..);
  res.({ : user });
}));

app.(errorHandler);

Validation Errors

Zod Validation

import { z, ZodError } from 'zod';

const createUserSchema = z.object({
  email: z.string().email('Invalid email format'),
  password: z.string().min(8, 'Password must be at least 8 characters'),
  name: z.string().min(2, 'Name must be at least 2 characters'),
});

function validateBody<T>(schema: z.Schema<T>) {
  return (req: Request, res: Response, next: NextFunction) => {
    try {
      req.body = schema.parse(req.body);
      next();
    } catch (error) {
      if (error instanceof ZodError) {
        const fields = error.errors.reduce((acc, err) => {
          acc[err.path.join('.')] = err.message;
          return acc;
        }, {}  <, >);
          (, fields);
      }
       error;
    }
  };
}


app.(, (createUserSchema), createUser);

Database Error Handling

import { DatabaseError, UniqueConstraintError } from 'pg';

function handleDatabaseError(error: Error): AppError {
  if (error instanceof UniqueConstraintError) {
    return new ConflictError('Resource already exists');
  }

  if (error.message.includes('foreign key')) {
    return new ValidationError('Referenced resource does not exist');
  }

  // Log unexpected database errors
  logger.error('Database error', { error: error.message });
  return new AppError('Database operation failed', 'DATABASE_ERROR', 500);
}

// Repository example
async function createUser(data: CreateUserInput): Promise<User> {
  try {
    return await db.users.create(data);
  }  (error) {
     (error);
  }
}

External Service Errors

class ExternalServiceError extends AppError {
  constructor(
    service: string,
    public originalError?: Error
  ) {
    super(
      `External service ${service} failed`,
      'EXTERNAL_SERVICE_ERROR',
      503
    );
  }
}

async function callExternalAPI(url: string) {
  try {
    const response = await fetch(url, {
      signal: AbortSignal.timeout(5000), // 5 second timeout
    });

    if (!response.ok) {
      throw new Error(`HTTP ${response.status}`);
    }

    return await response.json();
  } catch (error) {
    if (error.name === 'AbortError') {
      throw new ExternalServiceError('API', new Error('Timeout'));
    }
      (, error);
  }
}

Retry Logic

interface RetryOptions {
  maxAttempts: number;
  baseDelay: number;
  maxDelay: number;
  shouldRetry?: (error: Error) => boolean;
}

async function withRetry<T>(
  fn: () => Promise<T>,
  options: RetryOptions
): Promise<T> {
  const { maxAttempts, baseDelay, maxDelay, shouldRetry } = options;

  for (let attempt = 1; attempt <= maxAttempts; attempt++) {
    try {
      return await fn();
    } catch (error) {
      const canRetry = shouldRetry?.(error) ?? true;

      if (!canRetry || attempt === maxAttempts) {
        throw error;
      }

      const delay = Math.min(
        baseDelay * Math.pow(2, attempt - 1),
        maxDelay
      );

      logger.warn('Retrying operation', {
        attempt,
        maxAttempts,
        delay,
        error: error.message,
      });

      await sleep(delay);
    }
  }

    ();
}


 result =  (
   externalAPI.(),
  {
    : ,
    : ,
    : ,
    :  error  ,
  }
);

Logging Best Practices

// Structured logging
interface LogContext {
  requestId?: string;
  userId?: string;
  [key: string]: unknown;
}

const logger = {
  info: (message: string, context?: LogContext) => {
    console.log(JSON.stringify({ level: 'info', message, ...context }));
  },

  warn: (message: string, context?: LogContext) => {
    console.log(JSON.stringify({ level: 'warn', message, ...context }));
  },

  error: (message: string, context?: LogContext & { error?: string; stack?: string }) => {
    console.error(JSON.stringify({ level: 'error', message, ...context }));
  },
};

// Error logging middleware
 () {
  logger.(, {
    : req.[]  ,
    : req.?.,
    : req.,
    : req.,
    : error.,
    : error.,
  });
  (error);
}

Graceful Shutdown

async function gracefulShutdown(signal: string) {
  logger.info('Shutdown signal received', { signal });

  // Stop accepting new requests
  server.close(() => {
    logger.info('HTTP server closed');
  });

  // Close database connections
  await db.close();
  logger.info('Database connections closed');

  // Close other resources
  await redis.quit();
  logger.info('Redis connection closed');

  process.exit(0);
}

process.on('SIGTERM', () => gracefulShutdown('SIGTERM'));
process.on('SIGINT', () => gracefulShutdown('SIGINT'));

// Handle uncaught errors
process.on('uncaughtException', (error) => {
  logger.error('Uncaught exception', { error: error.message, stack: error.stack });
  gracefulShutdown('uncaughtException');
});

process.(,  {
  logger.(, { : (reason) });
  ();
});

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Error handling patterns for robust backend services