@fastify/error 4.2.0: createError

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify @fastify/error.createError in pkg:npm/%40fastify/error@4.2.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/%40fastify/error@4.2.0
Demonstrate these symbols/APIs:
  - @fastify/error.createError

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:c71f95fc6add0a9e3b1f9f48899fa6179087c66f3b26747b3cfc4965d23c7716","contract":["createError(code, message, statusCode = 500, Base = Error) returns a FastifyError constructor that uppercases code, formats message with util.format, and supports instantiation with or without new","Instances inherit from Base and satisfy instanceof for both the specific constructor and FastifyError, exposing code, name ('FastifyError'), statusCode (or undefined when statusCode is 0), and an optional cause extracted from a trailing { cause } argument","createError throws an Error when code or message is empty"],"goal":"verify @fastify/error.createError in pkg:npm/%40fastify/error@4.2.0","kind":"HOW","packages":["pkg:npm/%40fastify/error@4.2.0"],"schemaVersion":1,"symbols":["@fastify/error.createError"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"npm","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/%40fastify/error@4.2.0"],"schemaVersion":1,"subject":"pkg:npm/%40fastify/error@4.2.0","symbols":["@fastify/error.createError"],"verifierAdapter":"node-typescript@1"}

{
  "name": "fastify-error-sample",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "fastify-error-sample",
      "version": "1.0.0",
      "license": "MIT-0",
      "dependencies": {
        "@fastify/error": "4.2.0"
      }
    },
    "node_modules/@fastify/error": {
      "version": "4.2.0",
      "resolved": "https://registry.npmjs.org/@fastify/error/-/error-4.2.0.tgz",
      "integrity": "sha512-RSo3sVDXfHskiBZKBPRgnQTtIqpi/7zhJOEmAxCiBcM7d0uwdGdxLlsCaLzGs8v8NnxIRlfG0N51p5yFaOentQ==",
      "funding": [
        {
          "type": "github",
          "url": "https://github.com/sponsors/fastify"
        },
        {
          "type": "opencollective",
          "url": "https://opencollective.com/fastify"
        }
      ],
      "license": "MIT"
    }
  }
}

{
  "name": "fastify-error-sample",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "license": "MIT-0",
  "scripts": {
    "test": "node test/contract.mjs"
  },
  "dependencies": {
    "@fastify/error": "4.2.0"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify @fastify/error.createError in pkg:npm/%40fastify/error@4.2.0",
  "kind": "HOW",
  "packages": [
    "pkg:npm/%40fastify/error@4.2.0"
  ],
  "symbols": [
    "@fastify/error.createError"
  ]
}

import { createError, FastifyError } from '@fastify/error'

export function buildNotFoundError () {
  return createError('fst_err_not_found', 'Resource %s was not found', 404, TypeError)
}

export function buildInternalError () {
  return createError('FST_ERR_INTERNAL', 'Internal failure: %s')
}

export { createError, FastifyError }

import assert from 'node:assert/strict'
import { createError, FastifyError, buildNotFoundError, buildInternalError } from '../src/index.mjs'

const NotFoundError = buildNotFoundError()
const errWithNew = new NotFoundError('item-42')
assert.equal(errWithNew.code, 'FST_ERR_NOT_FOUND')
assert.equal(errWithNew.name, 'FastifyError')
assert.equal(errWithNew.statusCode, 404)
assert.equal(errWithNew.message, 'Resource item-42 was not found')
assert.equal(errWithNew.toString(), 'FastifyError [FST_ERR_NOT_FOUND]: Resource item-42 was not found')
assert.equal(errWithNew instanceof NotFoundError, true)
assert.equal(errWithNew instanceof FastifyError, true)
assert.equal(errWithNew instanceof TypeError, true)

const rootCause = new Error('disk read failed')
const InternalError = buildInternalError()
const errWithoutNew = InternalError('db-timeout', { cause: rootCause })
assert.equal(errWithoutNew.code, 'FST_ERR_INTERNAL')
assert.equal(errWithoutNew.statusCode, 500)
assert.equal(errWithoutNew.message, 'Internal failure: db-timeout')
assert.equal(errWithoutNew.cause, rootCause)
assert.equal(errWithoutNew instanceof InternalError, true)
assert.equal(errWithoutNew instanceof FastifyError, true)

const NoStatusError = createError('FST_ERR_NO_STATUS', 'No status', 0)
const noStatusInstance = new NoStatusError()
assert.equal(noStatusInstance.statusCode, undefined)

assert.throws(
  () => createError('', 'missing code'),
  { message: 'Fastify error code must not be empty' }
)
assert.throws(
  () => createError('FST_ERR_EMPTY_MSG', ''),
  { message: 'Fastify error message must not be empty' }
)