Testing React hooks and Components

Monday, July 8, 2024 - 7 min read

In our previous blog post, we discussed getting started with unit testing in JavaScript/TypeScript. check it if you missed it here Now that we have a testing environment set up, let's explore how to test React hooks and components.

Testing React Components

Imagine a simple Button component that accepts two props: children and onClick.


import { ReactNode, MouseEventHandler } from 'react';
 
const Button = ({
  children,
  onClick,
}: {
  children: ReactNode;
  onClick: MouseEventHandler<HTMLButtonElement>;
}) => {
  return <button onClick={onClick}>{children}</button>;
};
tsx

We want to ensure this button renders correctly and the onClick function gets called when clicked.

Testing Rendering


import { describe, expect, it, vi } from 'vitest';
import { render, screen } from '@testing-library/react';
import { Button } from './Button';
 
describe('<Button/>', () => {
  it('should render properly', () => {
    const testText = 'test text';
 
    render(<Button onClick={() => console.log('clicked')}>{testText}</Button>);
 
    expect(screen.getByText(testText)).toBeInTheDocument();
 
  });
});
tsx

describe is used here to define the <Button/> test suite
a test suite is a group of related tests
it is used to define a test
we render the <Button/> and expect it to be in the document

the test should run successfully ! congrats the button component renders properly.
now let's test the onClick

Testing the click functionality

in order to test a user behavior, we have to simulate the user action. luckily @testing-library/user-event provides exactly this


import userEvent from '@testing-library/user-event';
 
it('should respond to an onClick', async () => {
  const user = userEvent.setup();
  const onClick = vi.fn();
  const testText = 'test text';
 
  render(<Button onClick={onClick}>{testText}</Button>);
 
  await user.click(screen.getByText(testText));
 
  expect(onClick).toHaveBeenCalled();
});
tsx

we setup the user via userEvent.setup()
we mock an onClick function using vi.fn(). why ? mocking the function allows us to test it and check whether it got called or not and a bunch of other tests
we render the <Button/> and pass the onClick handler to it
we get the <Button/> via the screen and click it (note that user events in @testing-library/user-event are async calls so we must wait it)
then we expect the mocked onClicked to have been called

the test should run successfully!

Skip redundant tests:

We can skip checking things that are implied by other tests.
In our example, we had separate tests for rendering and button click functionality. However, testing interactions like clicks inherently verifies that the component is rendered correctly. When a button is clicked, it must be present in the DOM to be interactive.
We can now remove the render test and be confident that it's tested in the click functionality.

Testing React Hooks

Imagine a simple useCounter hook that manages a counter state and provides functions to increment and decrement it:


import { useState } from 'react';
 
export function useCounter() {
  const [count, setCount] = useState(0);
 
  const increment = () => setCount(count + 1);
  const decrement = () => setCount(count - 1);
 
  return { count, increment, decrement };
}
tsx

We want to ensure that

the hook works correctly and that count value is initialized with zero
the value increments correctly
the value decrements correctly

let's test that it initializes correctly and that count value is initialized with zero


import { expect, it } from 'vitest';
import { renderHook } from '@testing-library/react';
import { useCounter } from './useCounter';
 
it("should initialize with count 0", () => {
  const { result } = renderHook(() => useCounter());
  expect(result.current.count).toBe(0);
});
tsx

we use the renderHook from @testing-library/react to render the hook and test it in isolation without a component.
renderHook returns an object containing the property result to test the result of the render.
result contains an object with the property current which we use to test the current value in the render (note that it's preferred to test result.current immediately not assigning it to another variable to avoid stale closures)

Testing that it increments correctly


import { act } from '@testing-library/react';
 
it("should increment the count", () => {
  const { result } = renderHook(() => useCounter());
 
  act(() => {
    result.current.increment();
  });
 
  expect(result.current.count).toBe(1);
});
tsx

we must use the act method to wrap actions that cause a state update/s when using react-testing-library.

Testing that it decrements correctly


it("should decrement the count", () => {
  const { result } = renderHook(() => useCounter());
 
  act(() => {
    result.current.decrement();
  });
 
  expect(result.current.count).toBe(-1);
});
tsx

the tests should run successfully!

Testing React Hooks with browser events

Imagine a hook useIsOnline hook that detects if the browser is online or offline based on the browser events online & offline and also takes an optional props onOnline & onOffile that run when online & offline happens respectively.


import { useEffect, useState } from 'react';
 
const useIsOnline = (props?: { onOnline?: () => void; onOffline?: () => void }) => {
  const [isOnline, setIsOnline] = useState(true);
 
  useEffect(() => {
    const onIsOnline = () => {
      setIsOnline(true);
      props?.onOnline?.();
    };
    const onIsOffline = () => {
      setIsOnline(false);
      props?.onOffline?.();
    };
 
    window.addEventListener('online', onIsOnline);
    window.addEventListener('offline', onIsOffline);
 
    return () => {
      window.removeEventListener('online', onIsOnline);
      window.removeEventListener('offline', onIsOffline);
    };
  }, []);
 
  return isOnline;
};
tsx

We want to ensure that the hook works correctly so we want to test

it initializes with the state set to true (when first running the js in the browser, it must be online)
it should update the state to false when offline and call the onOffline prop
it should update the state to true when online and call the onOffline prop
it should remove the two event listeners when unmounted (to avoid leaky subscriptions)

so let's test that it initializes with true


import { expect, it } from 'vitest';
import { renderHook } from '@testing-library/react';
import { useIsOnline } from './useIsOnline';
 
it('should return initial online state', () => {
  const { result } = renderHook(() => useIsOnline());
 
  expect(result.current).toBe(true);
});
tsx

now let's test the offline functionality


import { expect, it, vi } from 'vitest';
import { act, renderHook } from '@testing-library/react';
import { useIsOnline } from './useIsOnline';
 
it('should update state and call onOffline when "offline" event occurs', () => {
  const onOffline = vi.fn();
  const { result } = renderHook(() => useIsOnline({ onOffline }));
 
  act(() => {
    window.dispatchEvent(new Event('offline'));
  });
 
  expect(onOffline).toHaveBeenCalledTimes(1);
  expect(result.current).toBe(false);
});
tsx

we dispatch the offline event manually to test it

now let's test the online functionality


  it('should update state and call onOnline when "online" event occurs', () => {
    const onOnline = vi.fn();
    const { result } = renderHook(() => useIsOnline({ onOnline }));
    // first we dispatch the offline to make sure that the value is not the initial one
    act(() => {
      window.dispatchEvent(new Event('offline'));
    });
 
    act(() => {
      window.dispatchEvent(new Event('online'));
    });
 
    expect(onOnline).toHaveBeenCalledTimes(1);
    expect(result.current).toBe(true);
});
tsx

now let's test that it removes the event listeners


  it('should clean up event listeners on unmount', () => {
    const removerListener = vi.spyOn(global, 'removeEventListener').mockImplementation(vi.fn());
    const { unmount } = renderHook(() => useIsOnline());
 
    unmount();
 
    expect(removerListener).toHaveBeenCalledTimes(2); // 2 events, each added and removed
});
tsx

we spy on the removeEventListener method via vi.spyOn and mock it's implementation via vi.fn to test it
renderHook returns an object containing the property unmount which we can use to test the un-mouting of the hooks
we call the unmount and expect the removeEventListener to be called twice one for the online and one for the offline event-listeners

you can now test most of the components and hooks

Congrats !