@badrap/result

A TypeScript result type taking cues from Rust's Result and Haskell's Either types. It's goals are:

Small, idiomatic API surface: Mix and match parts from Rust's Result and Haskell's Either types, but modify them to make the experience TypeScript-y (TypeScriptic? TypeScriptalicious?). Of course this is pretty subjective.
Coding errors should throw: While Result#map and Result#chain together somewhat resemble Promise#then but differ in that they don't implicitly wrap errors thrown in callbacks.
Be ergonomic but safe: Leverage TypeScript's type inference to make common cases simple while keeping type safety. This also helps to get a nice editor experience in e.g. Visual Studio Code.

Installation

$ npm i @badrap/result

Usage

import { Result } from "@badrap/result";

API

Result<T, E extends Error = Error> is a type that wraps either a value that is the result of a succesful computation and of type T, or an error of type E denoting a failed computation.

The type is actually an union of two types: Result.Ok<T, E> that wraps a success value and Result.Err<T, E> that wraps an error.

Result.ok / Result.err

Result.ok returns a new Result.Ok wrapping the given value, while Result.err returns a new Result.Err wrapping the given error.

const res = Result.ok(1);
res.isOk; // true

const res = Result.err(new Error());
res.isOk; // false

const res = Result.err(); // functionally equal to Result.err(new Error())
res.isOk; // false

Result.Ok has an additional property value containing the wrapped value. Similarly, Result.Err has the property error containing the wrapped error. They can be accessed after asserting to TypeScript's type checker that it's safe to do so. The isErr and isOk properties (see below) are handy for this.

const res = Math.random() < 0.5 ? Result.ok(1) : Result.err(new Error("oh no"));

if (res.isErr) {
  // TypeScript now knows that res is a Result.Err, and we can access res.error
  res.error; // Error("oh no")
}

if (res.isOk) {
  // TypeScript now knows that res is a Result.Ok, and we can access res.value
  res.value; // 1
}

Result#isOk / Result#isErr

Result#isOk and Result#isErr are complementary readonly properties. isOk is true for Result.Ok and false for Result.Err.

const ok = Result.ok(1);
ok.isOk; // true

const err = Result.err(new Error());
err.isOk; // false

isErr is the inverse of isOk: false for Result.Ok and true for Result.Err.

const ok = Result.ok(1);
ok.isErr; // false

const err = Result.err(new Error());
err.isErr; // true

Result#unwrap

Return the wrapped value for Result.Ok and throw the wrapped error for Result.Err. This can be modified for providing functions to map the value and error to some value.

const ok = Result.ok(1);
const err = Result.err(new Error("oh no"));

ok.unwrap(); // 1
err.unwrap(); // throws Error("oh no")

ok.unwrap((value) => value + 1); // 2
err.unwrap((value) => value + 2); // throws Error("oh no")

ok.unwrap(
  (value) => value + 1,
  (error) => 0,
); // 2
err.unwrap(
  (value) => value + 2,
  (error) => 0,
); // 0

As a small extra convenience the result types from the callbacks don't have to be the same. Here's an example Koa.js handler demonstrating this, using an imaginary validate function that returns a Result:

app.use(async ctx =>
  await validate(ctx.request.body).unwrap(
    async (value: any) => {
      ...
    },
    error => {
      ctx.status = 422;
      ctx.body = {
        message: "validation failed"
      };
    }
  )
);

Result#map

Return a new Result where the given function/functions have been applied to the wrapped value and error.

const ok = Result.ok(1);
const err = Result.err(new Error("oh no"));

ok.map((value) => value + 1).unwrap(); // 2
err.map((value) => value + 1).unwrap(); // throws Error("oh no")

ok.map(
  (value) => value + 1,
  (error) => new Error("mapped"),
).unwrap(); // 2
err
  .map(
    (value) => value + 1,
    (error) => new Error("mapped"),
  )
  .unwrap(); // throws Error("mapped")

Result#chain

const ok = Result.ok(1);
const err = Result.err(new Error("oh no"));

ok.chain((value) => Result.ok(value + 1)).unwrap(); // 2
err.chain((value) => Result.ok(value + 1)).unwrap(); // throws Error("oh no")

ok.chain(
  (value) => Result.ok(value + 1),
  (error) => Result.ok(0),
).unwrap(); // 2
err
  .chain(
    (value) => Result.ok(value + 1),
    (error) => Result.ok(0),
  )
  .unwrap(); // 0

Result.all

Return a new Result where the wrapped value is the collection of the wrapped values of the input array.

Result.all([Result.ok(1), Result.ok("test")]).unwrap(); // [1, "test"]

If any of the input results wraps an error then that result is returned as-is.

Result.all([Result.ok(1), Result.err(new Error("oh no"))]).unwrap(); // throws Error("oh no")

Non-array objects can also be given as arguments. In that case the wrapped output value is also an object.

Result.all({
  x: Result.ok(1),
  y: Result.ok("test"),
}).unwrap(); // { x: 1, y: "test" }

License

This library is licensed under the MIT license. See LICENSE.