@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 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:
  - 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:sha256:8394a58910c67cd36e4929603e29347089fd0b69bbb437b40ea84c4ab9f11ac8","contract":["createError(code, message, statusCode, Base) returns an error constructor that normalizes code to uppercase and formats message with util.format arguments","Calling the returned constructor with or without new produces an error instance with code, statusCode, name 'FastifyError', and instanceof checks passing for the constructor, FastifyError, and Base","Passing a trailing { cause } object sets the error cause property without consuming it as a format placeholder","statusCode defaults to 500 when omitted and becomes undefined when passed 0, while empty code or message throws an Error"],"goal":"verify createError in pkg:npm/%40fastify/error@4.2.0","kind":"HOW","packages":["pkg:npm/%40fastify/error@4.2.0"],"schemaVersion":1,"symbols":["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":["createError"],"verifierAdapter":"node-typescript@1"}

{
  "name": "sample",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample",
      "version": "1.0.0",
      "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": "sample",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "dependencies": {
    "@fastify/error": "4.2.0"
  }
}

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

import assert from 'node:assert/strict'
import { createError, FastifyError } from '@fastify/error'

const CustomError = createError('err_invalid_input', 'Invalid value %s for %s', 400, TypeError)

const errWithNew = new CustomError('foo', 'bar')
assert.equal(errWithNew.code, 'ERR_INVALID_INPUT')
assert.equal(errWithNew.name, 'FastifyError')
assert.equal(errWithNew.statusCode, 400)
assert.equal(errWithNew.message, 'Invalid value foo for bar')
assert.equal(errWithNew.toString(), 'FastifyError [ERR_INVALID_INPUT]: Invalid value foo for bar')
assert.equal(errWithNew instanceof CustomError, true)
assert.equal(errWithNew instanceof FastifyError, true)
assert.equal(errWithNew instanceof TypeError, true)
assert.equal(errWithNew instanceof Error, true)

const rootCause = new Error('underlying failure')
const errWithoutNew = CustomError('baz', 'qux', { cause: rootCause })
assert.equal(errWithoutNew.message, 'Invalid value baz for qux')
assert.equal(errWithoutNew.cause, rootCause)

const DefaultError = createError('ERR_DEFAULT', 'Something failed')
const defaultErr = new DefaultError()
assert.equal(defaultErr.statusCode, 500)

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

assert.throws(() => createError('', 'message'), /Fastify error code must not be empty/)
assert.throws(() => createError('ERR_CODE', ''), /Fastify error message must not be empty/)