Error Handling Patterns

Error Handling Patterns Skill

This skill covers type-safe error handling patterns for TypeScript.

When to Use

Use this skill when:

• Designing error handling strategies
• Creating APIs that can fail
• Implementing recoverable error flows
• Building robust applications

Core Principle

ERRORS ARE VALUES - Treat errors as first-class values that can be typed, returned, and handled explicitly.

Result Type Pattern

Basic Result Type

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 };
}

Using Result Type

interface User {
  id: string;
  name: string;
}

interface UserNotFoundError {
  type: 'USER_NOT_FOUND';
  userId: string;
}

interface DatabaseError {
  type: 'DATABASE_ERROR';
  message: string;
}

type GetUserError = UserNotFoundError | DatabaseError;

async function getUser(id: string): Promise<Result<User, GetUserError>> {
  try {
    const user = await db.users.findUnique({ where: { id } });
    if (!user) {
      return err({ type: 'USER_NOT_FOUND', userId: id });
    }
    return ok(user);
  } catch (e) {
    return err({ type: 'DATABASE_ERROR', message: String(e) });
  }
}

// Usage
const result = await getUser('123');
if (result.success) {
  console.log(result.data.name); // User
} else {
  switch (result.error.type) {
    case 'USER_NOT_FOUND':
      console.log(`User ${result.error.userId} not found`);
      break;
    case 'DATABASE_ERROR':
      console.log(`Database error: ${result.error.message}`);
      break;
  }
}

Discriminated Union Errors

Error Type Hierarchy

// Base error interface
interface BaseError {
  type: string;
  message: string;
  timestamp: Date;
}

// Specific error types
interface ValidationError extends BaseError {
  type: 'VALIDATION_ERROR';
  field: string;
  value: unknown;
}

interface NotFoundError extends BaseError {
  type: 'NOT_FOUND_ERROR';
  resource: string;
  id: string;
}

interface AuthorizationError extends BaseError {
  type: 'AUTHORIZATION_ERROR';
  requiredRole: string;
  userRole: string;
}

interface NetworkError extends BaseError {
  type: 'NETWORK_ERROR';
  url: string;
  statusCode?: number;
}

// Union of all errors
type AppError =
  | ValidationError
  | NotFoundError
  | AuthorizationError
  | NetworkError;

Error Factory Functions

function createValidationError(
  field: string,
  value: unknown,
  message: string,
): ValidationError {
  return {
    type: 'VALIDATION_ERROR',
    field,
    value,
    message,
    timestamp: new Date(),
  };
}

function createNotFoundError(resource: string, id: string): NotFoundError {
  return {
    type: 'NOT_FOUND_ERROR',
    resource,
    id,
    message: `${resource} with id ${id} not found`,
    timestamp: new Date(),
  };
}

Try-Catch Patterns

Type-Safe Catch Handling

function handleError(error: unknown): AppError {
  // Error instance
  if (error instanceof Error) {
    return {
      type: 'NETWORK_ERROR',
      url: '',
      message: error.message,
      timestamp: new Date(),
    };
  }

  // String error
  if (typeof error === 'string') {
    return {
      type: 'NETWORK_ERROR',
      url: '',
      message: error,
      timestamp: new Date(),
    };
  }

  // Unknown error
  return {
    type: 'NETWORK_ERROR',
    url: '',
    message: 'An unknown error occurred',
    timestamp: new Date(),
  };
}

async function fetchData<T>(url: string): Promise<Result<T, NetworkError>> {
  try {
    const response = await fetch(url);
    if (!response.ok) {
      return err({
        type: 'NETWORK_ERROR',
        url,
        statusCode: response.status,
        message: `HTTP ${response.status}`,
        timestamp: new Date(),
      });
    }
    const data = (await response.json()) as T;
    return ok(data);
  } catch (error) {
    return err({
      type: 'NETWORK_ERROR',
      url,
      message: error instanceof Error ? error.message : 'Unknown error',
      timestamp: new Date(),
    });
  }
}

