@vitest/spy 4.1.10: function and method spying with spyOn and fn

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: function and method spying with spyOn and fn
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/%40vitest/spy@4.1.10
Demonstrate these symbols/APIs:
  - fn
  - isMockFunction
  - spyOn

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:sha256:8b2267cce7bca535494ebc722543ee03f7d9b93ce6ebbdd6cc1d72a30b603509","contract":["fn creates mock functions that record invocations, arguments, and return values with default name vi.fn()","fn supports mockImplementationOnce for one-time overrides and falls back to base implementation","fn supports mockClear to reset call history without clearing implementation","fn supports mockReturnValue, mockReturnValueOnce, mockResolvedValueOnce, and mockRejectedValueOnce","spyOn wraps existing object methods and tracks invocations while delegating to original implementation","spyOn mockImplementation overrides method behavior with custom handler","spyOn mockRestore restores original method implementation and resets recorded calls","spyOn intercepts getter and setter property accessors with restore support","isMockFunction identifies mock functions and spy wrappers"],"goal":"function and method spying with spyOn and fn","kind":"HOW","packages":["pkg:npm/%40vitest/spy@4.1.10"],"schemaVersion":1,"symbols":["fn","isMockFunction","spyOn"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"esm","os":"linux","osVersionBucket":"24","packageManager":"npm","packageManagerVersion":"10.9.8","runtime":"node","runtimeVersion":"22.23.2","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/%40vitest/spy@4.1.10"],"schemaVersion":1,"subject":"pkg:npm/%40vitest/spy@4.1.10","symbols":["fn","isMockFunction","spyOn"],"verifierAdapter":"node-typescript@1"}

{
  "name": "sample-vitest-spy",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample-vitest-spy",
      "version": "1.0.0",
      "license": "MIT-0",
      "dependencies": {
        "@vitest/spy": "4.1.10"
      }
    },
    "node_modules/@vitest/spy": {
      "version": "4.1.10",
      "resolved": "https://registry.npmjs.org/@vitest/spy/-/spy-4.1.10.tgz",
      "integrity": "sha512-PLf/Ugvoq5wO/b4rwYCR1h2PSIdXz7wnkQFMiUpLdtM7l6pqVFcQIBEHyT1+l+cj7mNwAfZHzqXqDyjvOuwbDw==",
      "license": "MIT",
      "funding": {
        "url": "https://opencollective.com/vitest"
      }
    }
  }
}

{
  "name": "sample-vitest-spy",
  "version": "1.0.0",
  "private": true,
  "description": "Clean-room verification for fn and spyOn in @vitest/spy@4.1.10",
  "main": "test/contract.mjs",
  "type": "module",
  "scripts": {
    "test": "node test/contract.mjs"
  },
  "dependencies": {
    "@vitest/spy": "4.1.10"
  },
  "license": "MIT-0"
}

{
  "schemaVersion": 1,
  "goal": "function and method spying with spyOn and fn",
  "kind": "HOW",
  "packages": [
    "pkg:npm/%40vitest/spy@4.1.10"
  ],
  "symbols": [
    "fn",
    "isMockFunction",
    "spyOn"
  ]
}

import assert from 'node:assert';
import { fn, spyOn, isMockFunction } from '@vitest/spy';

async function testFnCreationAndCallTracking() {
  const mockAdder = fn((a, b) => a + b);
  assert.strictEqual(isMockFunction(mockAdder), true, 'fn instance should be recognized as mock function');
  assert.strictEqual(mockAdder.getMockName(), 'vi.fn()', 'default mock name in vitest spy v4 should be vi.fn()');

  mockAdder.mockName('customAdder');
  assert.strictEqual(mockAdder.getMockName(), 'customAdder', 'mockName should update mock function name');

  const result1 = mockAdder(10, 20);
  assert.strictEqual(result1, 30, 'mock implementation should return sum');
  assert.strictEqual(mockAdder.mock.calls.length, 1, 'mock should record 1 call');
  assert.deepStrictEqual(mockAdder.mock.calls[0], [10, 20], 'call arguments should match');
  assert.deepStrictEqual(mockAdder.mock.lastCall, [10, 20], 'lastCall should match latest invocation arguments');
  assert.deepStrictEqual(mockAdder.mock.results[0], { type: 'return', value: 30 }, 'mock result should record return');

