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3.0.5 • Public • Published


Option and Result as inspired by



import { Err, None, Ok, Option, OptionPromise, Result, ResultPromise, Some } 
from "";


"dependencies": {
	"rusty-core": "3.0.5"
import { Err, None, Ok, Option, OptionPromise, Result, ResultPromise, Some } 
from "rusty-core";

Table of contents


Inspired by the Rust core, which defines the Option and Result types that greatly improve the execution safety by preventing null pointer exceptions.

Also, great inspiration for the implementation came from the Monads library

This implementation supports the in-place modifiers of Option (insert, take), as well as combining of Promises to either type.

A note on version changes

  • Version 1 was a pure implementation of the Rust API, and lacked support for combining Promise<Option | Result> without awaiting them first.

  • Version 2 was a great improvement and introduced the mapOption and mapResult methods as mapOrElse counterparts where the callbacks may return Option/OptionPromise or Result/ResultPromise values. This broke the mapOrElse definition ov v1.

  • Version 3, I cleaned up the API types, but most imprtantly got rid of the optionFrom and resultFrom helper functions. They are now replaced by Some and Ok, but this also broke the definition of Some when the argument is undefined | null | Infinity | NaN

A note on (the lack of) unwrap/expect

Rust has two methods that might panic: unwrap and expect

let body = document.body.unwrap();
let title = body.get_attribute("title").expect("should have title attribute!");

Neither of these are implemented in this Javascript library. Use the combinator methods to handle all possibilities:

const title = document.body()
  .map( body => body.getAttribute("title) )
  .unwrapOr("*** No title given ***");

A note on Promises and async/await and combinator methods

Both Option and Result have several combinator methods, like andThen, orElse and map. Those methods accept one or more callback functions that can be async.

The examples below demonstrates how Promises and async callbacks can be combined with the andThen and mapOrElse.


type ToDo = { userId: number; id: number; title: string; completed: boolean };

function doFetch(url: string): ResultPromise<Response, string> {
  return Ok(
        Ok<Response, string>,
        (err) => Err<Response, string>(err.toString()),

function fetchJson(url: string): ResultPromise<ToDo, string> {
  return doFetch(url)
    .andThen(async (response) => {
      if (response.ok) return Ok<ToDo, string>(await response.json());
      else {return Err(
          `${response.status} ${response.statusText}: ${await response.text()}`,

    (err) => console.error("Failed:", err),
    (todo) => console.log("Success:", todo.title),

Example with non-Rust helper functions

function pauseIfNeeded(ms: Option<number>) {
  return ms.mapOrElse(
    () => Some(0),
    async (ms) => Some(await sleep(ms)),

Due to the second callback being async, the return type for that callback will be Promise<Option> But mapOrElse requires both callbacks to have the same result type.

error: TS2740 [ERROR]: Type 'Promise<Option<number>>' is missing the following properties from type 'Option<number>': getOrInsert, getOrInsertWith, insert, replace, and 19 more.
    async (ms) => Some(await sleep(ms)),
    at file:///projects/rusty-core/cli/doc.ts:10:19

    The expected type comes from the return type of this signature.
        fn: (some: T) => U,
        at file:///projects/rusty-core/src/option/combinators.ts:76:9

So one could surround the Some(0) with Promise.resolve. But then the return type would be Promise<Option>. Combining multiple Option and Result together would be cumbersome and error prone, as this requires constructs like

(await pauseIfNeeded(Some(2))).map( ... );

To improve the usability, mapOption and mapResult were added to return an OptionPromise or ResultPromise, in order to allow direct use of the combinators

function pauseIfNeeded(ms: Option<number>) {
  return ms.mapOption(
    () => SomePromise(0),
    async (ms) => Some(await sleep(ms)),

At first, it might be confusing when to use mapOrElse and when mapOption or mapResult. Conditional types are used to assist a little here. When mapOrElse would return something like Promise<Option> or Promise<Result>, the compiler will complain when the combining methods are used:

error: TS2339 [ERROR]: Property 'map' does not exist on type '"To return a Promise to an Option, use mapOption"'.

See the combinators test for more examples.


Type Option<T> represents an optional value: every Option is either Some and contains a value, or None, and does not. Option types are very common, as they have a number of uses:

- Initial values
- Return values for functions that are not defined over their entire input range (partial functions)
- Return value for otherwise reporting simple errors, where `None` is returned on error
- Optional fields
- Optional function arguments
- Nullable pointers
- Swapping things out of difficult situations


Option and OptionPromise are only interfaces and values of these types can only be created through Some/SomePromise or None/NonePromise.

