error-type

Create custom error types.

Features

• Simple API: errorType('errorName')
• Follows best practices
• Error properties can be set on initialization: new CustomError('message', { exampleProp: true })
• Polyfills error.cause on older Node.js and browsers
• Optional custom initialization logic

Examples

Custom error types

import errorType from 'error-type'

const UserError = errorType('UserError')
const SystemError = errorType('SystemError')

// Throwing with custom error types
try {
  throw new UserError('message')
} catch (error) {
  console.log(error.name) // 'UserError'
  console.log(error instanceof UserError) // true
}

Error properties

// Error properties can be set using the second argument
const userError = new UserError('message', { userId: 56 })
console.log(userError.userId) // 56

Error cause

// `error.cause` can be used even in older Node.js or browsers
try {
  doSomething()
} catch (cause) {
  throw new UserError('message', { cause })
}

Custom initialization logic

const DatabaseError = errorType('DatabaseError', (error, params) => {
  error.dbId = params.databaseId
})
const databaseError = new DatabaseError('message', { databaseId: 2 })
console.log(databaseError.dbId) // 2
console.log(databaseError.databaseId) // undefined

Install

npm install error-type

This package is an ES module and must be loaded using an import or import() statement, not require().

API

errorType(errorName, onCreate?)

errorName string
onCreate (error, params) => void
Return value: ErrorType

Custom initialization logic

onCreate(error, params) is optional and is called on new ErrorType('message', params).

By default, it sets any params as error properties. However, you can override it with any custom logic to validate, normalize params, etc.

Error type properties

Some error properties are the same for all instances of a given error type. In other words, those are properties of the error type, not of specific instances.

Those can be set by using a separate object with each error type's properties. They can be assigned using onCreate():

const ERROR_PROPS = {
  UserError: { isBug: false },
  DatabaseError: { isBug: false },
  CoreError: { isBug: true },
}

const onCreate = function (error, params) {
  Object.assign(error, params, ERROR_PROPS[error.name])
}

const UserError = errorType('UserError', onCreate)
const DatabaseError = errorType('DatabaseError', onCreate)
const CoreError = errorType('CoreError', onCreate)

Alternatively, the logic that catches/handles the error can retrieve those error properties instead:

const UserError = errorType('UserError')
const DatabaseError = errorType('DatabaseError')
const CoreError = errorType('CoreError')

const ERROR_PROPS = {
  UserError: { isBug: false },
  DatabaseError: { isBug: false },
  CoreError: { isBug: true },
}

try {
  doSomething()
} catch (error) {
  const isBug =
    error instanceof Error && error.name in ERROR_PROPS
      ? ERROR_PROPS[error.name].isBug
      : true
}

Best practices

Constructor

A common pattern for custom error types is:

class CustomError extends Error {
  constructor(message) {
    super(message)
    this.name = 'CustomError'
  }
}

However, this has several issues (which error-type handles):

• error.cause is not set
• Unlike native error types, error.name is:
  • Enumerable, although it should not. For example, for (const key in error) will iterate over name, which is unexpected.
  • Set on the error instance instead of its prototype. In Node.js, this sometimes results in the error name being printed as Error [CustomError] instead of CustomError.

Polyfills

Some Error polyfills (such as es-shims/error-cause prevent extending from it. This library includes some logic to fix this.

Related projects

• modern-errors: Handle errors like it's 2022 🔮
• error-serializer: Convert errors to/from plain objects
• normalize-exception: Normalize exceptions/errors
• merge-error-cause: Merge an error with its cause
• error-cause-polyfill: Polyfill error.cause
• handle-cli-error: 💣 Error handler for CLI applications 💥

Support

For any question, don't hesitate to submit an issue on GitHub.

Everyone is welcome regardless of personal background. We enforce a Code of conduct in order to promote a positive and inclusive environment.

Contributing

This project was made with ❤️. The simplest way to give back is by starring and sharing it online.

If the documentation is unclear or has a typo, please click on the page's Edit button (pencil icon) and suggest a correction.

If you would like to help us fix a bug or add a new feature, please check our guidelines. Pull requests are welcome!