@vltpkg/error-cause

Utility functions for Error creation to help enforce vlt’s Error.cause conventions.

Usage · Error Reporting · Conventions · Error Types

Why

Most node programs have a mishmash of error codes and various Error subtypes, all in different shapes, making error handling and reporting more difficult at the top level. This negatively impacts debugging and user experience.

The JavaScript Error constructor has a cause option which is supported since Node 16.9. We should use it!

This module makes that easy.

Usage

import { error, typeError } from '@vltpkg/error-cause'

// create an error when a lower-level thing fails
try {
  doSomethign()
} catch (er) {
  throw error('The something for the whatever failed', er)
}

// create an error with some extra information
if (!thing.valid) {
  throw error('the thing is not valid', {
    code: 'EINVAL',
    found: thing,
  })
}

// create an error from a lower-level error with extra info
try {
  doSomethign(thing)
} catch (er) {
  throw error('the thing is not valid', {
    code: 'EINVAL',
    found: thing,
    cause: er,
  })
}

// create an error and prune some stack frames
// use this when we want to report the location of a
// function call, not its internals.
const checkBar = () => {
  if (!bar) {
    // will report from the checkBar() call, not here.
    throw error('no bar', { found: bar, wanted: true }, checkBar)
  }
  // ...
}

The functions will create an error object with a cause property if set, and the type checks will ensure that the cause object matches vlt’s conventions.

Challenges of Error Reporting

• Provide enough information to be useful. On full inspection, we should ideally always get back to not just the initial error that was thrown, but also all locations where the error might have been caught and handled in some way.
• Do not provide more information than is useful. Eg, console.error(er) should not fill the entire scrollback buffer.
• New modules and libraries should have minimal friction in creating a new style of error when needed. This means, minimize the amount that any module needs to know about the errors raised by any other module, including especially top-level error handling.
• Some information about the error must be known to our top-level error handler, so that it can usefully report errors and suggest corrections.

Solution

• A strictly upheld convention of Error object creation using the cause property.
• Top level error handler can have special logic where necessary for known error codes, but will still be able to do something more useful when an Error object follows our conventions, even if it’s not a code that it knows.

Conventions

The following conventions should be followed for all Error creation and handling throughout the vlt codebase.

• If you can’t help, get out of the way. Just let throws pass through to the top when nothing can be done to assist.
• Add information by using thrown error as cause. Use a previously-thrown error as the cause option.
• Add even more info with a double-cause. If more info can be added to a prior throw, nest the cause properties like { some, other, info, cause: priorError }.
• Always set cause, even if no prior error. Use a plain-old JavaScript object following our field conventions.
• Rare exception: synthetic ErrnoException style errors. If we are doing something that is similar to a system operation, it’s sometimes ok to mimic node’s pattern.
• Do not subclass Error. Just create a plain old Error, and set the cause with additional information.

If you can’t help, don’t get in the way.

Whenever possible, if no remediation or extra information can usefully be added, it’s best to just not handle errors and let them be raised at the higher level. For example, instead of this:

let data
try {
  data = await readFile(someFile)
} catch (er) {
  throw new Error('could not read some file!')
}

this is preferred:

const data = await readFile(someFile)

Add information by using thrown error as cause.

If we can add information or do something else useful for the user in understanding the problem, do so by creating a new Error and setting the original thrown error as the cause.

let data
try {
  data = await readFile(someFile, 'utf8')
} catch (er) {
  // adds semantic information about what the file was for
  throw error('The lock file was not found', er)
}

Add even more info with a double-cause.

If we can add even more information, that should ideally not be put on the Error we throw, but on a cause object. Because cause objects can nest, we can do something like this:

let data
try {
  data = await readFile(someFile, 'utf8')
} catch (er) {
  throw error(`could not resolve '${name}'`, {
    // extra data about the situation
    // it's ok to put big noisy objects in here, not on the error
    // object itself!
    name,
    spec,
    target,
    // original error that was thrown
    cause: er,
  })
}

Always set cause, even if no prior error.