Custom Error Classes

Typed Error Classes

abstract class AppErrorBase extends Error {
  abstract readonly type: string;
  readonly timestamp: Date;

  constructor(message: string) {
    super(message);
    this.name = this.constructor.name;
    this.timestamp = new Date();
    Error.captureStackTrace(this, this.constructor);
  }
}

class ValidationErrorClass extends AppErrorBase {
  readonly type = 'VALIDATION_ERROR' as const;

  constructor(
    message: string,
    public readonly field: string,
    public readonly value: unknown,
  ) {
    super(message);
  }
}

class NotFoundErrorClass extends AppErrorBase {
  readonly type = 'NOT_FOUND_ERROR' as const;

  constructor(
    public readonly resource: string,
    public readonly id: string,
  ) {
    super(`${resource} with id ${id} not found`);
  }
}

Type Guard for Error Classes

function isValidationError(error: unknown): error is ValidationErrorClass {
  return error instanceof ValidationErrorClass;
}

function isNotFoundError(error: unknown): error is NotFoundErrorClass {
  return error instanceof NotFoundErrorClass;
}

function isAppError(
  error: unknown,
): error is ValidationErrorClass | NotFoundErrorClass {
  return isValidationError(error) || isNotFoundError(error);
}

Async Error Handling

Promise-Based Result

async function safeAsync<T>(
  fn: () => Promise<T>,
): Promise<Result<T, Error>> {
  try {
    const data = await fn();
    return ok(data);
  } catch (error) {
    return err(error instanceof Error ? error : new Error(String(error)));
  }
}

// Usage
const result = await safeAsync(() => fetchUser('123'));
if (result.success) {
  console.log(result.data);
}

Multiple Async Operations

async function getAllResults<T, E>(
  operations: Promise<Result<T, E>>[],
): Promise<Result<T[], E>> {
  const results = await Promise.all(operations);

  const errors = results.filter(
    (r): r is { success: false; error: E } => !r.success,
  );

  if (errors.length > 0) {
    return err(errors[0].error);
  }

  const data = results
    .filter((r): r is { success: true; data: T } => r.success)
    .map((r) => r.data);

  return ok(data);
}

Error Logging

Structured Error Logging

interface ErrorLog {
  type: string;
  message: string;
  timestamp: string;
  stack?: string;
  context?: Record<string, unknown>;
}

function logError(error: AppError, context?: Record<string, unknown>): void {
  const log: ErrorLog = {
    type: error.type,
    message: error.message,
    timestamp: error.timestamp.toISOString(),
    context,
  };

  if (error instanceof Error) {
    log.stack = error.stack;
  }

  console.error(JSON.stringify(log));
}

Error Recovery Patterns

Retry with Backoff

interface RetryOptions {
  maxAttempts: number;
  delayMs: number;
  backoffMultiplier: number;
}

async function withRetry<T, E>(
  fn: () => Promise<Result<T, E>>,
  options: RetryOptions,
): Promise<Result<T, E>> {
  let lastError: E | undefined;
  let delay = options.delayMs;

  for (let attempt = 1; attempt <= options.maxAttempts; attempt++) {
    const result = await fn();
    if (result.success) {
      return result;
    }

    lastError = result.error;

    if (attempt < options.maxAttempts) {
      await new Promise((resolve) => setTimeout(resolve, delay));
      delay *= options.backoffMultiplier;
    }
  }

  return err(lastError!);
}

Fallback Values

function withFallback<T, E>(
  result: Result<T, E>,
  fallback: T,
): T {
  return result.success ? result.data : fallback;
}

function withFallbackFn<T, E>(
  result: Result<T, E>,
  fallbackFn: (error: E) => T,
): T {
  return result.success ? result.data : fallbackFn(result.error);
}

Best Practices Summary

1. Use Result types for recoverable errors
2. Use discriminated unions for error types
3. Always handle unknown in catch clauses
4. Create factory functions for errors
5. Log errors with structured context
6. Use type guards for error narrowing
7. Throw only for unrecoverable errors

When to Throw vs Return Error

Throw for:

• Programming errors (bugs)
• Unrecoverable situations
• Constraint violations that should never happen

Return Error for:

• Expected failure cases
• User input validation
• Network/IO operations
• Business rule violations

Code Review Checklist

• All errors have explicit types
• Catch clauses handle unknown type
• Result types used for recoverable errors
• Error factory functions create consistent errors
• Discriminated unions enable exhaustive handling
• Errors are logged with context
• Recovery strategies implemented where appropriate

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