Mocking in tests

Mocking is a means of creating a facsimile, a puppet. This is generally done in a when 'a', do 'b' manner of puppeteering. The idea is to limit the number of moving pieces and control things that "don't matter". "mocks" and "stubs" are technically different kinds of "test doubles". For the curious mind, a stub is a replacement that does nothing (a no-op) but track its invocation. A mock is a stub that also has a fake implementation (the when 'a', do 'b'). Within this doc, the difference is unimportant, and stubs are referred to as mocks.

Tests should be deterministic: runnable in any order, any number of times, and always produce the same result. Proper setup and mocking make this possible.

Node.js provides many ways to mock various pieces of code.

This articles deals with the following types of tests:

There are different schools of thought about when to mock and when not to mock, the broad strokes of which are outlined below.

When and not to mock

There are 3 main mock candidates:

• Own code
• External code
• External system

Own code

This is what your project controls.

import foo from './foo.mjs';

export function main() {
  const f = foo();
}

Here, foo is an "own code" dependency of main.

Why

For a true unit test of main, foo should be mocked: you're testing that main works, not that main + foo work (that's a different test).

Why not

Mocking foo can be more trouble than worth, especially when foo is simple, well-tested, and rarely updated.

Not mocking foo can be better because it's more authentic and increases coverage of foo (because main's tests will also verify foo). This can, however, create noise: when foo breaks, a bunch of other tests will also break, so tracking down the problem is more tedious: if only the 1 test for the item ultimately responsible for the issue is failing, that's very easy to spot; whereas 100 tests failing creates a needle-in-a-haystack to find the real problem.

External code

This is what your project does not control.

import bar from 'bar';

export function main() {
  const f = bar();
}

Here, bar is an external package, e.g. an npm dependency.

Uncontroversially, for unit tests, this should always be mocked. For component and integration tests, whether to mock depends on what this is.

Why

Verifying that code that your project does not maintain works is not the goal of a unit test (and that code should have its own tests).

Why not

Sometimes, it's just not realistic to mock. For example, you would almost never mock a large framework such as react or angular (the medicine would be worse than the ailment).

External system

These are things like databases, environments (Chromium or Firefox for a web app, an operating system for a node app, etc), file systems, memory store, etc.

Ideally, mocking these would not be necessary. Aside from somehow creating isolated copies for each case (usually very impractical due to cost, additional execution time, etc), the next best option is to mock. Without mocking, tests sabotage each other:

import { db } from 'db';

export function read(key, all = false) {
  validate(key, val);

  if (all) return db.getAll(key);

  return db.getOne(key);
}

export function save(key, val) {
  validate(key, val);

  return db.upsert(key, val);
}

In the above, the first and second cases (the it() statements) can sabotage each other because they are run concurrently and mutate the same store (a race condition): save()'s insertion can cause the otherwise valid read()'s test to fail its assertion on items found (and read()'s can do the same thing to save()'s).

What to mock

Modules + units

This leverages mock from the Node.js test runner.

import assert from 'node:assert/strict';
import { before, describe, it, mock } from 'node:test';


describe('foo', { concurrency: true }, () => {
  let barMock = mock.fn();
  let foo;

  before(async () => {
    const barNamedExports = await import('./bar.mjs')
      // discard the original default export
      .then(({ default, ...rest }) => rest);

    // It's usually not necessary to manually call restore() after each
    // nor reset() after all (node does this automatically).
    mock.module('./bar.mjs', {
      defaultExport: barMock
      // Keep the other exports that you don't want to mock.
      namedExports: barNamedExports,
    });

    // This MUST be a dynamic import because that is the only way to ensure the
    // import starts after the mock has been set up.
    ({ foo } = await import('./foo.mjs'));
  });

  it('should do the thing', () => {
    barMock.mockImplementationOnce(function bar_mock() {/* … */});

    assert.equal(foo(), 42);
  });
});

APIs

A little-known fact is that there is a builtin way to mock fetch. undici is the Node.js implementation of fetch. It's shipped with node, but not currently exposed by node itself, so it must be installed (ex npm install undici).

import assert from 'node:assert/strict';
import { beforeEach, describe, it } from 'node:test';
import { MockAgent, setGlobalDispatcher } from 'undici';

import endpoints from './endpoints.mjs';

describe('endpoints', { concurrency: true }, () => {
  let agent;
  beforeEach(() => {
    agent = new MockAgent();
    setGlobalDispatcher(agent);
  });

  it('should retrieve data', async () => {
    const endpoint = 'foo';
    const code = 200;
    const data = {
      key: 'good',
      val: 'item',
    };

    agent
      .get('example.com')
      .intercept({
        path: endpoint,
        method: 'GET',
      })
      .reply(code, data);

    assert.deepEqual(await endpoints.get(endpoint), {
      code,
      data,
    });
  });

  it('should save data', async () => {
    const endpoint = 'foo/1';
    const code = 201;
    const data = {
      key: 'good',
      val: 'item',
    };

    agent
      .get('example.com')
      .intercept({
        path: endpoint,
        method: 'PUT',
      })
      .reply(code, data);

    assert.deepEqual(await endpoints.save(endpoint), {
      code,
      data,
    });
  });
});

Time

Like Doctor Strange, you too can control time. You would usually do this just for convenience to avoid artificially protracted test runs (do you really want to wait 3 minutes for that setTimeout() to trigger?). You may also want to travel through time. This leverages mock.timers from the Node.js test runner.

Note the use of time-zone here (Z in the time-stamps). Neglecting to include a consistent time-zone will likely lead to unexpected restults.

import assert from 'node:assert/strict';
import { describe, it, mock } from 'node:test';

import ago from './ago.mjs';

describe('whatever', { concurrency: true }, () => {
  it('should choose "minutes" when that\'s the closet unit', () => {
    mock.timers.enable({ now: new Date('2000-01-01T00:02:02Z') });

    const t = ago('1999-12-01T23:59:59Z');

    assert.equal(t, '2 minutes ago');
  });
});

This is especially useful when comparing against a static fixture (that is checked into a repository), such as in snapshot testing.