Instead of this:

throw Object.assign(new Error('could not resolve'), {
  code: 'ERESOLVE',
  from,
  spec,
  registry,
})

Do this instead:

throw error('could not resolve', {
  code: 'ERESOLVE',
  from,
  spec,
  registry,
})

This makes any big objects easily skipped if we want to just output the error with console.error() or something, but still preserves any debugging information that might be useful all the way down the chain.

Rare exception: synthetic ErrnoException style errors.

In some rare low-level cases, there are operations we perform that are very similar to a node filesystem operation.

For example, the @vltpkg/which module raises an error that is intentionally similar to node’s filesystem ENOENT errors, because that is semantically sensible.

In those cases, the error must follow node’s conventions as close as possible. If we feel the need to add additional information beyond a known system error code, string path, etc., or if the message isn’t one that is typically raised by the underlying system, then it’s a good sign that we ought to be creating an Error with a cause so that it can be reported more usefully.

In such cases, this is fine:

// identical to the error thrown by node's fs
throw Object.assign(new Error('not found'), {
  path: someFile,
  code: 'ENOENT',
})

But this is way out of bounds and makes no sense:

throw Object.assign(new Error('could not resolve'), {
  code: 'EPERM',
  spec,
  config: someHugeConfigObjectOrSomething,
})

Do not copy properties from a lower-level error or cause onto the new cause object. That is unnecessary, and obscures the origin of problems. Instead, just include the lower-level error as the cause property. If you already have a low-level error, you don’t need to invent a synthetic one!

For example, do not do this:

let data
try {
  data = await readFile(lockFile, 'utf8')
} catch (er) {
  throw error('lockfile not found', {
    code: er.code,
    path: er.path,
  })
}

Instead, do this:

let data
try {
  data = await readFile(lockFile, 'utf8')
} catch (er) {
  throw new Error('lockfile not found', { cause: er })
}

Do not subclass Error.

Just use the Error classes defined in the language. Additional information about error causes should be on the cause property, not implicit in the constructor type.

I.e. do not do this:

class VersionError extends Error {
  version?: Version
  constructor(version: Version | string) {
    super('Could not version')
    this.version = Version.parse(version)
  }
}
// ...
throw new VersionError(myVersion)

Instead, do this:

throw error('Could not version', { version })

cause Field Conventions

All of these are optional. Additional fields may be used where appropriate, and should be added to this list over time.

• cause - The cause field within a cause object should always be an Error object that was previously thrown. Note that the cause on an Error itself might also be a previously thrown error, if no additional information could be usefully added beyond improving the message.
• name - String. The name of something.
• offset - Number. The offset in a Buffer or file where we are trying to read or write.
• registry - String or URL. A package registry.
• code - This must be a string if set, and should only be present if it’s one of our creation, not a code raised on a system error. Eg, ERESOLVE, not ENOENT.
• path - The target of a file system operation.
• target - path on disk that is being written or extracted to
• spec - a @vltpkg/spec.Spec object relevant to the operation that failed.
• from - string. The file path origin of a resolution that failed, for example in the case of relative file: specifiers.
• status - Number or null. Either the exit code of a process or an HTTP response status code.
• signal - NodeJS.Signals string or null, indicating the signal that terminated a process.
• validOptions - Array of valid options when something is not a valid option. (For use in did you mean X? output.)
• todo - String message indicating what bit of work this might be a part of, what feature needs to be implemented, etc. Eg, { todo: 'nested workspace support' }.
• wanted - A desired value that was not found, or a regular expression or other pattern describing it.
• found - The actual value, which was not wanted.
• max - A maximum value, which was exceeded.
• min - A minimum value, which was not met.
• response - An HTTP response or @vltpkg/registry-client.CacheEntry
• url - A string or URL object
• repository - String git repository remote
• version - string or @vltpkg/semver.Version
• range - string or @vltpkg/semver.Range
• manifest - @vltpkg/pick-manifest.Manifest
• packument - @vltpkg/pick-manifest.Packument
• cwd - The current working directory of a process that failed

Error Types

• If there is a type problem with an argument, for example a string was expected and a number was provided, throw a TypeError. Do not use it for a value that is the correct type but otherwise invalid, such as a string argument that is actually a string but does not match an expected pattern.
• If the type is fine, but a parsed string is invalid and not parseable, use SyntaxError.
• In all other cases, use Error.