@ehmpathy/error-fns

error-fns

Standardized helpful errors and methods for simpler, safer, and easier to read code.

Purpose

Standardize on helpful errors for simpler, safer, easier to read code

• extend the HelpfulError for observable and actionable error messages
• leverage the UnexpectedCodePath to eliminate complexity with narrowed codepaths
• leverage the BadRequestError to make it clear when your logic successfully rejected a request
• test that logic throws errors ergonomically with getError

install

npm install --save @ehmpathy/error-fns

use

UnexpectedCodePathError

The UnexpectedCodePath error is probably the most common type of error you'll throw.

It's common in business logic that you'll face a scenario that is technically possible but logically shouldn't occur.

For example, lets say you're writing on-vehicle code to check the tire pressures of a vehicle.

// given the tires to check
const tires = Tire[];

// first get the tire pressures for each
const tirePressures: number[] = tires.map(tire => getTirePressure(tire));

// now get the lowest tire pressure
const lowestTirePressure: number | undefined = tires.sort()[0];

In this case, its technically possible that lowestTirePressure could be undefined: there could not be any tires.

However, this is definitely an unexpected code path for our application. We can just halt our logic if we reach here, since we dont need to solve for it.

// sanity check that we do have a tire pressure
if (lowestTirePressure === undefined)
  throw new UnexpectedCodePath('no tire pressures found. can not compute lowest tire pressure', { tires });

With this, the type of lowestTirePressure has been narrowed from number | undefined to just number, so you wont have any type errors anymore.

Further, if this case does occur in real life, then it will be really easy to debug what happened and why. Your error message will include the tires input that caused the problem making this a breeze to debug. No more could not read property 'x' of undefined!

BadRequestError

The BadRequestError is probably the next most common type of error you'll throw.

It's common in business logic that callers will try to execute your logic with inputs that are simply logically not valid. The user may not understand that their input is not valid or there may just be a bug upstream that is resulting in invalid requests.

For example, imagine you have an api that returns the liked songs of a user

const getLikedSongsByUser = ({ userUuid }: { userUuid: string }) => {
  // lookup the user
  const user = await userDao.findByUuid({ uuid: userUuid });

  // if the user does not exist, this is an invalid request. we shouldn't be asked to lookup songs for fake users
  if (!user)
    throw new BadRequestError('user does not exist for uuid', { userUuid });

  // use a property of the user to lookup their favorite songs
  const songs = await spotifyApi.getLikesForUser({ spotifyUserId: user.spotifyUserId });
}

Whatever the reason for a caller making a logically invalid request, it's important to distinguish when your code is at fault versus when the request is at fault.

This is particularly useful when monitoring error rates. Its important to distinguish whether your software failed to execute or whether it successfully rejected the request for observability in monitoring for issues. The BadRequestError enables us to do this easily

For example, libraries such as the simple-lambda-handlers leverage BadRequestErrors to ensure that a bad request both successfully returns an error to the caller but is not marked as an lambda invocation error.

HelpfulError

The HelpfulError is the backbone of this pattern and is what you'll extend whenever you want to create a custom error.

The purpose of this error is to be as helpful as possible to whoever has to read it when its thrown.

To fulfill this goal, the error makes it very easy to specify what the issue was as well as any other information that may be helpful to understanding why it occurred at the time. It then pretty prints this information to make it easy to read when observing.

throw new HelpfulError(
  'the message of the error goes here',
  {
    context,
    relevantInfo,
    potentiallyHelpfulVariables,
    goHere,
  }
)

getError

The getError method is the cherry-on-top of this library.

When writing tests for logic that throws an error in certain situations, you may want to verify that the code indeed throws this error in a test.

The getError utility makes it really easy to assert that the expected error is thrown.

Under the hood, it executes or awaits the logic or promise you give it as input, catches the error that is throw, or throws a NoErrorThrownError of its own. It does the legwork of handling all three cases you may need to use it in and defining the return type correctly.

usecase 1: synchronous logic

const doSomething = () => { throw new HelpfulError('found me'); }

const error = getError(() => doSomething())
expect(error).toBeInstanceOf(HelpfulError);
expect(error.message).toContain('found me')

usecase 2: asynchronous logic

const doSomething = async () => { throw new HelpfulError('found me'); }

const error = await getError(() => doSomething())
expect(error).toBeInstanceOf(HelpfulError);
expect(error.message).toContain('found me')

usecase 3: a promise

const doSomething = async () => { throw new HelpfulError('found me'); }

const error = await getError(doSomething())
expect(error).toBeInstanceOf(HelpfulError);
expect(error.message).toContain('found me')