Testing with Preact Testing Library

The Preact Testing Library is a lightweight wrapper around preact/test-utils. It provides a set of query methods for accessing the rendered DOM in a way similar to how a user finds elements on a page. This approach allows you to write tests that do not rely on implementation details. Consequently, this makes tests easier to maintain and more resilient when the component being tested is refactored.

Unlike Enzyme, Preact Testing Library must be called inside a DOM environment.

Installation

Install the testing-library Preact adapter via the following command:

npm install --save-dev @testing-library/preact

Note: This library relies on a DOM environment being present. If you're using Jest it's already included and enabled by default. If you're using another test runner like Mocha or Jasmine you can add a DOM environment to node by installing jsdom.

Usage

Suppose we have a Counter component which displays an initial value, with a button to update it:

import { h } from 'preact';
import { useState } from 'preact/hooks';

export function Counter({ initialCount }) {
  const [count, setCount] = useState(initialCount);
  const increment = () => setCount(count + 1);

  return (
    <div>
      Current value: {count}
      <button onClick={increment}>Increment</button>
    </div>
  );
}

We want to verify that our Counter displays the initial count and that clicking the button will increment it. Using the test runner of your choice, like Jest or Mocha, we can write these two scenarios down:

import { expect } from 'expect';
import { h } from 'preact';
import { render, fireEvent, screen, waitFor } from '@testing-library/preact';

import Counter from '../src/Counter';

describe('Counter', () => {
  test('should display initial count', () => {
    const { container } = render(<Counter initialCount={5}/>);
    expect(container.textContent).toMatch('Current value: 5');
  });

  test('should increment after "Increment" button is clicked', async () => {
    render(<Counter initialCount={5}/>);

    fireEvent.click(screen.getByText('Increment'));
    await waitFor(() => {
      // .toBeInTheDocument() is an assertion that comes from jest-dom.
      // Otherwise you could use .toBeDefined().
      expect(screen.getByText("Current value: 6")).toBeInTheDocument();
    });
  });
});

You may have noticed the waitFor() call there. We need this to ensure that Preact had enough time to render to the DOM and flush all pending effects.

test('should increment counter", async () => {
  render(<Counter initialCount={5}/>);

  fireEvent.click(screen.getByText('Increment'));
  // WRONG: Preact likely won't have finished rendering here
  expect(screen.getByText("Current value: 6")).toBeInTheDocument();
});

Under the hood, waitFor repeatedly calls the passed callback function until it doesn't throw an error anymore or a timeout runs out (default: 1000ms). In the above example we know that the update is completed, when the counter is incremented and the new value is rendered into the DOM.

We can also write tests in an async-first way by using the "findBy" version of the queries instead of "getBy". Async queries retry using waitFor under the hood, and return Promises, so you need to await them.

test('should increment counter", async () => {
  render(<Counter initialCount={5}/>);

  fireEvent.click(screen.getByText('Increment'));

  await screen.findByText('Current value: 6'); // waits for changed element

  expect(screen.getByText("Current value: 6")).toBeInTheDocument(); // passes
});

Finding Elements

With a full DOM environment in place, we can verify our DOM nodes directly. Commonly tests check for attributes being present like an input value or that an element appeared/disappeared. To do this, we need to be able to locate elements in the DOM.

Using Content

The Testing Library philosophy is that "the more your tests resemble the way your software is used, the more confidence they can give you".

The recommended way to interact with a page is by finding elements the way a user does, through the text content.

