ts-error-handling

TypeScript Error Handling

A fully typed error handling library for TypeScript, inspired by Rust's Result type and neverthrow. This library provides a type-safe way to handle errors without throwing exceptions, enabling railway-oriented programming patterns.

Features

• Fully Typed: Leverage TypeScript's type system for complete type safety
• Narrow Type Guards: Automatic type narrowing with isOk and isErr
• Zero Dependencies: Lightweight and self-contained
• Rust-Inspired API: Familiar patterns from Rust's Result type
• Railway-Oriented Programming: Chain operations safely with map, andThen, and more
• Async Support: First-class support for async operations
• Comprehensive Utilities: Combine, traverse, sequence, and more

Installation

# bun
bun add ts-error-handling

# npm
npm install ts-error-handling

# pnpm
pnpm add ts-error-handling

Quick Start

import type { Result } from 'ts-error-handling'
import { err, ok } from 'ts-error-handling'

function divide(a: number, b: number): Result<number, string> {
  if (b === 0) {
    return err('Division by zero')
  }
  return ok(a / b)
}

const result = divide(10, 2)

if (result.isOk) {
  console.log(result.value) // 5
}
else {
  console.log(result.error) // Type-safe error handling
}

Core API

Creating Results

import { err, ok } from 'ts-error-handling'

const success = ok(42) // Result<number, never>
const failure = err('error') // Result<never, string>

Type Narrowing

const result: Result<number, string> = ok(42)

if (result.isOk) {
  // TypeScript knows result is Ok<number, string>
  console.log(result.value) // number
}
else {
  // TypeScript knows result is Err<number, string>
  console.log(result.error) // string
}

Transforming Results

// map: Transform the Ok value
ok(21).map(x => x * 2) // Ok(42)
err('failed').map(x => x * 2) // Err('failed')

// mapErr: Transform the Err value
ok(42).mapErr(e => e.toUpperCase()) // Ok(42)
err('failed').mapErr(e => e.toUpperCase()) // Err('FAILED')

// andThen: Chain Result-returning operations
ok(21)
  .andThen(x => ok(x * 2))
  .andThen(x => x > 40 ? ok(x) : err('too small')) // Ok(42)

// orElse: Recover from errors
err('failed')
  .orElse(e => ok(0)) // Ok(0)

Pattern Matching

const message = result.match({
  ok: value => `Success: ${value}`,
  err: error => `Error: ${error}`
})

Unwrapping Values

ok(42).unwrap() // 42
err('failed').unwrap() // throws Error

ok(42).unwrapOr(0) // 42
err('failed').unwrapOr(0) // 0

ok(42).unwrapOrElse(e => 0) // 42
err('failed').unwrapOrElse(e => 0) // 0

ok(42).expect('should work') // 42
err('failed').expect('should work') // throws with custom message

Utility Functions

Exception Handling

import { fromPromise, tryCatch, tryCatchAsync } from 'ts-error-handling'

// Synchronous exception handling
const result = tryCatch(
  () => JSON.parse(jsonString),
  error => `Parse error: ${error}`
)

// Async exception handling
const asyncResult = await tryCatchAsync(
  async () => await fetch('/api/data'),
  error => new ApiError(error)
)

// Convert Promise to Result
const promiseResult = await fromPromise(
  fetch('/api/users/1'),
  error => `Failed to fetch: ${error}`
)

Combining Results

import { all, combine, combineWithAllErrors } from 'ts-error-handling'

// Combine - stops at first error
const result1 = combine([
  validateEmail(email),
  validatePassword(password),
  validateAge(age)
]) // Result<[string, string, number], string>

// Combine with all errors collected
const result2 = combineWithAllErrors([
  validateEmail(email),
  validatePassword(password),
  validateAge(age)
]) // Result<[string, string, number], string[]>

// Alternative syntax using all()
const result3 = all([ok(1), ok(2), ok(3)]) // Result<number[], string>

Array Operations

import { partition, traverse, traverseAsync } from 'ts-error-handling'

// Transform array with Result-returning function
const numbers = traverse(['1', '2', '3'], (s) => {
  const n = Number.parseInt(s)
  return Number.isNaN(n) ? err('Invalid') : ok(n)
}) // Result<number[], string>

// Async version
const users = await traverseAsync([1, 2, 3], async (id) => {
  return fetchUser(id)
}) // Promise<Result<User[], ApiError>>

// Partition into successes and failures
const results = [ok(1), err('error'), ok(2)]
const [successes, failures] = partition(results)
// successes: [1, 2], failures: ['error']

Working with Nullable Values

import { fromNullable } from 'ts-error-handling'

interface Config {
  apiKey?: string
}

function getApiKey(config: Config): Result<string, string> {
  return fromNullable(config.apiKey, 'API key is required')
}

Advanced Utilities

import {
  any,
  filter,
  flatten,
  getOrElse,
  swap,
  tap,
  toPromise,
  unwrapOrThrow
} from 'ts-error-handling'

// Flatten nested Results
const nested: Result<Result<number, string>, string> = ok(ok(42))
const flat = flatten(nested) // Result<number, string>

