try-to-result

A TypeScript-first emulation of the try‑operator proposed by proposal-try-operator, including a polyfill for the Result type.

Installation

npm install try-to-result

or

yarn add try-to-result

Usage

Wrap any synchronous or asynchronous computation in Result.try. It never throws – instead it returns a discriminated union you can safely destructure, like tuple.

Basic usage

import Result from 'try-to-result';

function div(a: number, b: number): number {
  if (b !== 0) return a / b;
  throw new Error('Division by zero');
}

const [ok, error, value] = Result.try(() => div(4, 2));

if (ok) {
  console.log(value * 2); // 4
} else {
  console.error('Failed:', error);
}

or the same but without creating a new function:

import Result from 'try-to-result';

const [ok, error, value] = await Result.try(div, 4, 2);

if (ok) {
  console.log(value * 2); // 4
} else {
  console.error('Failed:', error);
}

Promises and async functions

You can also use Result.try with promises and async functions:

import Result from 'try-to-result';

type User = { id: string; name: string };

async function fetchUser(id: string): Promise<User | null> {
  const [ok, error, res] = await Result.try(
    fetch(`https://api.example.com/users/${id}`),
  );

  if (!ok) {
    // no response received
    throw new Error(`Failed to fetch user: ${error}`, { cause: error });
  }

  if (res.status === 404) {
    // user not found
    return null;
  }

  if (res.status !== 200) {
    // handle other HTTP errors
    throw new Error(`HTTP error on fetching user: ${res.status}`);
  }

  const [okJson, jsonError, data] = await Result.try(res.json());

  if (!okJson) {
    // failed to parse JSON response
    throw new Error(`Failed to parse user data: ${jsonError}`, { cause: jsonError });
  }

  return data as User;
}

Accepting plain synchronous values

const result = Result.try(42);
// result = Result.ok(42)

Returning Results

Instead of exceptions, you can return a typed Result from your functions:

import Result from 'try-to-result';

type DivError = 
  | 'ERR_DIV_BY_ZERO' // x / 0, x != 0
  | 'ERR_IND_FORM';   // 0 / 0

function div(a: number, b: number): Result<number, DivError> {
  if (b !== 0) return Result.ok(a / b);
  if (a !== 0) return Result.error('ERR_DIV_BY_ZERO');
  return Result.error('ERR_IND_FORM');
}

const result = div(4, 0);

if (result.ok) {
  console.log(result.value);
} else {
  console.error(result.error); // 'ERR_DIV_BY_ZERO'
}

Returning Async Results

import Result from 'try-to-result';

type FetchError =
  | { code: 'NETWORK_ERROR', cause: unknown }
  | { code: 'HTTP_ERROR', status: number }
  | { code: 'PARSE_JSON_ERROR', cause: unknown }
  | { code: 'ABORTED' };

type FetchJsonResult<T> = Result<T, FetchError>;

async function fetchJson<T = unknown>(
  url: string,
  init?: RequestInit,
): Promise<FetchJsonResult<T>> {
  const [ok, error, res] = await Result.try(fetch(url, init));

  if (!ok) {
    // no response received
    return Result.error(
      (error as any)?.name === 'AbortError'
        ? { code: 'ABORTED' }
        : { code: 'NETWORK_ERROR', cause: error },
    );
  }

  if (!res.ok) {
    // handle HTTP errors
    return Result.error({ code: 'HTTP_ERROR', status: res.status });
  }

  const [okJson, jsonError, data] = await Result.try(res.json());

  if (!okJson) {
    // failed to parse JSON response
    return Result.error({ code: 'PARSE_JSON_ERROR', cause: jsonError });
  }

  return Result.ok(data as T);
}

type User = { id: string; name: string; companyId: string };

export async function fetchUser(id: string): Promise<User | null> {
  const [ok, error, data] = await fetchJson(
    `https://api.example.com/users/${id}`,
  );

  if (ok) return data as User;

  if (error.code === 'ABORTED') {
    return null;
  }

  if (error.code !== 'HTTP_ERROR') {
    throw new Error(`Failed to fetch user: ${error.code}`, {
      cause: error.cause,
    });
  }

  if (error.status === 404) {
    return null;
  }

  throw new Error(`HTTP error on fetching user: ${error.status}`);
}

In the above example, fetchJson returns a Promise resolving to a Result and delegates error handling to the caller. Thanks to return value of fetchJson is strictly typed, the fetchUser function can confidently handle all error cases.

