@fastify/error


A small utility, used by Fastify itself, for generating consistent error objects across your codebase and plugins.

$3

``
npm i @fastify/error
`

$3

The module exports a function that you can use for consistent error objects, it takes 4 parameters:

`js
createError(code, message [, statusCode [, Base [, captureStackTrace]]])
`

- code (string, required) - The error code, you can access it later with error.code. For consistency, we recommend prefixing plugin error codes with FST_
- message (string, required) - The error message. You can also use interpolated strings for formatting the message.
- statusCode (number, optional) - The status code that Fastify will use if the error is sent via HTTP.
- Base (ErrorConstructor, optional) - The base error object that will be used. (eg TypeError, RangeError)
- captureStackTrace (boolean, optional) - Whether to capture the stack trace or not.

`js
const createError = require('@fastify/error')
const CustomError = createError('ERROR_CODE', 'Hello')
console.log(new CustomError()) // error.message => 'Hello'
`

How to use an interpolated string:
`js
const createError = require('@fastify/error')
const CustomError = createError('ERROR_CODE', 'Hello %s')
console.log(new CustomError('world')) // error.message => 'Hello world'
`

How to add cause:
`js
const createError = require('@fastify/error')
const CustomError = createError('ERROR_CODE', 'Hello %s')
console.log(new CustomError('world', {cause: new Error('cause')}))
// error.message => 'Hello world'
// error.cause => Error('cause')
`

$3

It is possible to limit your error constructor with a generic type using TypeScript:

`ts
const CustomError = createError<[string]>('ERROR_CODE', 'Hello %s')
new CustomError('world')
//@ts-expect-error
new CustomError(1)
`

$3

All errors created with createError will be instances of the base error constructor you provided, or Error if none was provided.

`js
const createError = require('@fastify/error')
const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError)
const customError = new CustomError('world')

console.log(customError instanceof CustomError) // true
console.log(customError instanceof TypeError) // true
console.log(customError instanceof Error) // true
`

All instantiated errors are instances of the FastifyError class, which can be required directly from the module.

`js
const { createError, FastifyError } = require('@fastify/error')
const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError)
const customError = new CustomError('world')

console.log(customError instanceof FastifyError) // true
`

A FastifyError created by createError can extend another FastifyError while maintaining correct instanceof behavior.

`js
const { createError, FastifyError } = require('@fastify/error')

const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError)
const ChildCustomError = createError('CHILD_ERROR_CODE', 'Hello %s', 500, CustomError)

const customError = new ChildCustomError('world')

console.log(customError instanceof ChildCustomError) // true
console.log(customError instanceof CustomError) // true
console.log(customError instanceof FastifyError) // true
console.log(customError instanceof TypeError) // true
console.log(customError instanceof Error) // true
`

If fastify-error is installed multiple times directly or as a transitive dependency, instanceof checks for errors created by createError will still work correctly across these installations, as long as their error codes (e.g., FST_ERR_CUSTOM_ERROR) are identical.

`js
const { createError, FastifyError } = require('@fastify/error')

// CustomError from @fastify/some-plugin is created with createError and
// has its own @fastify/error installation as dependency. CustomError has
// FST_ERR_CUSTOM_ERROR as code.
const { CustomError: CustomErrorFromPlugin } = require('@fastify/some-plugin')

const CustomError = createError('FST_ERR_CUSTOM_ERROR', 'Hello %s', 500)

const customError = new CustomError('world')
const customErrorFromPlugin = new CustomErrorFromPlugin('world')

console.log(customError instanceof CustomError) // true
console.log(customError instanceof CustomErrorFromPlugin) // true
console.log(customErrorFromPlugin instanceof CustomError) // true
console.log(customErrorFromPlugin instanceof CustomErrorFromPlugin) // true
`

Changing the code of an instantiated Error will not change the result of the instanceof operator.

`js
const { createError, FastifyError } = require('@fastify/error')

const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError)
const AnotherCustomError = createError('ANOTHER_ERROR_CODE', 'Hello %s', 500, CustomError)

const customError = new CustomError('world')
customError.code = 'ANOTHER_ERROR_CODE'

console.log(customError instanceof CustomError) // true
console.log(customError instanceof AnotherCustomError) // false
``

License

Licensed under MIT.