  mockAdder.mockImplementationOnce((a, b) => a * b);
  const result2 = mockAdder(5, 6);
  assert.strictEqual(result2, 30, 'mockImplementationOnce should override implementation for one call');

  const result3 = mockAdder(2, 3);
  assert.strictEqual(result3, 5, 'mock should fallback to original implementation after one-time override');
  assert.strictEqual(mockAdder.mock.calls.length, 3, 'mock should have recorded 3 calls');

  mockAdder.mockClear();
  assert.strictEqual(mockAdder.mock.calls.length, 0, 'mockClear should clear call history');
  assert.strictEqual(mockAdder(1, 1), 2, 'implementation should remain intact after clear');
}

async function testFnReturnAndResolutionOverrides() {
  const valueMock = fn();

  valueMock.mockReturnValue('default-val');
  valueMock.mockReturnValueOnce('first-val');
  valueMock.mockReturnValueOnce('second-val');

  assert.strictEqual(valueMock(), 'first-val', 'first call should return first-val');
  assert.strictEqual(valueMock(), 'second-val', 'second call should return second-val');
  assert.strictEqual(valueMock(), 'default-val', 'subsequent calls should return default-val');

  const asyncMock = fn();
  asyncMock.mockResolvedValueOnce('async-ok');
  asyncMock.mockRejectedValueOnce(new Error('async-fail'));

  const resolved = await asyncMock();
  assert.strictEqual(resolved, 'async-ok', 'mockResolvedValueOnce should resolve with value');

  await assert.rejects(
    async () => { await asyncMock(); },
    { message: 'async-fail' },
    'mockRejectedValueOnce should reject with error'
  );
}

async function testSpyOnMethodInterceptionAndRestore() {
  const service = {
    multiplier: 2,
    process(n) {
      return n * this.multiplier;
    }
  };

  const spy = spyOn(service, 'process');
  assert.strictEqual(isMockFunction(spy), true, 'spyOn returns a mock function');

  const output1 = service.process(7);
  assert.strictEqual(output1, 14, 'original method should execute by default');
  assert.strictEqual(spy.mock.calls.length, 1, 'spy should record invocation');
  assert.deepStrictEqual(spy.mock.calls[0], [7], 'spy should record argument 7');

  spy.mockImplementation((n) => n + 100);
  const output2 = service.process(7);
  assert.strictEqual(output2, 107, 'spy should override implementation');

  spy.mockRestore();
  const output3 = service.process(7);
  assert.strictEqual(output3, 14, 'mockRestore should restore original method');
  assert.strictEqual(spy.mock.calls.length, 0, 'mockRestore resets mock calls and stops interception');
}

async function testSpyOnGettersAndSetters() {
  const store = {
    _token: 'alpha',
    get token() {
      return this._token;
    },
    set token(val) {
      this._token = val;
    }
  };

  const getSpy = spyOn(store, 'token', 'get').mockReturnValue('intercepted-token');
  assert.strictEqual(store.token, 'intercepted-token', 'getter spy should intercept property read');
  assert.strictEqual(getSpy.mock.calls.length, 1, 'getter spy should record access');

  getSpy.mockRestore();
  assert.strictEqual(store.token, 'alpha', 'restored getter should return underlying value');

  let setterCalls = 0;
  const setSpy = spyOn(store, 'token', 'set').mockImplementation((val) => {
    setterCalls += 1;
    store._token = val.toUpperCase();
  });

  store.token = 'beta';
  assert.strictEqual(setterCalls, 1, 'setter spy should intercept property assignment');
  assert.strictEqual(store._token, 'BETA', 'custom setter implementation should execute');

  setSpy.mockRestore();
  store.token = 'gamma';
  assert.strictEqual(store._token, 'gamma', 'restored setter should use original accessor');
}

async function main() {
  await testFnCreationAndCallTracking();
  await testFnReturnAndResolutionOverrides();
  await testSpyOnMethodInterceptionAndRestore();
  await testSpyOnGettersAndSetters();
  console.log('Contract tests passed successfully.');
}

main().catch((err) => {
  console.error('Contract test failed:', err);
  process.exit(1);
});