None / NonePromise

None creates an Option that has no associated value. It might be useful to pass a type argument:

const token = None<string>();
token.insert(12); // will give a compile error that the argument must be string

Some / SomePromise

Some creates an Option which has an associated value. The type argument can be inferred from the argument. Actually, Some is a bit special, as it also might return a None value:

// All the statements below return a None


SomePromise and NonePromise both create an OptionPromise, which has a similar interface as optin. Variables with the OptionPromise interface are, of course, intended in async/promise methods. In general, when a function returns Promise<Option<T>>, the actual return value will have the OptionPromise interface. One can await such a value and get the Option. Since OptionPromise has the same combinator function names as Option, these can be concattenated directly

// @sleep: (s: string) => Promise<nummber>

    () => None<number>(),
    (n) => Some(n % 2 == 0 ? 2 : 1),

    () => NonePromise<number>(),
    async (s) => Some(await calculate(s) % 2 == 0 ? 2 : 1),

Both statements will log 2 to the console. The first starts as an OptionPromise due to calculate being a promise. Because mapOption requires both callback methods to return the same type, the second statement uses NonePromise to prevent a compiler error.


function divide(numerator: number, denominator: number): Option<number> {
  if (denominator === 0) {
    return None();
  } else {
    return Some(numerator / denominator);

// The return value of the function is an option
const result = divide(2.0, 3.0);

// Pattern match to retrieve the value
const message = result.mapOrElse(
  () => "Cannot divide by 0",
  (some) => `Result: ${some}`,

console.log(message); // "Result: 0.6666666666666666"

// This can al be done using combinators
  Some(2.0 / 3.0)
    .map((some) => `Result: ${some}`)
    .unwrapOr("Cannot divide by 0"),

Original implementation:

A note on None()

The Rust core library defines None to be assignable to any Option<T>. Since the below example also comes from the core library, we had to implement None as a function: None()

const x = None<number>();
const y = x.getOrInsert(5);
assertEquals(y, 5);
assertEquals(x, Some(5));

isSome() => boolean

Returns true if the option is a Some value.

isNone() => boolean

Returns true if the option is a None value.

unwrapOr(optb: T) => T

Returns the contained value or optb.

map(fn: (val: T) => U) => Option<U>

Maps an Option<T> to Option<U> by applying a function to a contained value.

mapOrElse(def: () => U, fn: (val: T) => U) => U

Computes a default function result (if None), or applies a different function to the contained value (if Some).

andThen(fn: (val: T) => Option<U>) => Option<U>

Returns None if the option is None, otherwise calls fn with the wrapped value and returns the result.

Some languages call this operation flatmap.

okOrElse<E>(fn: () => E): Result<T, E>

Transforms the Option<T> into a {Result<T, E>,mapping Some<T>(v) to Ok<T,E>(v)} and None to Err(fn()).

or(optb: Option<T>) => Option<T>

Returns the option if it contains a value, otherwise returns optb.

and(optb: Option<T>) => Option<T>

Returns None if the option is None, otherwise returns optb.

insert(value: T): T

Inserts value into the option, then returns a mutable reference to it.

If the option already contains a value, the old value is dropped.

replace(value: T): Option<T>

Replaces the actual value in the option by the value given in parameter, returning the old value if present, leaving a Some in its place without deinitializing either one.

take(): Option<T>

Takes the value out of the option, leaving a None in its place.


Type Result<T,E> represents an result value: every Result is either Ok and contains a value of type T, or Err, which holds an error value of type E. When using Result throwing Errors is no longer necessary. Just make sure that Result values are properly mapped to other values, or other error types.


Result and ResultPromise are only interfaces and values of these types can only be created through Ok/OkPromise or Err/ErrPromise. Any type can be used for both Ok and Err and it is even allowed to have the same type for both:

function httpStatus(status: number): Result<number, number> {
  return status >= 200 && status < 400 ? Ok(status) : Err(status);

Ok / OkPromise

Ok creates a Result which has an associated value. The type argument can be inferred from the argument.

Err / ErrPromise

Err creates a Result which has an associated error value. The type argument can be inferred from the argument.



class CannotDivideByZero {}

function divide(
  numerator: number,
  denominator: number,
): Result<number, CannotDivideByZero> {
  if (denominator === 0) {
    return Err(new CannotDivideByZero());
  } else {
    return Ok(numerator / denominator);

// The return value of the function is always a result
for (const result of [divide(7, 0), divide(2.0, 3.0)]) {
    (_) => console.error("Cannot divide by zero"),
    (ok) => console.log(`Result: ${ok}`),
// "Cannot divide by zero"
// "Result: 0.6666666666666666"

A note on Ok()

When a Result has a void Ok type (Result<void,unknown>), Ok must be called as Ok()

Original implementation:

isOk() => boolean

Returns true if the result is a Ok value.

isErr() => boolean

Returns true if the result is an Err value.

unwrapOr(res: T) => T

Returns the contained value when it is an Ok, otherwise res.

map(fn: (val: T) => U) => Result<U,E>

Maps an Result<T,E> to Result<U,E> by applying a function to a contained Ok value, otherwise the Err.

mapOrElse(fn: (val: T) => U) => U

Maps a Result<T, E> to U by applying fallback function default to a contained Err value, or function fn to a contained Ok value.

This function can be used to unpack a successful result while handling an error.


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