Mocking Fundamentals

This section implements mocking in vanilla JS without help from testing frameworks and helper functions like Jest. The goal is to better understand how mocking works behind the scenes.

What is Mocking?

The idea behind mocking comes when you have some module that is too expensive to use directly. For example, there could be asynchronous functions involved, or it could be processing a credit card payment, which is too expensive to actually test.

So, you create a fake or mock version of the module, so you can test the module without incurring the costs.

Monkey-patching: Overriding Object Properties

The process of monkey-patching is to

Override the object method with your own mock method.
Cleanup that mock method by reassigning the key to the original method after you're done your test. (This is so other tests aren't affected by what happens in the target test.)

In the example below, we have a utils object with helper functions that we need to monkey-patch:

const assert = require('assert');
const thumbWar = require('../thumb-war');
const utils = require('../utils');

// Override/monkey patch
const originalGetWinner = utils.getWinner; // store original
utils.getWinner = (p1, p2) => p1; // this method is now deterministic for easier testing

const winner = thumbWar('Kent C. Dodds', 'Ken Wheeler');
assert.strictEqual(winner, 'Kent C. Dodds');

// Cleanup
utils.getWinner = originalGetWinner;

Creating a Mock Factory Function

Jest version

A mock function is a special function that stores properties as it's called, making the function easier to test. We will be recreating some of the functionality of jest.fn.

In Jest, here's some cool things you can do:

utils.getWinner = jest.fn((p1, p2) => p1); // custom implementation

const winner = thumbWar('Kent C. Dodds', 'Ken Wheeler');

// You can test number of times called
expect(utils.getWinner).toHaveBeenCalledTimes(2);
// You can test arguments passed in calls
expect(utils.getWinner.mock.calls).toEqual([
  ['Dan', 'John'],
  ['Dan', 'John'],
]);

Our version

Here's a bare-bones mock factory function. It creates a mock function that utilizes a user-provided implementation of that function. That implementation replaces the original implementation:

const fn = implementation => {
  const mockFn = (...args) => {
    return implementation(...args);
  };
  return mockFn;
};

We want to add the following features to our mock function:

Stores number of times it's been called.
Stores the arguments passed into each call.

const fn = implementation => {
  const mockFn = (...args) => {
    mockFn.numCalls++;
    mockFn.mockFn.mock.calls.push(args);
    return implementation(...args);
  };

  // Initialize starting values
  mockFn.numCalls = 0;
  mockFn.mock = { calls: [] };

  return mockFn;
};

Now we can perform tests to make sure our mock function ran as many times as we expected and with the arguments we expected.

const originalWinner = utils.getWinner;
utils.getWinner = fn((p1, p2) => p1); // deterministic

utils.getWinner('Dan', 'John');
utils.getWinner('Dan', 'John');

expect(utils.getWinner.numCalls).toBe(2);
expect(utils.getWinner.mocks.calls).toEqual([
  ['Dan', 'John'],
  ['Dan', 'John'],
]);

// Cleanup
utils.getWinner = originalWinner;

Upgrading Our Monkey-patch with spyOn

Our monkey-patch solution of storing originalWinner and restoring it during cleanup is a bit messy. We can instead use spyOn and mockRestore to do that for us.

Jest version

In Jest, here's the code:

// Overrides original function with mock function at 'getWinner' key
jest.spyOn(utils, 'getWinner');
// Adds custom implementation to pre-existing mock function (instead of during initialization)
utils.getWinner.mockImplementation((p1, p2) => p1);

// Run tests...

// Cleanup
utils.getWinner.mockRestore();

Our version

To recreate the above functionality, here are the requirements:

spyOn needs to override the original function with a mock function generated by fn.
- Note: fn needs to accept zero arguments here because the custom implementation comes after.
We need a mockRestore method that resets the function back to its original state.
We need a mockImplementation method that updates and/or sets the custom implementation.

Note: Our solution below uses closures to solve 2 and 3.