Usage of typed results with Result.do

Result.do makes control flow easier by letting you unpack results inline with a generator:

type DivError = 
  | 'ERR_DIV_BY_ZERO' // x / 0, x != 0
  | 'ERR_IND_FORM';   // 0 / 0

function div(a: number, b: number): Result<number, DivError> {
  if (b !== 0) return Result.ok(a / b);
  if (a !== 0) return Result.error('ERR_DIV_BY_ZERO');
  return Result.error('ERR_IND_FORM');
}

type SqrError = 'ERR_NEGATIVE_SQUARE_ROOT';

function sqrt(x: number): Result<number, SqrError> {
  if (x < 0) return Result.error('ERR_NEGATIVE_SQUARE_ROOT');
  return Result.ok(Math.sqrt(x));
}

const result = Result.do(function* (_) {
  const x = yield* _(div(4, 2)); // unpack Result from div
  const y = yield* _(sqrt(x));    // unpack Result from sqrt
  return y * 2;                  // return final value
});

In this example, the yield* _(result) unpacks the Result values, only if they are Result.ok. If the result is Result.error, the control flow stops and returns that error. The function Result.do is in charge of returning yielded values as Result.error.

For async workflows, use an async function* and await:

type User = { id: string; name: string, companyId: string };

const result = await Result.do(async function* (_) {
  const user = yield* _(await fetchJson<User>('/users/1'));
  const company = yield* _(await fetchJson<Company>(
    `/companies/${user.companyId}`
  ));

  return `${user.name} works at ${company.name}`;
});

console.log(result); // Result.ok('John Doe works at Example Corp')

Specification

API	Description
Result.ok(value)	Creates a successful result.
Result.ok()	Creates a void successful result.
Result.error(error)	Creates an error result.
Result.error()	Creates a void error result.
Result.try(value)	Wraps a plain value in Result.ok.
Result.try(promise)	Converts a Promise into Promise<Result<T>>.
Result.try(fn, ...args)	Calls a function (sync or async) with arguments, capturing thrown errors or rejections.
Result.collect(results)	Collects multiple results into a single result of array of values. Returns the first error if any result is an ErrorResult.
Result.do(function*)	Structured error handling with generator-based flow.
Result.do(async function*)	Structured error handling with async generator-based flow.

Type Result

type Result<T, E = unknown> = ValueResult<T> | ErrorResult<E>;

Class Result

class Result {
  static ok(): ValueResult<void>;
  static ok<T>(value: T): ValueResult<T>;
  static error(): ErrorResult<void>;
  static error<E>(error: E): ErrorResult<E>;
  static try<T>(value: T): ValueResult<T>;
  static try<T>(fn: () => T): ValueResult<T>;
  static try<T>(promise: Promise<T>): Promise<ValueResult<T>>;
  static try<T>(fn: (...args: any[]) => T, ...args: any[]): ValueResult<T>;
  static collect<T, E>(results: Result<T, E>[]): Result<T[], E>;
  static do<T>(generator: (yield: <R>(result: Result<R>) => Result<R>) => Generator<Result<T>>): Result<T>;
  static do<T>(generator: (yield: <R>(result: Promise<Result<R>>) => Promise<Result<R>>) => AsyncGenerator<Result<T>>): Promise<Result<T>>;
}

The Result is not a constructor. So, you cannot use new Result(). Instead, use the static methods: Result.ok, Result.error to create results, and Result.try, Result.collect, and Result.do to work with them.

Result.ok: ValueResult<T>

Represents a successful value:

type ValueResult<T> = readonly [
  ok: true,
  error: undefined,
  value: T
] & { readonly ok: true; readonly value: T };

Constructed with:

Result.ok(value)

Result.error: ErrorResult<T>

Represents an error value:

type ErrorResult<E> = readonly [
  ok: false,
  error: E,
  value: undefined
] & { readonly ok: false; readonly error: E };

Constructed with:

Result.error(error)

Result.try

Wraps a value, function, or promise:

• Result.try(value) → wraps a plain value
• Result.try(fn) → executes a function and captures exceptions
• Result.try(promise) → converts a Promise into Promise<Result<T>>
• Result.try(fn, ...args) → calls a function with given arguments and wraps the result

Returns:

• Result<T> for synchronous values
• Promise<Result<T>> for async functions/promises

Alternative import:

import { _try } from 'try-to-result';

Result.collect

Collects multiple results into a single result of an array of values. If all results are successful, it returns Result.ok([values]).

const result = Result.collect([Result.ok(1), Result.ok(2), Result.ok(3)]);
// result = Result.ok([1, 2, 3])

If any result is an error, it returns the first error encountered:

const result = Result.collect([Result.ok(1), Result.ok(2), Result.error('Error')]);
// result = Result.error('Error')

Result.do

Simplifies working with nested results using generator functions:

• For synchronous generators:

  const result = Result.do(function* (_) { 
    const value = yield* _(Result.ok(42));
  });

  Returns a Result.

  yield* _(result) unpacks a Result.

• For asynchronous generators:

  const result = await Result.do(async function* (_) { 
    const value = yield* _(await Promise.resolve(Result.ok(42)));
  });

  Returns a Promise<Result>.

  yield* _(await Promise<someResult>) unpacks a Result.

If someResult is Result.error, control flow stops and returns that error.