Unit Tests

As we mention in the overview page of the testing section, we include the Jest testing framework and the React Native Testing Library for unit tests, along with mocks for most libraries.

The following guide is not a tutorial on how to write tests using React Native Testing Library and Jest, but rather a guide on how to write tests with this starter and some best practices to follow. If you are not familiar with testing, we recommend reading the official documentation of Jest and React Native Testing Library to get familiar with them.

Also worth mentioning that we should aim to test the following:

• Business logic: Test component and function utilities that contain business logic. Form validation, data manipulation and calculations, etc.

• Complex components: Test components that contain complex logic. For example, components that contain a lot of conditional rendering, or components that contain a lot of state management logic.

Writing tests

Let’s start by writing a simple test for our login screen. We will test the following login form component:

./src/components/login-form.tsx

import { zodResolver } from '@hookform/resolvers/zod';
import React from 'react';
import type { SubmitHandler } from 'react-hook-form';
import { useForm } from 'react-hook-form';
import * as z from 'zod';

import { Button, ControlledInput, Text, View } from '@/ui';

const schema = z.object({
  name: z.string().optional(),
  email: z
    .string({
      required_error: 'Email is required',
    })
    .email('Invalid email format'),
  password: z
    .string({
      required_error: 'Password is required',
    })
    .min(6, 'Password must be at least 6 characters'),
});

export type FormType = z.infer<typeof schema>;

export type LoginFormProps = {
  onSubmit?: SubmitHandler<FormType>;
};

export const LoginForm = ({ onSubmit = () => {} }: LoginFormProps) => {
  const { handleSubmit, control } = useForm<FormType>({
    resolver: zodResolver(schema),
  });
  return (
    <View className="flex-1 justify-center p-4">
      <Text testID="form-title" className="pb-6 text-center text-2xl">
        Sign In
      </Text>

      <ControlledInput
        testID="name"
        control={control}
        name="name"
        label="Name"
      />

      <ControlledInput
        testID="email-input"
        control={control}
        name="email"
        label="Email"
      />
      <ControlledInput
        testID="password-input"
        control={control}
        name="password"
        label="Password"
        placeholder="***"
        secureTextEntry={true}
      />
      <Button
        testID="login-button"
        label="Login"
        onPress={handleSubmit(onSubmit)}
      />
    </View>
  );
};

Tip

To make it easier to write tests, we are separating the form from any other logic related to API calls, navigation, etc. This way, we can test the form logic without having to worry about other things.

As you may have noticed, writing code with the intention of writing tests for it will assist us in producing cleaner code.

Now, let’s write a test for the login form component. We will test the following:

• The form renders correctly.
• Show the correct error messages on invalid or missing data.
• Submit the form with valid data and make sure that the onSubmit function is called with the correct data.

First, let’s create a new file called login-form.test.tsx in the src/screens/login directory. Then, add the following code to it:

./src/components/login-form.test.tsx

import React from 'react';

import { cleanup, fireEvent, render, screen, waitFor } from '@/core/test-utils';

import type { LoginFormProps } from './login-form';
import { LoginForm } from './login-form';

afterEach(cleanup);

const onSubmitMock: jest.Mock<LoginFormProps['onSubmit']> = jest.fn();

describe('LoginForm Form ', () => {
  it('renders correctly', async () => {
    render(<LoginForm />);
    expect(await screen.findByText(/Sign in/i)).toBeOnTheScreen();
  });

  it('should display required error when values are empty', async () => {
    render(<LoginForm />);

    const button = screen.getByTestId('login-button');
    expect(screen.queryByText(/Email is required/i)).not.toBeOnTheScreen();
    fireEvent.press(button);
    expect(await screen.findByText(/Email is required/i)).toBeOnTheScreen();
    expect(screen.getByText(/Password is required/i)).toBeOnTheScreen();
  });

  it('should display matching error when email is invalid', async () => {
    render(<LoginForm />);

    const button = screen.getByTestId('login-button');
    const emailInput = screen.getByTestId('email-input');
    const passwordInput = screen.getByTestId('password-input');

    fireEvent.changeText(emailInput, 'yyyyy');
    fireEvent.changeText(passwordInput, 'test');
    fireEvent.press(button);

    expect(screen.queryByText(/Email is required/i)).not.toBeOnTheScreen();
    expect(await screen.findByText(/Invalid Email Format/i)).toBeOnTheScreen();
  });

  it('Should call LoginForm with correct values when values are valid', async () => {
    render(<LoginForm onSubmit={onSubmitMock} />);

    const button = screen.getByTestId('login-button');
    const emailInput = screen.getByTestId('email-input');
    const passwordInput = screen.getByTestId('password-input');

    fireEvent.changeText(emailInput, '[email protected]');
    fireEvent.changeText(passwordInput, 'password');
    fireEvent.press(button);
    await waitFor(() => {
      expect(onSubmitMock).toHaveBeenCalledTimes(1);
    });
    // undefined because we don't use second argument of the  SubmitHandler
    expect(onSubmitMock).toHaveBeenCalledWith(
      {
        email: '[email protected]',
        password: 'password',
      },
      undefined
    );
  });
});

As you may notice from the code, we are importing a bunch of things from the @/core/test-utils directory. This is a simple file that exports everything from the @testing-library/react-native library and overrides the render function to wrap the component with the providers we need. This way, we don’t have to import the providers in every test file.

Tip

You can update this file to add any other providers you need to wrap your components with as well as any other utility functions you need to use in your tests.

src/core/test-utils.tsx

import '@shopify/flash-list/jestSetup';

import { BottomSheetModalProvider } from '@gorhom/bottom-sheet';
import { NavigationContainer } from '@react-navigation/native';
import type { RenderOptions } from '@testing-library/react-native';
import { render } from '@testing-library/react-native';
import type { ReactElement } from 'react';
import React from 'react';

const createAppWrapper = () => {
  return ({ children }: { children: React.ReactNode }) => (
    <BottomSheetModalProvider>
      <NavigationContainer>{children}</NavigationContainer>
    </BottomSheetModalProvider>
  );
};

const customRender = (
  ui: ReactElement,
  options?: Omit<RenderOptions, 'wrapper'>
) => {
  const Wrapper = createAppWrapper(); // make sure we have a new wrapper for each render
  return render(ui, { wrapper: Wrapper, ...options });
};

export * from '@testing-library/react-native';
export { customRender as render };

Now that we have our test file ready, let’s run it and see what happens. To run the test, run the following command:

pnpm  test
pnpm test:watch # To run the tests in watch mode

Tests on CI with GitHub actions

It’s important to run tests on CI in addition to local testing. This ensures that our code doesn’t break when we push it to Github. We have added a GitHub action that runs tests for every push to the main branch or new pull request. It reports the results to GitHub through annotations and provides a summary of the tests along with coverage.

Here is an example of the output of the GitHub action:

Tip

Aiming for 100% test coverage is not always a good idea and doesn’t always make sense. Same with testing views and components that only render a UI

More tests

For more complex logic and components, we recommend taking a look at this amazing project which provides a lot of examples and best practices for testing React Native apps using React Native Testing Library and Jest:

⚡️ React Native Testing