Vitest + React Testing Library for Remix & React Router v7 (with TypeScript): A Complete Setup Guide

Sujal — Mon, 17 Mar 2025 18:34:52 +0000

As a frontend developer working with Remix v2 and React Router v7, setting up a testing environment can be tricky. After battling through configuration issues and TypeScript conflicts, I've created this guide to solve three specific challenges:

Configuring Vitest to work with Remix's path aliases
Properly mocking React Router's useOutletContext hook with TypeScript support
Handling window.matchMedia errors when testing UI components (especially with libraries like Mantine)

Let's dive into a complete solution that addresses all these issues!

Required Dependencies

First, install these packages:

npm install -D vitest @testing-library/react @testing-library/jest-dom @testing-library/user-event jsdom

Optional but recommended:

npm install -D @vitest/ui @vitest/coverage-v8

Package.json Scripts

Add these testing scripts:

{
  "scripts": {
    "test": "vitest",
    "test:ui": "vitest --ui",
    "coverage": "vitest run --coverage"
  }
}

Configuration Files

Create a separate vitest.config.ts in your project root:

import path from 'path';
import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true,
    environment: 'jsdom',
    setupFiles: ['./vitest.setup.ts'],
    include: ['**/*.{test,spec}.{ts,tsx}']
  },
  resolve: {
    alias: {
      '~': path.resolve(__dirname, './app')
    }
  }
});

Create vitest.setup.ts:

import '@testing-library/jest-dom';
import { cleanup } from '@testing-library/react';
import { afterEach } from 'vitest';

afterEach(() => {
  cleanup();
});

// Mock window.matchMedia for required components
Object.defineProperty(window, 'matchMedia', {
  writable: true,
  value: vi.fn().mockImplementation(query => ({
    matches: false,
    media: query,
    onchange: null,
    addListener: vi.fn(),
    removeListener: vi.fn(),
    addEventListener: vi.fn(),
    removeEventListener: vi.fn(),
    dispatchEvent: vi.fn(),
  })),
});

Test Organization

For a feature-based folder structure (remix-flat-routes):

app/routes/feature+/
├── __tests__/
│   ├── routes/
│   │   └── index.test.tsx
│   ├── components/
│   │   └── FeatureCard.test.tsx
│   └── hooks/
│       └── useFeature.test.ts

This structure offers several advantages:

Tests are co-located with the features they test, making them easier to find
The separation between route, component, and hook tests clarifies the test's purpose
It follows the same organization as your source code, maintaining consistency
When using flat routes, this structure ensures tests are excluded from the route generation

Mocking React Router's `useOutletContext`

One of the trickiest parts of testing Remix components is mocking the useOutletContext hook:

import { render, screen } from '@testing-library/react';
import { describe, expect, it, vi } from 'vitest';
import { MantineProvider } from '@mantine/core';

import { OutletContext } from '~/types';
import YourComponent from '../_components/YourComponent';

// Mock useOutletContext with TypeScript type safety
// replace 'react-router' with '@remix-run/react' if using remix
vi.mock('react-router', () => ({
  // Preserve all original exports from react-router
  ...vi.importActual('react-router'), // or '@remix-run/react'

  // Override only the useOutletContext function
  // The 'satisfies' operator ensures type safety without changing the return type
  useOutletContext: () => ({ language: 'en' } satisfies Partial<OutletContext>) // Using Partial allows us to mock only what we need
}));

describe('YourComponent', () => {
  it('should render correctly', () => {
    render(<YourComponent />);

    // Your assertions here
  });
});

A Complete Component Test Example

Here's a real-world example testing a component that displays course metadata:

import { MantineProvider } from '@mantine/core';
import { render, screen } from '@testing-library/react';
import { describe, expect, it, vi } from 'vitest';

import { OutletContext } from '~/types';
import CourseMetaDataSummary from '../_components/CourseMetaDataSummary';

vi.mock('react-router', () => ({
  ...vi.importActual('react-router'),
  useOutletContext: () => ({ language: 'en' } satisfies Partial<OutletContext>)
}));

describe('CourseMetaDataSummary', () => {
  it('should render lessons count as 8', () => {
    render(
      <MantineProvider>
        <CourseMetaDataSummary lessons_count={8} members_count={20} />
      </MantineProvider>
    );

    const lessonsCount = screen.getByText('8 Lessons');
    expect(lessonsCount).toBeInTheDocument();
  });

  it('should render members count as 20', () => {
    render(
      <MantineProvider>
        <CourseMetaDataSummary lessons_count={8} members_count={20} />
      </MantineProvider>
    );

    const membersCount = screen.getByText('20 Members');
    expect(membersCount).toBeInTheDocument();
  });

  it('should render 1 lesson and 1 member', () => {
    render(
      <MantineProvider>
        <CourseMetaDataSummary lessons_count={1} members_count={1} />
      </MantineProvider>
    );

    const lessonsCount = screen.getByText('1 Lesson');
    const membersCount = screen.getByText('1 Member');

    expect(lessonsCount).toBeInTheDocument();
    expect(membersCount).toBeInTheDocument();
  });
});

Common Gotchas & Solutions

Path resolution issues
- Add the resolve.alias configuration to match your tsconfig paths
Mocking useOutletContext
- Use vi.importActual to preserve the rest of the module
- Use TypeScript's satisfies operator for type safety
Components requiring window.matchMedia
- Add the matchMedia mock to vitest.setup.ts

DEV Community: Sujal

Vitest + React Testing Library for Remix & React Router v7 (with TypeScript): A Complete Setup Guide

Required Dependencies

Package.json Scripts

Configuration Files

Test Organization

Mocking React Router's `useOutletContext`

A Complete Component Test Example

Common Gotchas & Solutions

References

DEV Community: Sujal

Vitest + React Testing Library for Remix & React Router v7 (with TypeScript): A Complete Setup Guide

Required Dependencies

Package.json Scripts

Configuration Files

Test Organization

Mocking React Router's useOutletContext

A Complete Component Test Example

Common Gotchas & Solutions

References

Mocking React Router's `useOutletContext`