Package Exports

@fastify/error
@fastify/error/index.js

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (@fastify/error) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

@fastify/error

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

Install

npm i @fastify/error

Usage

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

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.

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

How to use an interpolated string:

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:

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')

TypeScript

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

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

instanceof

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

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.

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.

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.

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.

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.