Functional programming patterns for TypeScript.

Requirements

• TypeScript 5.0+

Installation

# Install core
npm install @deessejs/fp

# Or using pnpm
pnpm add @deessejs/fp

# Or using yarn
yarn add @deessejs/fp

Usage

import { ok, err, isOk, isErr, Result } from '@deessejs/fp'

// Ok - Normal Operation Result
const ok = ok({ id: 1, name: 'John' })
if (isOk(ok)) {
  console.log(ok.value.name) // TypeScript knows this is User
}

// Err - Domain Errors (Business Logic)
const notFound = err({
  name: 'NOT_FOUND',
  args: { id: 123 }
})
if (isErr(notFound)) {
  console.log(notFound.error.name) // "NOT_FOUND"
  console.log(notFound.error.args.id) // 123
}

// Result - Union Type
function getUser(id: number): Result<User> {
  const user = findUser(id)
  if (!user) {
    return err({ name: 'NOT_FOUND', args: { id } })
  }
  return ok(user)
}

const result = getUser(123)
if (isOk(result)) {
  console.log(result.value.name)
} else if (isErr(result)) {
  console.log(result.name) // "NOT_FOUND"
}

Features

• Result Type - Successful operation result with value or error
• Maybe Type - Optional values (Some/None)
• Try Type - Exception handling wrapper
• Type Guards - isOk, isErr, isSome, isNone for narrowing
• Fully Typed - Comprehensive TypeScript support
• Minimal Dependencies - Only Zod for runtime validation, no other external deps
• 100% Test Coverage - Thoroughly tested

API Reference

Types

| Type | Description | |------|-------------| | Result<T, E> | Successful operation result with value or error | | Maybe<T> | Optional value (Some or None) | | Try<T> | Exception handling wrapper |

Functions

| Function | Description | |----------|-------------| | ok<T>(value) | Create an Ok result | | err<E>(options) | Create an Err result | | some<T>(value) | Create a Some | | none() | Create a None | | try<T>(fn) | Wrap a function in Try | | flattenMaybe(maybe) | Flatten a nested Maybe |

Type Guards

| Guard | Description | |-------|-------------| | isOk(result) | Check if result is Ok (narrows type) | | isErr(result) | Check if result is Err (narrows type) | | isSome(maybe) | Check if maybe is Some (narrows type) | | isNone(maybe) | Check if maybe is None (narrows type) |

TypeScript Signature Examples

// Ok result
ok<T>(value: T): Ok<T>

// Err with typed args
err<E>(options: { name: string; args: E }): Err<Error<E>>

// Result type
type Result<T, E = Err<unknown>> = Ok<T> | Err<E>

Why This Package?

These patterns are useful in any TypeScript project:

• Backend services
• CLI tools
• Workers
• Even frontend (with care)

Having them as a separate package means:

• Users can use FP patterns without the full API framework
• Smaller bundle size for simple projects
• The patterns are well-tested and reusable
• Can be used in any project, not just @deessejs ecosystem

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

Author

• Nesalia Inc.

Security

If you discover any security vulnerabilities, please send an e-mail to [email protected].

License

MIT License - see the LICENSE file for details.