// Default empty function is what allow fn() call with no arguments
function fn(impl = () => {}) {
  const mockFn = (...args) => {
    mockFn.mock.calls.push(args);
    return impl(...args);
  };
  mockFn.mock = { calls: [] };
  // Using closure, we can update our implementation to solve 3
  mockFn.mockImplementation = newImpl => (impl = newImpl);
  return mockFn;
}

function spyOn(object, property) {
  // By storing original implementation, we can override with a mock function, solving 1
  const originalImpl = object[property];
  object[property] = fn();

  // Then using closure, we can restore to the original implementation to solve 2
  object[property].mockRestore = () => {
    object[property] = originalImpl;
  };
}

Mocking a Module

As it turns out, monkey-patching only works with CommonJS. It doesn't work for ES6 modules.

// CommonJS import
const utils = require('../utils');

// ES6 module import
import utils from '../utils';

Jest version

To solve this, Jest introduces the mock method. It has 2 parameters:

Relative path to module, and
Module factory function that returns a mocked version of the module.

import utils from '../utils';

// Overrides module
jest.mock('../utils', () => {
  return {
    getWinner: jest.fn((p1, p2) => p1),
  };
});

// Run tests...

// Cleans up metadata in mock function
utils.getWinner.mockReset();

Our version

The way that Jest kind of implements mock behind the scenes is to utilize require.cache, an object that stores information about modules in node. Then when you import that module into your file, it will import the mock version instead.

The goal in our vanilla version is to basically

Manually inject our mock module into require.cache, making sure to include our mock functions as well.
Delete the mock module from require.cache after we're done testing.

const mock = (relativePath, factory) => {
  const resolvedPath = require.resolve(relativePath);
  require.cache[resolvedPath] = {
    id: resolvedPath,
    filename: resolvedPath,
    loaded: true,
    exports: factory(),
  };

  return () => {
    delete require.cache[resolvedPath];
  };
};

const cleanup = mock('../utils', () => {
  return {
    getWinner: fn((p1, p2) => p1),
  };
});

const utils = require('../utils');

// Run tests...

cleanup();

Things to note:

require.resolve gets the path for the file.
We return a cleanup function to remove the mocked module.
The mocked module needs to be created before importing it into the file.

Pro tip: Jest actually will run mock before any other line of code when running your tests for you. This is a convenience it provides, which is especially useful given that ES modules are always hoisted to the top of the file.

Very often, you use a module in multiple test files. In this case, you want to externalize your mocked module and re-use it across tests.

Jest version

Jest allows you to share your mocked module by accessing a __mocks__ directory at the same directory level as your module file.

.
+-- utils.js
+-- __mocks__
| +--utils.js

When you call jest.mock, it will pull your module from the __mocks__ directory instead of the original file.

// In test file
// Notice that we don't need to pass a mock module factory function
jest.mock('../utils');

// In __mocks__/utils.js
module.exports = {
  getWinner: jest.fn((p1, p2) => p1),
};

Our version

To implement this ourselves, we need to do the following in our test:

Cache the mocked module by running require. This will make it show up in require.cache.
Add the mocked module to require.cache at the real module's path.
Finally, when we require our module, it will import the mocked module instead.

// 1. Cache
require('../__mock__/utils');
// 2. Replace
const mockPath = require.resolve('../__mock__/utils');
const utilsPath = require.resolve('../utils');
require.cache[utilsPath] = require.cache[mockPath];

// 3. Import
const utils = require('../utils');

And that's it! (Of course, this is a huge simplification of what Jest is actually doing. Jest takes full control of the module system.)

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Last updated 3 years ago

What is Mocking?

Monkey-patching: Overriding Object Properties

Creating a Mock Factory Function

Jest version

Our version

Upgrading Our Monkey-patch with spyOn

Jest version

Our version

Mocking a Module

Jest version

Our version

Sharing Mocked Modules

Jest version

Our version