jest-util 30.4.1: isError

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 jest-util.isError in pkg:npm/jest-util@30.4.1
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/jest-util@30.4.1
Demonstrate these symbols/APIs:
  - jest-util.isError

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.

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'use strict';

const { isError } = require('jest-util');

function checkError(value) {
  return isError(value);
}

module.exports = {
  checkError,
  isError
};

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{
  "name": "sample-jest-util-iserror",
  "version": "1.0.0",
  "private": true,
  "description": "verify jest-util.isError in pkg:npm/jest-util@30.4.1",
  "main": "index.js",
  "scripts": {
    "test": "node test/contract.cjs"
  },
  "dependencies": {
    "jest-util": "30.4.1"
  },
  "license": "MIT-0"
}

{
  "schemaVersion": 1,
  "goal": "verify jest-util.isError in pkg:npm/jest-util@30.4.1",
  "kind": "HOW",
  "packages": [
    "pkg:npm/jest-util@30.4.1"
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  "symbols": [
    "jest-util.isError"
  ]
}

'use strict';

const assert = require('assert');
const { isError } = require('jest-util');
const { checkError } = require('../index.js');

// 1. Standard Error instances
const standardError = new Error('standard error');
assert.strictEqual(isError(standardError), true, 'isError must return true for Error instance');
assert.strictEqual(checkError(standardError), true, 'checkError must return true for Error instance');

const typeError = new TypeError('type error');
assert.strictEqual(isError(typeError), true, 'isError must return true for TypeError instance');

const rangeError = new RangeError('range error');
assert.strictEqual(isError(rangeError), true, 'isError must return true for RangeError instance');

const syntaxError = new SyntaxError('syntax error');
assert.strictEqual(isError(syntaxError), true, 'isError must return true for SyntaxError instance');

// 2. Custom Error classes inheriting from Error
class CustomAppError extends Error {
  constructor(message, code) {
    super(message);
    this.name = 'CustomAppError';
    this.code = code;
  }
}
const customError = new CustomAppError('custom failed', 500);
assert.strictEqual(isError(customError), true, 'isError must return true for custom Error subclass');
assert.strictEqual(checkError(customError), true, 'checkError must return true for custom Error subclass');

// 3. Non-error values must return false
assert.strictEqual(isError(null), false, 'isError must return false for null');
assert.strictEqual(isError(undefined), false, 'isError must return false for undefined');
assert.strictEqual(isError('Error message'), false, 'isError must return false for string');
assert.strictEqual(isError(404), false, 'isError must return false for number');
assert.strictEqual(isError(true), false, 'isError must return false for boolean');
assert.strictEqual(isError(Symbol('err')), false, 'isError must return false for Symbol');
assert.strictEqual(isError({}), false, 'isError must return false for plain object');
assert.strictEqual(isError([]), false, 'isError must return false for array');

// 4. Object mimicking Error structure but not an Error instance
const fakeError = {
  name: 'Error',
  message: 'fake error message',
  stack: 'Error: fake error message\n    at Object.<anonymous>'
};
assert.strictEqual(isError(fakeError), false, 'isError must return false for object mimicking Error');

console.log('Contract tests passed successfully.');
process.exit(0);