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Create custom error types.



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( // '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 {
} catch (cause) {
  throw new UserError('message', { cause })

Custom initialization logic

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


npm install error-type

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


errorType(errorName, onCreate?)

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

Custom initialization logic

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

By default, it sets any options as error properties. However, you can override it with any custom logic to validate, normalize options, 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, options) {
  Object.assign(error, options, ERROR_PROPS[])

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

Best practices

A common pattern for custom error types is:

class CustomError extends Error {
  constructor(message) {
    super(message) = 'CustomError'

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

  • error.cause is not set
  • Unlike native error types, 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.


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

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