The Power of Mocking in Unit Tests

June 16, 2023 - 7 min read
Testing is often an afterthought when writing code. It can be confusing, slow down development, and if implemented poorly provides questionable utility. The following are some tips for preemptively writing and structuring your code so that it is easy to unit test.

Mocking Test Boundaries

When using React, composition is key. This often means that if you want to test a component, you will by default be rendering all of the children of that component. This can be problematic in a large application where you can end up rendering dozens or even hundreds of components.
Here is an example of a component that we want to write a unit test for. It is a Search component that contains a search input, a button that triggers the search, and a component that is the list of all search results.
import { SearchInput } from 'components/SearchInput'; import { SearchResults } from 'components/SearchResults'; interface Props { search: () => void; } function Search({ search }: Props) { return ( <Flex> <SearchInput /> <Button data-testid="search-button" onClick={() => search()}> Search </Button> <SearchResults /> </Flex> ); }
Since SearchInput and SearchResults are separate components, ideally the unit tests for those should be written separately. This leaves us with testing the button to make sure that when it is clicked, the search function is called. We don't want to render the other components, otherwise they may potentially fail the unit test and we wouldn't know if the Search component caused the failure, or if it was caused by another component deeper down the component tree.
Using module mocking we can prevent the search input and search results from rendering so that we can write a unit test that only deals with the search button. The following snippet uses Jest's mock function to render stubbed components with a test-id.
jest.mock('components/SearchInput', () => ({ SearchInput: () => <div data-testid="search-input" />, })); jest.mock('components/SearchResults', () => ({ SearchResults: () => <div data-testid="search-results" />, }));
Now, when we render our Search component, we end up with something that looks like this:
<Flex> <div data-testid="search-input" /> <Button data-testid="search-button" onClick={() => search()}> Search </Button> <div data-testid="search-results" /> </Flex>
At this point it is very easy to write a unit test that checks the search function is called when the button is clicked.
// provide a mocked function for the search prop const props = { search: jest.fn(), }; const { getByTestId } = render(<Search {...props} />); // find the button by test id const button = getByTestId('search-button'); // fire a click event for the button; // check that the search prop has been called 1 time expect(;

Mocking Environment Variables

If you structure your fetching code in React hooks, you will probably end up with hooks that look like this:
async function fetchUsers() { const response = await fetch(`${process.env.API_BASE_URL}/users`); return await response.json(); } function useFetchUsers() { // using react-query return useQuery(['users'], () => fetchUsers()); }
It can be annoying to mock environment variables like process.env.API_BASE_URL. A better way to structure this code is to have a separate file for environment variables that you can import as a module.
Environment variables module:
// config/constants.ts export const API_BASE_URL = process.env.API_BASE_URL || '';
Then you can update the fetchUsers function to look like this:
import { API_BASE_URL } from 'config/constants'; async function fetchUsers() { const response = await fetch(`${API_BASE_URL}/users`); return await response.json(); }
With this setup, it is very easy to use jest.mock to change the value of API_BASE_URL to whatever you'd like, and skip needing to fiddle with mocking process.env values.
jest.mock('config/constants', () => ({ API_BASE_URL: 'whatever-value-youd-like', }));

Use Props Instead of Complex Code Directly in UI Components

When creating a UI in React, it is often tempting to grab any hooks or values that you need and include them alongside your UI components. This becomes very annoying when writing unit tests because you'll find that you need to mock a bunch of different modules and functionality before you can render your UI. It is much easier to separate your components so that your UI component accepts all of the props it needs to render the UI as a pure function.

🚫 Including everything in the UI component

function UsersList() { // translation hook for i18n const { t } = useTranslation(); // hook to fetch users const { users, isLoading, isError } = useFetchUsers(); return ( <Flex direction="column"> <Flex> <Flex>{t('firstName', 'First Name')}</Flex> <Flex>{t('lastName', 'Last Name')}</Flex> </Flex> { => ( <Flex key={}> <Flex>{user.firstName}</Flex> <Flex>{user.lastName}</Flex> </Flex> ))} </Flex> ); }

✅ Passing props to a pure UI component

interface Props { t: TranslationFunction; users: User[]; isLoading: boolean; isError: boolean; } function UsersList({ t, users, isLoading, isError }: Props) { return ( <Flex direction="column"> <Flex> <Flex>{t('firstName', 'First Name')}</Flex> <Flex>{t('lastName', 'Last Name')}</Flex> </Flex> { => ( <Flex key={}> <Flex>{user.firstName}</Flex> <Flex>{user.lastName}</Flex> </Flex> ))} </Flex> ); }
With the UI component set up like this, it is very easy to write different testing scenarios by tweaking the props that are passed.
const baseProps = { t: mockTranslationFn(), users: [], isLoading: false, isError: false, }; test('when loading', () => { const props = { ...baseProps, isLoading: true, }; // render the UI component... }); test('when error', () => { const props = { ...baseProps, isError: true, }; // render the UI component... }); test('when users', () => { const props = { ...baseProps, users: [{ id: 1, firstName: 'test', lastName: 'user' }], }; // render the UI component... });
This setup is part of a larger strategy of structuring your React code as separate Container and UI components. I will discuss this topic in a future blog post.

Mock Implementations

Sometimes it is not enough to mock a module. Instead, you'd like to mock a module and have multiple implementations of that module to test different scenarios. Let's say we have a hook that fetches users and returns an array of users and some loading and error state.
import { useFetchUsers } from 'hooks/useFetchUsers'; function ContainerComponent() { // we need to mock this hook const { users, isLoadingUsers, isErrorUsers } = useFetchUsers(); }
Lets start by using jest.mock to mock the module.
By default, jest will mock the module, but it won't provide us with useful return values to write unit tests against. In order to do that, we need to set up different mock implementations of the hook.
jest.mock('hooks/useFetchUsers'); import { useFetchUsers } from 'hooks/useFetchUsers'; // necessary for Typescript to understand this is a mock const mockedUseFetchUsers = jest.mocked(useFetchUsers); test('when returns users', () => { mockedUseFetchUsers.mockReturnValue({ // return users users: [{ id: 1, firstName: 'test', lastName: 'user' }], isFetchingUsers: false, isErrorUsers: false, }); // write a test against this scenario }); test('when error', () => { mockedUseFetchUsers.mockReturnValue({ users: [], isFetchingUsers: false, // error is true isErrorUsers: true, }); // write a test against this scenario });
These are just some of the many things you can do with mocking when writing unit tests. Hopefully these tips help you to write better unit tests!