Error Handling Patterns

Core Principles

1. Fail fast - Detect and report errors early
2. Fail safely - Errors shouldn't crash the application
3. Fail informatively - Provide actionable error messages
4. Recover gracefully - Handle expected failures

Custom Error Classes

// Base application error
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;
    Error.captureStackTrace(this, this.constructor);
  }

  toJSON() {
    return {
      code: this.code,
      message: this.message,
      ...(this.details && { details: this.details }),
    };
  }
}

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

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

export class UnauthorizedError extends AppError {
  constructor(message = 'Authentication required') {
    super(message, 'UNAUTHORIZED', 401);
  }
}

export class ForbiddenError extends AppError {
  constructor(message = 'Insufficient permissions') {
    super(message, 'FORBIDDEN', 403);
  }
}

Result Type Pattern

type Result<T, E = Error> =
  | { success: true; data: T }
  | { success: false; error: E };

// Helper functions
function ok<T>(data: T): Result<T, never> {
  return { success: true, data };
}

function err<E>(error: E): Result<never, E> {
  return { success: false, error };
}

// Usage
async function findUser(id: string): Promise<Result<User, NotFoundError>> {
  const user = await db.users.findUnique({ where: { id } });
  if (!user) {
    return err(new NotFoundError('User', id));
  }
  return ok(user);
}

// Consuming
const result = await findUser('123');
if (!result.success) {
  console.error(result.error.message);
  return;
}
const user = result.data; // Type-safe User

Error Boundaries (React)

import { Component, ErrorInfo, ReactNode } from 'react';

interface Props {
  children: ReactNode;
  fallback?: ReactNode;
  onError?: (error: Error, info: ErrorInfo) => void;
}

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);
  }

  render() {
    if (this.state.hasError) {
      return this.props.fallback ?? (
        <div role="alert">
          <h2>Something went wrong</h2>
          <pre>{this.state.error?.message}</pre>
        </div>
      );
    }
    return this.props.children;
  }
}

Async Error Handling

// Wrapper for async functions
function tryCatch<T>(
  promise: Promise<T>
): Promise<[null, T] | [Error, null]> {
  return promise
    .then((data) => [null, data] as [null, T])
    .catch((error) => [error as Error, null]);
}

// Usage
const [error, user] = await tryCatch(fetchUser(id));
if (error) {
  handleError(error);
  return;
}
// user is guaranteed to be defined here

// Multiple operations
async function processOrder(orderId: string) {
  const [orderError, order] = await tryCatch(getOrder(orderId));
  if (orderError) return err(orderError);

  const [paymentError] = await tryCatch(processPayment(order));
  if (paymentError) {
    await tryCatch(rollbackOrder(order));
    return err(paymentError);
  }

  return ok(order);
}

Retry Pattern

interface RetryOptions {
  maxAttempts: number;
  delayMs: number;
  backoff?: 'linear' | 'exponential';
  shouldRetry?: (error: Error) => boolean;
}

async function withRetry<T>(
  fn: () => Promise<T>,
  options: RetryOptions
): Promise<T> {
  const { maxAttempts, delayMs, backoff = 'exponential', shouldRetry } = options;
  let lastError: Error;

  for (let attempt = 1; attempt <= maxAttempts; attempt++) {
    try {
      return await fn();
    } catch (error) {
      lastError = error as Error;

      if (shouldRetry && !shouldRetry(lastError)) {
        throw lastError;
      }

      if (attempt < maxAttempts) {
        const delay = backoff === 'exponential'
          ? delayMs * Math.pow(2, attempt - 1)
          : delayMs * attempt;
        await sleep(delay);
      }
    }
  }

  throw lastError!;
}

// Usage
const data = await withRetry(() => fetchFromAPI(), {
  maxAttempts: 3,
  delayMs: 1000,
  shouldRetry: (err) => err.message.includes('timeout'),
});

Validation Errors

import { z } from 'zod';

function validateInput<T>(schema: z.ZodSchema<T>, input: unknown): Result<T, ValidationError> {
  const result = schema.safeParse(input);

  if (!result.success) {
    const details = result.error.flatten().fieldErrors;
    return err(new ValidationError('Invalid input', details));
  }

  return ok(result.data);
}

// Usage
const userSchema = z.object({
  email: z.string().email(),
  age: z.number().min(18),
});

const result = validateInput(userSchema, req.body);
if (!result.success) {
  return res.status(400).json(result.error.toJSON());
}

Logging Errors

interface ErrorContext {
  userId?: string;
  requestId?: string;
  path?: string;
  [key: string]: unknown;
}

function logError(error: Error, context: ErrorContext = {}) {
  const payload = {
    timestamp: new Date().toISOString(),
    name: error.name,
    message: error.message,
    stack: error.stack,
    ...(error instanceof AppError && { code: error.code }),
    ...context,
  };

  console.error(JSON.stringify(payload));

  // Send to error tracking service
  if (process.env.NODE_ENV === 'production') {
    // Sentry.captureException(error, { extra: context });
  }
}

Best Practices

1. Create specific error types for different failure modes
2. Use Result types for expected failures (validation, not found)
3. Throw errors for unexpected failures (bugs, system errors)
4. Include context in error messages for debugging
5. Log at boundaries - API handlers, event processors
6. Validate at boundaries - external input, API responses
7. Implement retry for transient failures
8. Use error boundaries in React for graceful degradation