@usersatoshi/results

A lightweight TypeScript Result type for safe error handling. Write predictable, composable code without throwing exceptions.

Features

• ✅ Lightweight - Zero dependencies, minimal bundle size
• ✅ Type-safe - Full TypeScript support with discriminated unions
• ✅ Runtime-agnostic - Works with Node.js, Bun, Deno, and browsers
• ✅ Composable - Chain operations with map, andThen, and more
• ✅ Async support - Convert promises to Results with fromPromise

Installation

npm install @usersatoshi/results
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Or with your preferred package manager:

bun add @usersatoshi/results
yarn add @usersatoshi/results
pnpm add @usersatoshi/results
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Quick Start

import { ok, err, type Result } from '@usersatoshi/results';

const enum ErrorKind { NotFound = 0, Unauthorized = 1 }
type AppError = { kind: ErrorKind };

// Create an Ok result
const success: Result<number, AppError> = ok(42);

// Create an Err result
const failure: Result<number, AppError> = err({ kind: ErrorKind.NotFound });

// Use type guards (methods, not properties)
if (success.isOk()) {
  console.log(success.value); // 42
}

if (failure.isErr()) {
  console.log(failure.error); // { kind: 0 }
}
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API

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

Creates an Ok result wrapping a successful value.

const result = ok(100);
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err<E extends { kind: number }>(error: E): Err<E>

Creates an Err result wrapping an error.

const result = err({ kind: 1 as const, message: "Something went wrong" });
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fromPromise<T, E>(promise: Promise<T>, onErr: (error: unknown) => E): Promise<Result<T, E>>

Converts a Promise into a Result, mapping rejections with your error handler.

const result = await fromPromise(
  fetch('/api/data'),
  (error) => ({ kind: 0 as const, message: String(error) })
);
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Result Methods

Both Ok and Err implement the Result interface with methods for chaining operations:

• isOk() - Type guard: narrows to Ok<T>
• isErr() - Type guard: narrows to Err<E>
• map(fn) - Transform the success value; propagates Err unchanged
• mapErr(fn) - Transform the error value; propagates Ok unchanged
• andThen(fn) - Chain Result-returning operations (flatMap)
• orElse(fn) - Recover from an error with a new Result
• match(onOk, onErr) - Exhaustively handle both variants
• unwrap() - Extract the value, throws ResultError if Err
• unwrapOr(default) - Extract the value or return a default
• BaseResult.all(results) - Collect array of Results; returns first Err or Ok<T[]>
• BaseResult.any(results) - Return first Ok, or last Err if all fail

Examples

Error Handling

const enum ErrorKind { DivisionByZero = 0 }
type MathError = { kind: ErrorKind };

function divide(a: number, b: number): Result<number, MathError> {
  if (b === 0) {
    return err({ kind: ErrorKind.DivisionByZero });
  }
  return ok(a / b);
}

const result = divide(10, 2);

if (result.isOk()) {
  console.log(`Result: ${result.value}`); // Result: 5
}
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Chaining Operations

ok(5)
  .map(x => x * 2)
  .andThen(x => x > 5 ? ok(x) : err({ kind: 0 as const }))
  .match(
    (value) => console.log(`Success: ${value}`),
    (error) => console.log(`Error: ${error.kind}`)
  );
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Async Operations

const result = await fromPromise(
  fetch('/api/users'),
  (error) => ({ kind: 1 as const, message: String(error) })
);

if (result.isOk()) {
  const users = await result.value.json();
  console.log(users);
}
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Collecting Multiple Results

const results = [ok(1), ok(2), ok(3)];
const combined = BaseResult.all(results);

if (combined.isOk()) {
  console.log(combined.value); // [1, 2, 3]
}
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Browser Compatibility

Works in all modern browsers that support ES2020+.

License

Apache-2.0

Repository

• Main: https://git.usersatoshi.com/usersatoshi/results.git
• Mirror: https://github.com/usersatoshi/results