CODE-GUIDELINES.md

Guidelines for Podman Desktop Code

Production code

Unit tests code

Use vi.mocked, not a generic myFunctionMock

If you define a mock with const myFunctionMock = vi.fn(); its type is Mock<Procedure>, which is a generic type.

For example, do not write this, or Typescript won't be able to detect that you passed an object instead of a string to mockResolvedValue:

const windowMethodMock = vi.fn();

Object.defineProperty(global, 'window', {
  value: {
    windowMethod: windowMethodMock,
  },
});

test('...', () => {
  windowMethodMock.mockResolvedValue({ msg: 'a string' }); // here, Typescript is not able to detect that the type is wrong
});

Instead, you can write vi.mocked(window.windowMethod).mock..., and Typescript will check that you correctly pass a string to mockResolvedValue:

Object.defineProperty(global, 'window', {
  value: {
    windowMethod: vi.fn(),
  },
});

test('...', () => {
  vi.mocked(window.windowMethod).mockResolvedValue('a string');
});

Mock complete modules, spy on parts of module for specific tests

When testing a module, you have to decide for each imported module if you mock the entire module or if you spy on specific functions of the module for specific tests and keep the real implementation for the other functions.

System modules (node:fs, etc) are most generally mocked, so you are sure that unit tests are executed in isolation of the system. For internal modules, it's up to you to decide if you want to mock them or not, depending on the coverage you want for the unit tests.

Mock a complete module

Mock completely an imported module with vi.mock('/path/to/module), and define mock implementation for each test with vi.mocked(function).mock...().

Use vi.resetAllMocks() in the top-level beforeEach to reset all mocks to a no-op function returning undefined before to start each test.

import { existsSync } from 'node:fs';
import { beforeEach, describe, expect, test, vi } from 'vitest';

// completely mock the fs module, to be sure to
// run the tests in complete isolation from the filesystem
vi.mock('node:fs');

beforeEach(() => {
  vi.resetAllMocks();
});

describe('the file exists', () => {
  beforeEach(() => {
    vi.mocked(existsSync).mockReturnValue(true);
  });

  test('file exists', () => {
    // existsSync is mocked to return true
    expect(codeCheckingIfFileExists('/file/not/found')).toBeTruthy();
  });
});

describe('the file does not exist', () => {
  beforeEach(() => {
    vi.mocked(existsSync).mockReturnValue(false);
  });

  test('root does not exists', () => {
    // existsSync is mocked to return false
    expect(codeCheckingIfFileExists('/')).toBeFalsy();
  });
});

test('file existence is not defined', () => {
  // a no-op mock returning undefined is called
  expect(codeCheckingIfFileExists('/file/not/found')).toBeUndefined();
});

Spy on a function for a specific test

When you want to mock only one or a small number of functions of a module (for example a function of the module you are testing, or a function of an helper module from which you want to use real implementation for some functions) for a particular test, you can use vi.spyOn(module, 'function') to mock only function and keep the original implementation for the rest of the module.

To be sure that the spied function is restored to its original implementation for the other tests, use vi.restoreAllMocks() in the top-level beforeEach.

// helpers.ts
export function f1(): boolean {
  return true;
}

// mymodule.ts
import { f1 } from './helpers.js';

export class MyModuleToTest {
  f2(): boolean {
    return f1();
  }
}

// mymodule.spec.ts
import { beforeEach, describe, expect, test, vi } from 'vitest';
import { MyModuleToTest } from './mymodule.js';
import * as helpers from './helpers.js';

let myModuleToTest: MyModuleToTest;

beforeEach(() => {
  myModuleToTest = new MyModuleToTest();

  // restore f1 to its original implementation
  vi.restoreAllMocks();
});

describe('f1 returns false', () => {
  beforeEach(() => {
    vi.spyOn(helpers, 'f1').mockReturnValue(false);
  });

  test('f2 returns false', () => {
    expect(myModuleToTest.f2()).toBeFalsy();
    expect(helpers.f1).toHaveBeenCalledOnce();
  });
});

test('f2 returns true', () => {
  // use the original implementation of f1
  expect(myModuleToTest.f2()).toBeTruthy();
  // this won't work, as f1 is not spied for this test
  // expect(helpers.f1).toHaveBeenCalledOnce();
});