Testing React App Using Vitest & React Testing Library

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The Problem

When developing an application, we can’t ensure our app is bug-free. We will test our app before going into production. Often, while creating new features or modifying existing code, we can unintentionally broke previous code and produce errors. It would be useful to know this during the development process. Imagine if we don’t knew the broken feature and that is main feature of our application, If this happens, it can lead to user frustration and damage the application’s reputation.

Error illustration

Solution

To minimize this from happening, we can implement automated testing. With this, we can quickly identify any bugs that occur and take immediate action to fix them. I don’t want to dwell on what automated testing is, you can read more about it here.

Benefit of implementing Automate Testing

  • Early Bug Detection
    By running tests automatically, bugs can be detected earlier in the development process, making them easier and cheaper to fix.

  • Increased Confidence
    With automated testing in place, developers and stakeholders have more confidence that the application functions as expected, improving the overall quality of the final product.

Several things need to be considered before carrying out automated testing in our application:

  • Messy code isn’t testable
    We need to know how to write clean code in React, such as breaking large components down into smaller parts and abstracting logic. Implementing automated testing encourages developers to write clean code.

  • Clarity regarding feature specifications
    It can be very tiring if we have written test code but the features change often. We would then need to update the test code repeatedly, which takes a lot of time.

So, I will share my learning journey about testing our React application, including how to set it up and a little about how to implement it. Enough explaining, Let’s get started.

Tools Used

There is tools used to test our application, I will explain according to my understanding about these tools.

Vitest

We will write unit test & integration test in our application. We can use Vitest as test runner, it will run our all test code then give the results.

React Testing Library (RTL)

Using React Testing Library we can test our React component by user perspective.

Mock Service Worker (MSW)

While testing our app in integration test test we don’t interact to other services like API server. We need to “fake” that API response using Mock Service Worker.

User Events

This is a companion for React Testing Libary is used for simulate user interactions

JSDOM

While testing we no need to start React project instead using JSDOM to emulate browser

Jest DOM

This is like an utility to ensure the html elements behavior are as we expect

Setup

  1. Create React App using Vite
npm create vite@latest react-test -- -- template react-ts
cd react-test
npm install
npm run dev
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  1. Install Vitest
npm install -D Vitest
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  1. Install React Testing Library (RTL)
npm install --save-dev @testing-library/react @testing-library/dom @types/react @types/react-dom
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  1. Install JSDOM
npm i -D jsdom
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  1. Install Jest DOM
npm install --save-dev @testing-library/jest-dom
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  1. Install User Event
npm install --save-dev @testing-library/user-event
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  1. Add script run Vitest in package.json
 "scripts": { "test": "vitest" }
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  1. Configure vite.config.ts
/// <reference types="vitest" />
import { defineConfig } from 'vite';
import react from '@vitejs/plugin-react'; // https://vitejs.dev/config/
export default defineConfig({ plugins: [react()], test: { globals: true, // Make Vitest variable can be accessed globaly environment: 'jsdom', // Change default testing environment to jsdom setupFiles: './src/tests/setup.ts', // Setup file },
});
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  1. Configure tsconfig.app.json
 "compilerOptions": { "types": ["vitest/globals"] // Make code editor has type hinting for Vitest },
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  1. Install Mock Service Worker (MSW)
npm install msw@latest --save-dev
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  1. Create handler.ts in src/tests/mocks
import { http, HttpResponse } from 'msw'; export const handlers: any = [];
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  1. Create node.ts in src/tests/mocks for mocking API
import { setupServer } from 'msw/node';
import { handlers } from './handlers'; export const server = setupServer(...handlers);
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  1. Create setup.ts in src/tests
import '@testing-library/jest-dom';
import { server } from './mocks/node'; beforeEach(() => { // Enable mocking server.listen();
}); afterEach(() => { // Disable mocking server.resetHandlers();
}); afterAll(() => { // Clean up once the tests are done server.close();
});
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Now we can start to write test code for our app

Basic Unit Testing

To start i will give a simple example:

// sum.ts
export const sum = (a: number, b: number): number => { return a + b;
};
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We will test this function by creating sum.test.ts alongside sum.js

// sum.test.ts
import { sum } from './sum'; describe('sum function', () => { test('should return the sum of two numbers', () => { expect(sum(1, 2)).toBe(3); expect(sum(-1, 1)).toBe(0); expect(sum(0, 0)).toBe(0); expect(sum(5, 7)).toBe(12); });
});
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Explanation

  1. describe: grouping some related tests with sum function
  2. test: is a block that contains specific test
  3. expect: this is the way to make an assertion. expect expects the result of sum(a, b) to be equal to the value we specified with toBe.
  4. toBe: is one of Vitest matcher which is used to express expectations in unit tests.