// Side effects without modifying Result
const logged = tap(
  result,
  value => console.log('Success:', value),
  error => console.error('Error:', error)
)

// Swap Ok and Err
const swapped = swap(ok(42)) // Err(42)
const swapped2 = swap(err('failed')) // Ok('failed')

// Filter based on predicate
const filtered = filter(
  ok(42),
  n => n > 0,
  'must be positive'
) // Ok(42) if > 0, else Err('must be positive')

// Convert to exception at boundaries
const value = unwrapOrThrow(result, e => new Error(e))

// Convert to Promise
const promise = toPromise(result) // Promise<T>

// Get value or compute default
const value2 = getOrElse(result, e => defaultValue)

// Return first Ok, or last Err
const first = any([err('e1'), ok(42), err('e2')]) // Ok(42)

Async Operations

import { parallel, sequence } from 'ts-error-handling'

// Execute operations sequentially (stops at first error)
const sequential = await sequence([
  async () => fetchUser(1),
  async () => fetchUser(2),
  async () => fetchUser(3)
]) // Result<User[], ApiError>

// Execute operations in parallel
const parallelResult = await parallel([
  async () => fetchUser(1),
  async () => fetchUser(2),
  async () => fetchUser(3)
]) // Result<User[], ApiError>

Why Use Result Types?

• Explicit Error Handling: Errors are part of the function signature
• No Hidden Control Flow: No try/catch blocks, exceptions are values
• Type Safety: The compiler enforces error handling
• Composability: Chain operations without nested error handling
• Self-Documenting: Function signatures show what can fail and how

Examples

See the examples.ts file for comprehensive examples including:

• Async operations
• Form validation with error collection
• Railway-oriented programming
• API client implementation
• Array operations with traverse
• Sequential and parallel execution

Complete API Reference

Core Functions

Function	Description
ok<T, E>(value: T)	Creates a successful Result
err<T, E>(error: E)	Creates a failed Result
isResult<T, E>(value: unknown)	Type guard to check if value is a Result

Result Methods

Method	Description
.map<U>(fn: (value: T) => U)	Transform the Ok value
.mapErr<F>(fn: (error: E) => F)	Transform the Err value
.andThen<U>(fn: (value: T) => Result<U, E>)	Chain Result-returning operations
.orElse<F>(fn: (error: E) => Result<T, F>)	Recover from errors
.match<U>(patterns: {...})	Pattern matching on Ok/Err
.unwrap()	Get value or throw
.unwrapOr(defaultValue: T)	Get value or return default
.unwrapOrElse(fn: (error: E) => T)	Get value or compute default
.expect(msg: string)	Get value or throw with custom message

Exception Handling

Function	Description
tryCatch<T, E>(fn, errorHandler?)	Wrap function that might throw
tryCatchAsync<T, E>(fn, errorHandler?)	Wrap async function that might throw
fromPromise<T, E>(promise, errorHandler?)	Convert Promise to Result

Combining Results

Function	Description
combine<T[]>(results)	Combine Results, stop at first error
combineWithAllErrors<T[]>(results)	Combine Results, collect all errors
all<T, E>(results)	Collect all Ok values or return first Err
any<T, E>(results)	Return first Ok or last Err

Array Operations

Function	Description
traverse<T, U, E>(items, fn)	Map over array with Result-returning function
traverseAsync<T, U, E>(items, fn)	Async version of traverse
partition<T, E>(results)	Split Results into [successes, failures]

Async Operations

Function	Description
sequence<T, E>(operations)	Execute async operations sequentially
parallel<T, E>(operations)	Execute async operations in parallel

Transformations

Function	Description
flatten<T, E>(result)	Flatten nested Result<Result<T, E>, E>
swap<T, E>(result)	Swap Ok and Err values
filter<T, E>(result, predicate, error)	Filter Result based on predicate

Utilities

Function	Description
fromNullable<T, E>(value, error)	Convert nullable to Result
tap<T, E>(result, onOk?, onErr?)	Side effects without modifying Result
unwrapOrThrow<T, E>(result, mapError?)	Convert to exception at boundaries
toPromise<T, E>(result, mapError?)	Convert Result to Promise
getOrElse<T, E>(result, fn)	Get value or compute default

Type Utilities

Type	Description
Result<T, E>	Union of Ok<T, E> and Err<T, E>
ResultAsync<T, E>	Promise<Result<T, E>>
InferOkType<T>	Extract Ok type from Result
InferErrType<T>	Extract Err type from Result

Testing

bun test

Changelog

Please see our releases page for more information on what has changed recently.

Contributing

Please see CONTRIBUTING for details.

Community

For help, discussion about best practices, or any other conversation that would benefit from being searchable:

Discussions on GitHub

For casual chit-chat with others using this package:

Join the Stacks Discord Server

Postcardware

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Our address: Stacks.js, 12665 Village Ln #2306, Playa Vista, CA 90094, United States 🌎

Sponsors

We would like to extend our thanks to the following sponsors for funding Stacks development. If you are interested in becoming a sponsor, please reach out to us.

• JetBrains
• The Solana Foundation

License

The MIT License (MIT). Please see LICENSE for more information.

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