@vitest/expect 2.1.9: isA

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 @vitest/expect.isA in pkg:npm/%40vitest/expect@2.1.9
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/%40vitest/expect@2.1.9
Demonstrate these symbols/APIs:
  - @vitest/expect.isA

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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import { isA } from '@vitest/expect';

/**
 * Checks whether a given value matches the expected type name tag.
 *
 * @param {string} typeName - Expected internal [[Class]] tag, e.g. "String", "Array", "Map".
 * @param {unknown} value - Target value to inspect.
 * @returns {boolean} True if the value's string representation matches "[object <typeName>]".
 */
export function checkType(typeName, value) {
  return isA(typeName, value);
}

export { isA };

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{
  "name": "sample-vitest-expect-is-a",
  "version": "1.0.0",
  "description": "Clean-room sample verifying @vitest/expect.isA",
  "license": "MIT-0",
  "type": "module",
  "scripts": {
    "test": "node test/contract.mjs"
  },
  "dependencies": {
    "@vitest/expect": "2.1.9"
  }
}

{
  "schemaVersion": 1,
  "goal": "verify @vitest/expect.isA in pkg:npm/%40vitest/expect@2.1.9",
  "kind": "HOW",
  "packages": [
    "pkg:npm/%40vitest/expect@2.1.9"
  ],
  "symbols": [
    "@vitest/expect.isA"
  ]
}

import assert from 'node:assert/strict';
import { isA } from '@vitest/expect';
import { checkType } from '../index.js';

// 1. Primitive types verification
assert.equal(isA('String', 'hello world'), true);
assert.equal(isA('String', ''), true);
assert.equal(isA('Number', 42), true);
assert.equal(isA('Number', 0), true);
assert.equal(isA('Number', NaN), true);
assert.equal(isA('Number', Infinity), true);
assert.equal(isA('Boolean', true), true);
assert.equal(isA('Boolean', false), true);
assert.equal(isA('Undefined', undefined), true);
assert.equal(isA('Null', null), true);
assert.equal(isA('Symbol', Symbol('sym')), true);
assert.equal(isA('BigInt', 9007199254740991n), true);

// 2. Built-in object structures
assert.equal(isA('Array', [1, 2, 3]), true);
assert.equal(isA('Array', []), true);
assert.equal(isA('Object', { key: 'value' }), true);
assert.equal(isA('Object', {}), true);
assert.equal(isA('Date', new Date()), true);
assert.equal(isA('RegExp', /abc/i), true);
assert.equal(isA('Promise', Promise.resolve(1)), true);
assert.equal(isA('Error', new Error('test err')), true);

// 3. Collection instances
assert.equal(isA('Map', new Map()), true);
assert.equal(isA('Set', new Set()), true);
assert.equal(isA('WeakMap', new WeakMap()), true);
assert.equal(isA('WeakSet', new WeakSet()), true);

// 4. Function variants
assert.equal(isA('Function', function named() {}), true);
assert.equal(isA('Function', () => {}), true);
assert.equal(isA('AsyncFunction', async () => {}), true);
assert.equal(isA('GeneratorFunction', function* () {}), true);

// 5. Custom Symbol.toStringTag
const customTagged = { [Symbol.toStringTag]: 'CustomTag' };
assert.equal(isA('CustomTag', customTagged), true);
assert.equal(checkType('CustomTag', customTagged), true);

// 6. Negative assertions / type discrimination
assert.equal(isA('Array', { 0: 'a', length: 1 }), false);
assert.equal(isA('Object', [1, 2, 3]), false);
assert.equal(isA('Number', '42'), false);
assert.equal(isA('String', 42), false);
assert.equal(isA('Function', {}), false);
assert.equal(isA('AsyncFunction', function () {}), false);

console.log('All contract assertions passed.');