To start Vitest we can type

npm run test

in terminal and see the result:
Testing result

Testing React Component

I will make a counter component for example

// src/components/Counter.tsx
import { useState } from 'react'; const Counter = () => { const [count, setCount] = useState(0); return ( <div> <button type='button' data-testid="decrease" onClick={() => setCount((currentValue) => currentValue - 1)} > Decrease </button> <h1 data-testid="current">{count}</h1> <button type='button' data-testid="increase" onClick={() => setCount((currentValue) => currentValue + 1)} > Increase </button> </div> );
}; export default Counter;
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We test Counter.tsx by creating Counter.test.tsx

// src/components/Counter.tsx
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import Counter from './Counter'; describe('Counter component', () => { it('Should render correctly', () => { render(<Counter />); // Assert element exists expect(screen.getByTestId('decrease')).toBeInTheDocument(); expect(screen.getByTestId('increase')).toBeInTheDocument(); expect(screen.getByTestId('current')).toBeInTheDocument(); // Assert rendering initial value expect(screen.getByTestId('current')).toHaveTextContent('0'); }); it('Should decrease and incrementing correctly'), async () => { render(<Counter />); const user = userEvent.setup(); // Decreasing count const decreaseButton = screen.getByTestId('decrease'); await user.click(decreaseButton); expect(screen.getByTestId('current')).toHaveTextContent('0'); // Increasing count const increaseButton = screen.getByTestId('increase'); await user.click(increaseButton); expect(screen.getByTestId('current')).toHaveTextContent('1'); };
});
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Testing Component That Do Api Call

I will use Jsonplaceholder free api

// components/Users.tsx import { useEffect, useState } from 'react'; type User = { id: number; name: string;
}; /** * User list */
const Users = () => { const [data, setData] = useState<null | User[]>(null); const [error, setError] = useState<null | unknown>(null); const [loading, setLoading] = useState(false); /** * Fetch users */ const fetchUsers = async () => { setLoading(true); try { const response = await fetch('https://jsonplaceholder.typicode.com/users'); const result = await response.json(); setData(result); setError(null); } catch (error) { setData(null); setError(error); } finally { setLoading(false); } }; useEffect(() => { fetchUsers(); }, []); if (loading) { return <div data-testid='loading'>Loading...</div>; } if (error) { return <div data-testid='error'>Error when fetching users...</div>; } return ( <ul data-testid='users'> {data?.map((user) => ( <li data-testid='user' key={user.id} > {user.name} </li> ))} </ul> );
}; export default Users;
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Adding handler

// src/tests/mocks/handlers.ts
import { http, HttpResponse } from 'msw'; export const handlers: any = [ http.get('https://jsonplaceholder.typicode.com/users', () => { return HttpResponse.json([ { id: 1, name: 'Arza', }, { id: 2, name: 'Zaarza', }, ]); }),
];
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// src/components/Users.test.tsx import { render, screen, waitForElementToBeRemoved } from '@testing-library/react';
import Users from './Users';
import { server } from '../tests/mocks/node';
import { HttpResponse, http } from 'msw'; describe('Users component', () => { it('Should render correctly', () => { render(<Users />); // Assert element exists expect(screen.getByTestId('loading')).toBeInTheDocument(); }); it('Should render users correctly after loading', async () => { render(<Users />); // Wait for loading const loading = screen.getByTestId('loading'); await waitForElementToBeRemoved(loading); // Assert rendering users const userItem = screen.getAllByTestId('user'); expect(userItem).toHaveLength(2); }); it('Should render error correctly', async () => { // Mock error server.use( http.get('https://jsonplaceholder.typicode.com/users', async () => { return new HttpResponse(null, { status: 500, }); }) ); render(<Users />); // Assert rendering error expect(await screen.findByTestId('error')).toBeInTheDocument(); });
});
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