jest-mock 29.7.0: mock function creation and call tracking with jest-mock

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: mock function creation and call tracking with jest-mock
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/jest-mock@29.7.0
Demonstrate these symbols/APIs:
  - fn
  - 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.

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{
  "name": "sample-jest-mock",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "jest-mock": "29.7.0"
  }
}

{
  "schemaVersion": 1,
  "goal": "mock function creation and call tracking with jest-mock",
  "kind": "HOW",
  "packages": [
    "pkg:npm/jest-mock@29.7.0"
  ],
  "symbols": [
    "fn",
    "spyOn"
  ]
}

import { fn, spyOn } from "jest-mock";

/**
 * Creates a mock function that tracks invocations and return values.
 */
export function createTrackedHandler(implementation) {
  return implementation ? fn(implementation) : fn();
}

/**
 * Creates a stub function that returns predefined sequential values followed by a default.
 */
export function createSequenceStub(sequence, fallbackValue) {
  const mockFunc = fn();
  for (const item of sequence) {
    mockFunc.mockReturnValueOnce(item);
  }
  if (fallbackValue !== undefined) {
    mockFunc.mockReturnValue(fallbackValue);
  }
  return mockFunc;
}

/**
 * Attaches a spy to an object method to observe invocations.
 */
export function attachMethodSpy(target, methodName) {
  return spyOn(target, methodName);
}

/**
 * Attaches a spy to an object method and substitutes custom implementation.
 */
export function attachCustomImplementationSpy(target, methodName, implementation) {
  const spy = spyOn(target, methodName);
  spy.mockImplementation(implementation);
  return spy;
}

/**
 * Restores a spy to the original target method implementation.
 */
export function restoreMethodSpy(spy) {
  spy.mockRestore();
}

/**
 * Retrieves the recorded call history and results from a mock or spy.
 */
export function getMockCallSummary(mockOrSpy) {
  return {
    callCount: mockOrSpy.mock.calls.length,
    calls: mockOrSpy.mock.calls,
    results: mockOrSpy.mock.results.map(r => r.value)
  };
}

import assert from "node:assert/strict";
import { fn, spyOn } from "jest-mock";
import {
  createTrackedHandler,
  createSequenceStub,
  attachMethodSpy,
  attachCustomImplementationSpy,
  restoreMethodSpy,
  getMockCallSummary
} from "../src/index.mjs";

// Test 1: fn creates a callable mock function tracking arguments and return values in mock.calls and mock.results
{
  const handler = createTrackedHandler((x, y) => x + y);
  
  assert.equal(typeof handler, "function");
  assert.equal(handler.mock.calls.length, 0);

  const res1 = handler(10, 20);
  const res2 = handler(5, 15);

  assert.equal(res1, 30);
  assert.equal(res2, 20);

  const summary = getMockCallSummary(handler);
  assert.equal(summary.callCount, 2);
  assert.deepEqual(summary.calls, [[10, 20], [5, 15]]);
  assert.deepEqual(summary.results, [30, 20]);

  assert.equal(handler.mock.results[0].type, "return");
  assert.equal(handler.mock.results[0].value, 30);
  assert.equal(handler.mock.results[1].type, "return");
  assert.equal(handler.mock.results[1].value, 20);
}

// Test 2: fn mockReturnValue and mockReturnValueOnce support default and sequential return values
{
  const stub = createSequenceStub(["first", "second"], "fallback");

  assert.equal(stub(), "first");
  assert.equal(stub(), "second");
  assert.equal(stub(), "fallback");
  assert.equal(stub(), "fallback");

  assert.equal(stub.mock.calls.length, 4);

  // Direct fn test for mockReturnValue
  const defaultMock = fn().mockReturnValue(42);
  assert.equal(defaultMock(), 42);
  assert.equal(defaultMock(), 42);
}

// Test 3: fn mockImplementation overrides execution logic dynamically
{
  const operation = fn((val) => val * 2);
  assert.equal(operation(5), 10);

  operation.mockImplementation((val) => val * 10);
  assert.equal(operation(5), 50);

  operation.mockImplementationOnce((val) => `custom:${val}`);
  assert.equal(operation(5), "custom:5");
  assert.equal(operation(5), 50);
}

// Test 4: spyOn wraps an existing object method to intercept and record invocations without altering default behavior
{
  const calculator = {
    multiplier: 3,
    multiply(a, b) {
      return a * b * this.multiplier;
    }
  };

  const spy = attachMethodSpy(calculator, "multiply");

  assert.equal(spy.mock.calls.length, 0);
  const result = calculator.multiply(2, 4);
  assert.equal(result, 24); // original behavior preserved with context

  assert.equal(spy.mock.calls.length, 1);
  assert.deepEqual(spy.mock.calls[0], [2, 4]);
  assert.equal(spy.mock.results[0].value, 24);

  restoreMethodSpy(spy);
}

// Test 5: spyOn mockImplementation temporarily overrides method behavior and mockRestore restores original implementation
{
  const greeter = {
    greet(name) {
      return `Hello, ${name}!`;
    }
  };

  assert.equal(greeter.greet("Alice"), "Hello, Alice!");

  const spy = attachCustomImplementationSpy(greeter, "greet", (name) => `Hi, ${name}.`);
  assert.equal(greeter.greet("Alice"), "Hi, Alice.");
  assert.equal(spy.mock.calls.length, 1);
  assert.deepEqual(spy.mock.calls[0], ["Alice"]);

  restoreMethodSpy(spy);
  assert.equal(greeter.greet("Alice"), "Hello, Alice!");
}

console.log("All jest-mock contract assertions passed successfully.");