How to isolate tests in React Testing Library?

Question

My tests are affecting each other. I'm using the default create-react-app setup with Typescript. All tests run fine individually but when I run all tests the last one fails (both in IntelliJ and npm test). The assertion that fails finds a value that was caused by the previous test.

Now I have read articles such as Test Isolation with React but I am not sharing any values between my tests. I also read about the cleanUp function and tried adding beforeEach(cleanup) and beforeAll(cleanUp), but I didn't found a working solution yet besides putting each test in a separate file. I feel the solution should be pretty simple.

I've quickly generated a create-react-app with TypeScript to reproduce the issue in a small as possible project: https://github.com/Leejjon/breakingtests

My App.tsx

import React from 'react';
import './App.css';
import {BrowserRouter, Link, Route} from 'react-router-dom';

const About: React.FC = () => {
    return (
        <div>
            <h1 id="pageHeader">About page</h1>
            <p>This is the about page</p>
        </div>
    );
};

const Home: React.FC = () => {
    return (
        <div>
            <h1 id="pageHeader">Home page</h1>
            <p>This is the home page</p>
        </div>
    );
};

const News: React.FC = () => {
    return (
        <div>
            <h1 id="pageHeader">News page</h1>
            <p>This is the news page</p>
        </div>
    );
};

const App: React.FC = () => {
    return (
        <div className="App">
            <BrowserRouter>
                <Link id="linkToHome" to="/">Home</Link><br/>
                <Link id="linkToNews" to="/news">News</Link><br/>
                <Link id="linkToAbout" to="/about">About</Link>

                <Route exact path="/" component={Home}/>
                <Route exact path="/news" component={News}/>
                <Route exact path="/about" component={About}/>
            </BrowserRouter>
        </div>
    );
};


export default App;

My App.test.tsx:

import React from 'react';
import {render, fireEvent, waitForElement} from '@testing-library/react';
import App from './App';

describe('Test routing', () => {
    test('Verify home page content', () => {
        const {container} = render(<App/>);
        const pageHeaderContent = container.querySelector("#pageHeader")
            ?.firstChild
            ?.textContent;
        expect(pageHeaderContent).toMatch('Home page');
    });

    test('Navigate to news', async () => {
        const {container} = render(<App/>);

        const pageHeaderContent = container.querySelector("#pageHeader")
            ?.firstChild
            ?.textContent;
        expect(pageHeaderContent).toMatch('Home page');

        const linkToNewsElement: Element = (container.querySelector('#linkToNews') as Element);
        fireEvent.click(linkToNewsElement);
        const pageHeaderContentAfterClick = await waitForElement(() => container.querySelector('#pageHeader')?.firstChild?.textContent);
        expect(pageHeaderContentAfterClick).toMatch('News page');
    });

    test('Navigate to about', async () => {
        const {container} = render(<App/>);

        const pageHeaderContent = container.querySelector("#pageHeader")
            ?.firstChild
            ?.textContent;
        expect(pageHeaderContent).toMatch('Home page');

        const linkToAboutElement: Element = (container.querySelector('#linkToAbout') as Element);
        fireEvent.click(linkToAboutElement);
        const pageHeaderContentAfterClick = await waitForElement(() => container.querySelector('#pageHeader')?.firstChild?.textContent);
        expect(pageHeaderContentAfterClick).toMatch('About page');
    });
});

Answer 1

I found out by adding console.log(document.location.href); that the location is not reset. Which makes sense.

The code below resets the url. I could enter any domain to fix my tests, for example http://blabla/ will also work.

beforeEach(() => {
    delete window.location;
    // @ts-ignore
    window.location = new URL('http://localhost/');
});

In TypeScript this gives an error: TS2739: Type 'URL' is missing the following properties from type 'Location': ancestorOrigins, assign, reload, replace. I didn't know how to fix this so I suppressed it it for now.