[Testing] JavaScript Mocking Fundamentals

Ensure Functions are Called Correctly with JavaScript Mocks

Often when writing JavaScript tests and mocking dependencies, you’ll want to verify that the function was called correctly. That requires keeping track of how often the function was called and what arguments it was called with. That way we can make assertions on how many times it was called and ensure it was called with the right arguments.

Function to be mocked: utils.js

// returns the winning player or null for a tie
// Let's pretend this isn't using Math.random() but instead
// is making a call to some third party machine learning
// service that has a testing environment we don't control
// and is unreliable so we want to mock it out for tests.
function getWinner(player1, player2) {
  const winningNumber = Math.random();
  return winningNumber < 1 / 3
    ? player1
    : winningNumber < 2 / 3
      ? player2
      : null;
}

module.exports = {getWinner};

Implementaion: thumbwar.js

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

function thumbWar(player1, player2) {
  const numberToWin = 2;
  let player1Wins = 0;
  let player2Wins = 0;
  while (player1Wins < numberToWin && player2Wins < numberToWin) {
    const winner = utils.getWinner(player1, player2);
    if (winner === player1) {
      player1Wins++;
    } else if (winner === player2) {
      player2Wins++;
    }
  }
  return player1Wins > player2Wins ? player1 : player2;
}

module.exports = thumbWar;

Testing:

const thumbWar = require("./thumbwar");
const utils = require("./utils");
const assert = require("assert");

test("returns winner", () => {
  const originalGetWinner = utils.getWinner;
  utils.getWinner = jest.fn((p1, p2) => p1); // eslint-disable-line no-unused-vars
  const winner = thumbWar("KCD", "KW");
  expect(winner).toBe("KCD");
  // check the params are correct
  expect(utils.getWinner.mock.calls).toEqual([["KCD", "KW"], ["KCD", "KW"]]);
  // check the fn has been called number of times
  expect(utils.getWinner).toHaveBeenCalledTimes(2);
  // check each time call the fn with the correct params
  expect(utils.getWinner).toHaveBeenNthCalledWith(1, "KCD", "KW");
  expect(utils.getWinner).toHaveBeenNthCalledWith(2, "KCD", "KW");

  utils.getWinner = originalGetWinner;
});

Here we are using 'jest.fn' to mock the function.

We can also create a mock fn by ourselves.

function fn(impl) {
  const mockFn = (...args) => {
    mockFn.mock.calls.push(args);
    return impl(...args);
  };
  mockFn.mock = {calls: []};
  return mockFn;
}

test("returns winner: fn", () => {
  const originalGetWinner = utils.getWinner;
  utils.getWinner = fn((p1, p2) => p1); // eslint-disable-line no-unused-vars
  const winner = thumbWar("KCD", "KW");
  assert.strictEqual(winner, "KCD");
  assert.deepStrictEqual(utils.getWinner.mock.calls, [
    ["KCD", "KW"],
    ["KCD", "KW"],
  ]);
  utils.getWinner = originalGetWinner;
});

Restore the Original Implementation of a Mocked JavaScript Function with jest.spyOn

With our current usage of the mock function we have to manually keep track of the original implementation so we can cleanup after ourselves to keep our tests idempotent (moonkey patching). Let’s see how jest.spyOn can help us avoid the bookkeeping and simplify our situation.

test("returns winner", () => {
  //const originalGetWinner = utils.getWinner;
  //utils.getWinner = jest.fn((p1, p2) => p1); // eslint-disable-line no-unused-vars
  jest.spyOn(utils, "getWinner");
  utils.getWinner.mockImplementation((p1, p2) => p1); // eslint-disable-line no-unused-vars
  const winner = thumbWar("KCD", "KW");
  expect(winner).toBe("KCD");
  expect(utils.getWinner.mock.calls).toEqual([["KCD", "KW"], ["KCD", "KW"]]);
  expect(utils.getWinner).toHaveBeenCalledTimes(2);
  expect(utils.getWinner).toHaveBeenNthCalledWith(1, "KCD", "KW");
  expect(utils.getWinner).toHaveBeenNthCalledWith(2, "KCD", "KW");

  // utils.getWinner = originalGetWinner;
  utils.getWinner.mockRestore();
});

Here we are using jest.spyOn function.

We can also write spyOn function by ourselves.

function fn(impl = () => {}) {
  const mockFn = (...args) => {
    mockFn.mock.calls.push(args);
    mockFn.mockImplementation = newImpl => (impl = newImpl);
    return impl(...args);
  };
  mockFn.mock = {calls: []};
  return mockFn;
}

function spyOn(obj, prop) {
  // store the origianl fn
  const originalValue = obj[prop];
  // assign new mock fn
  obj[prop] = fn;
  // add restore fn
  obj[prop].mockRestore = () => (obj[prop] = originalValue);
}

test("returns winner: fn", () => {
  spyOn(utils, "getWinner");
  utils.getWinner.mockImplementation = fn((p1, p2) => p1); // eslint-disable-line no-unused-vars
  const winner = thumbWar("KCD", "KW");
  assert.strictEqual(winner, "KCD");
  assert.deepStrictEqual(utils.getWinner.mock.calls, [
    ["KCD", "KW"],
    ["KCD", "KW"],
  ]);
  utils.getWinner.mockRestore();
});

Mock a JavaScript module in a test

So far we’re still basically monkey-patching the utils module which is fine, but could lead to problems in the future, especially if we want to mock a ESModule export which doesn’t allow this kind of monkey-patching on exports. Instead, let’s mock the entire module so when our test subject requires the file they get our mocked version instead.

To mock a whole module. we can use 'jest.mock':

const thumbWar = require("./thumbwar");
const utils = require("./utils");
const assert = require("assert");

jest.mock("./utils", () => {
  return {
    getWinner: jest.fn((p1, p2) => p1), // eslint-disable-line no-unused-vars
  };
});

test("returns winner", () => {

  const winner = thumbWar("KCD", "KW");
  expect(winner).toBe("KCD");
  expect(utils.getWinner.mock.calls).toEqual([["KCD", "KW"], ["KCD", "KW"]]);
  expect(utils.getWinner).toHaveBeenCalledTimes(2);
  expect(utils.getWinner).toHaveBeenNthCalledWith(1, "KCD", "KW");
  expect(utils.getWinner).toHaveBeenNthCalledWith(2, "KCD", "KW");

  utils.getWinner.mockReset();
});

Now we don't need to mock the 'getWinner' function inside test, 'jest.mock' can be used anywhere, jest will make sure it mock will be hoisted to the top.

Make a shared JavaScript mock module

Often you’ll want to mock the same file throughout all the tests in your codebase. So let’s make a shared mock file in Jest's __mocks__ directory which Jest can load for us automatically.

__mocks__/utils.js:

module.exports = {
  getWinner: jest.fn((p1, p2) => p1), // eslint-disable-line no-unused-vars
};

const thumbWar = require("../thumbwar");
const utils = require("../utils");
const assert = require("assert");

jest.mock("../utils");

test("returns winner", () => {
  const winner = thumbWar("KCD", "KW");
  expect(winner).toBe("KCD");
  expect(utils.getWinner.mock.calls).toEqual([["KCD", "KW"], ["KCD", "KW"]]);
  expect(utils.getWinner).toHaveBeenCalledTimes(2);
  expect(utils.getWinner).toHaveBeenNthCalledWith(1, "KCD", "KW");
  expect(utils.getWinner).toHaveBeenNthCalledWith(2, "KCD", "KW");

  utils.getWinner.mockReset();
});