andrejewski / result

Typescript result type

@andrejewski/result

A result type written in TypeScript

npm install @andrejewski/result

import { Result, ok, err } from '@andrejewski/result'

function parseJSON (json: string): Result<unknown, SyntaxError> {
  try {
    return ok(JSON.parse(json))
  } catch (error) {
    if (error instanceof SyntaxError) {
      return err(error)
    }

    throw error
  }
}

const parseResult = parseJSON('{"foo": "bar"}')
const fooResult = parseResult.andThen(value => {
  if (!(value && value.foo && typeof value.foo === 'string')) {
    err(new Error('Field `foo` must be of type string'))
  }

  return ok(value.foo)
})

const foo = fooResult.match({
  ok (value) {
    return value
  },
  err (error) {
    throw error
  }
})

Documentation

See the type definitions and code comments for detailed documentation.

Cool features

Limited constructors

There's only two ways to create result types: ok and err. Nice and short: no need to use new Result(x, y), new Ok(x), etc.

Type narrowing

To let Typescript help us, we enable type guards via discriminated unions on multiple fields, so we can write clean branches:

import { Result, ok } from '@andrejewski/result'

const result = ok(1) as Result<number, never>

// Type guard using `type` property
if (result.type === 'ok') {
  // We can now access `value`
  console.log(result.value)
} else {
  // We can now access `error`
  console.log(result.error)
}

// Type guard using `isOk` property
if (result.isOk) {
  // We can now access `value`
  console.log(result.value)
} else {
  // We can now access `error`
  console.log(result.error)
}

// Type guard using `isErr` property
if (result.isErr) {
  // We can now access `error`
  console.log(result.error)
} else {
  // We can now access `value`
  console.log(result.value)
}

A lack of features

Hey now, lack of features is totally a feature!

No unwrap

You'll notice I didn't add some "niceties" like Result#unwrap, which throws the err component and returns the ok component of a result. Including those methods makes it too easy to use them internally whereas they should really only be used, if at all, at the edges of a system. Folks can build there own on top but the friction of doing so is a feature.

No toJSON

We don't provide any common mechanism for JSON de/serialization because it has many quirks. These can be built on top but don't need to be in this package.

No try sugar

If you are coming from Rust, you'll have loved the result? unwrapping sugar. Unfortunately that type of sugar, even as written as unwrap(result) instead of a macro, is not going to be helped by TypeScript. Simply put, there's no good way to have a good error type derived from successive calls of a function:

const result = Result.try(unwrap => {
  const a = unwrap(fnErrorsA())
  const b = unwrap(fnErrorsB(a))
  return ok(nonResultFn(b))
})

Here, the best we could do is result having type Result<typeof b, unknown> but that's not good enough! I don't wanna have to re-check for all the potential errors.
So we don't include any syntactic sugar to make this easier, preferring correctness and precise types.

Why not use X?

There are alternatives to this package's take on result type. Here are some and why I'm not using them:

result:

• Large surface area: tons of methods that aren't necessary
• .then and .node smatter against Promise and callback asynchrony
• No TypeScript types

result-js

• No TypeScript types
• Verbose methods for checking type e.g. result.isOk() that can't type guard
• Verbose constructors fromSuccess and fromError which mismatch ok/err naming
• Many unwrap* methods which make it too easy to use exceptions

typescript-result

• Poor Typescript types
• Couples the err type to Error which need not be the case
• Puts the err type on the leftmost type parameter which is very confusing to read
• Provides weird Result.safe method

ts-results

• Includes an Option type which isn't needed in JavaScript/Typescript
• Member ok.val is the same in err.val meaning you don't need to narrow the type to access the value, and you might forget to do that
• Includes expect and unwrap methods which encourage exceptions