Error-type NPM

Create custom error types.

Features

Simple API: errorType('errorName')
Follows best practices
Error properties can be set on initialization: new CustomError('message', { props: { example: 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 DatabaseError = errorType('DatabaseError')

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

Error properties

const userError = new UserError('message', { props: { 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, { props }) => {
  error.dbId = props.databaseId
})
const databaseError = new DatabaseError('message', { props: { 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: CustomError

Custom initialization logic

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

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

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.