You can find a guide to picking the right query on the 'Which query should I use' page of the Testing Library docs. The simplest query is getByText, which looks at elements' textContent. There are also queries for label text, placeholder, title attributes, etc. The getByRole query is the most powerful in that it abstracts over the DOM and allows you to find elements in the accessibility tree, which is how your page is read by a screen reader. Combining role and accessible name covers many common DOM traversals in a single query.

import { render, fireEvent, screen } from '@testing-library/preact';

test('should be able to sign in', async () => {
  render(<MyLoginForm />);

  // Locate the input using textbox role and the accessible name,
  // which is stable no matter if you use a label element, aria-label, or
  // aria-labelledby relationship
  const field = await screen.findByRole('textbox', { name: 'Sign In' });

  // type in the field
  fireEvent.change(field, { value: 'user123' });
})

Sometimes using text content directly creates friction when the content changes a lot, or if you use an internationalization framework that translates text into different languages. You can work around this by treating text as data that you snapshot, making it easy to update but keeping the source of truth outside the test.

test('should be able to sign in', async () => {
  render(<MyLoginForm />);

  // What if we render the app in another language, or change the text? Test fails.
  const field = await screen.findByRole('textbox', { name: 'Sign In' });
  fireEvent.change(field, { value: 'user123' });
})

Even if you don't use a translation framework, you can keep your strings in a separate file and use the same strategy as in the example below:

test('should be able to sign in', async () => {
  render(<MyLoginForm />);

  // We can use our translation function directly in the test
  const label = translate('signinpage.label', 'en-US');
  // Snapshot the result so we know what's going on
  expect(label).toMatchInlineSnapshot(`Sign In`);

  const field = await screen.findByRole('textbox', { name: label });
  fireEvent.change(field, { value: 'user123' });
})

Using Test IDs

Test IDs are data attributes added to DOM elements to help in cases where selecting content is ambiguous or unpredictable, or to decouple from implementation details like DOM structure. They can be used when none of the other methods of finding elements make sense.

function Foo({ onClick }) {
  return (
    <button onClick={onClick} data-testid="foo">
      click here
    </button>
  );
}

// Only works if the text stays the same
fireEvent.click(screen.getByText('click here'));

// Works if we change the text
fireEvent.click(screen.getByTestId('foo'));

Debugging Tests

To debug the current DOM state you can use the debug() function to print out a prettified version of the DOM.

const { debug } = render(<App />);

// Prints out a prettified version of the DOM
debug();

Supplying custom Context Providers

Quite often you'll end up with a component which depends on shared context state. Common Providers typically range from Routers, State, to sometimes Themes and other ones that are global for your specific app. This can become tedious to set up for each test case repeatedly, so we recommend creating a custom render function by wrapping the one from @testing-library/preact.

// helpers.js
import { render as originalRender } from '@testing-library/preact';
import { createMemoryHistory } from 'history';
import { FooContext } from './foo';

const history = createMemoryHistory();

export function render(vnode) {
  return originalRender(
    <FooContext.Provider value="foo">
      <Router history={history}>
        {vnode}
      </Router>
    </FooContext.Provider>
  );
}

// Usage like usual. Look ma, no providers!
render(<MyComponent />)

Testing Preact Hooks

With @testing-library/preact we can also test the implementation of our hooks! Imagine that we want to re-use the counter functionality for multiple components (I know we love counters!) and have extracted it to a hook. And we now want to test it.

import { useState, useCallback } from 'preact/hooks';

const useCounter = () => {
  const [count, setCount] = useState(0);
  const increment = useCallback(() => setCount(c => c + 1), []);
  return { count, increment };
}

Like before, the approach behind it is similar: We want to verify that we can increment our counter. So we need to somehow call our hook. This can be done with the renderHook()-function, which automatically creates a surrounding component internally. The function returns the current hook return value under result.current, which we can use to do our verifications:

import { renderHook, act } from '@testing-library/preact';
import useCounter from './useCounter';

test('should increment counter', () => {
  const { result } = renderHook(() => useCounter());

  // Initially the counter should be 0
  expect(result.current.count).toBe(0);

  // Let's update the counter by calling a hook callback
  act(() => {
    result.current.increment();
  });

  // Check that the hook return value reflects the new state.
  expect(result.current.count).toBe(1);
});

For more information about @testing-library/preact check out https://github.com/testing-library/preact-testing-library .