next-page-tester

Next page tester

The missing DOM integration testing tool for Next.js.

Given a Next.js route, this library will return an instance of the matching page component instantiated with the properties derived by Next.js' routing system and server side data fetching.

import { render, screen, fireEvent } from '@testing-library/react';
import { getPage } from 'next-page-tester';

describe('Blog page', () => {
  it('renders blog page', async () => {
    const { page } = await getPage({
      route: '/blog/1',
    });

    render(page);
    expect(screen.getByText('Blog')).toBeInTheDocument();

    fireEvent.click(screen.getByText('Link'));
    await screen.findByText('Linked page');
  });
});

What

The idea behind this library is to enable DOM integration tests on Next.js pages along with server side data fetching and routing.

The testing approach suggested here consists of manually mocking external API's dependencies and get the component instance matching a given route.

Next page tester will take care of:

• resolving provided routes into matching page component
• calling Next.js data fetching methods (getServerSideProps, getInitialProps or getStaticProps) if the case
• set up a mocked next/router provider initialized with the expected values (to test useRouter and withRouter)
• wrapping page with custom _app component
• instantiating page component with expected page props
• Emulate client side navigation via Link, router.push, router.replace

Options

Property	Description	type	Default
route (mandatory)	Next route (must start with /)	string	-
req	Access default mocked request object (getServerSideProps only)	res => res	-
res	Access default mocked response object (getServerSideProps only)	req => req	-
router	Access default mocked Next router object	router => router	-
useCustomApp	Use custom App component	boolean	true
nextRoot	Absolute path to Next's root folder	string	auto detected

Notes

• Data fetching methods' context req and res objects are mocked with node-mocks-http
• Next page tester can be used with any testing framework/library (not tied to Testing library)
• It might be necessary to install @types/react-dom and @types/webpack when using Typescript in strict mode due to this bug

Error: Not implemented: window.scrollTo

Next.js Link components invoke window.scrollTo on click which is not implemented in JSDOM environment. In order to fix the error you should provide your own window.scrollTo mock.

Next.js versions support

next-page-tester focuses on supporting only the last major version of Next.js:

next-page-tester	next.js
v0.1.0 - v0.7.0	v9.X.X
v0.8.0 +	v10.X.X

Todo's

• Consider adding custom Document support
• Consider reusing Next.js code parts (not only types)
• Consider supporting Next.js trailingSlash option

Changelog

0.8.0

Breaking Changes

• Dropped support for Next.js 9
• getPage promise returns an object with a page prop holding the expected page element (previously page element was directly returned)
• useCustomApp option defaults to true

New Features

• Provide support for Next.js 10