Testing React Applications

Introduction to Testing in React

Testing is a critical part of building reliable and maintainable applications. In React, testing helps ensure that components render as expected, handle user interactions properly, and behave correctly as your app evolves.

There are several types of tests:

  • Unit tests: Test individual functions or components in isolation.
  • Integration tests: Test how components and logic work together.
  • End-to-end (E2E) tests: Simulate real user scenarios using tools like Cypress or Playwright.

In this post, we'll focus on unit and component testing using Jest and React Testing Library (RTL).

Unit Testing with Jest

Jest is a JavaScript testing framework developed by Meta, and it's the default testing library in many React setups. It provides a test runner, assertion library, mocking capabilities, and snapshot testing.

Installing Jest

If you’re using Create React App (CRA), Jest is already configured. For Vite or custom setups:

npm install --save-dev jest babel-jest @babel/preset-env @babel/preset-react

Example: Unit Test for a Function


// utils/math.js
export function add(a, b) {
  return a + b;
}

// __tests__/math.test.js
import { add } from '../utils/math';

test('adds two numbers', () => {
  expect(add(2, 3)).toBe(5);
});
  

Running Tests

Add a script to package.json:

"test": "jest"

Then run:

npm test

Component Testing with React Testing Library

React Testing Library (RTL) encourages testing components the way users interact with them—by querying the DOM. It pairs well with Jest and is widely adopted for React UI testing.

Installing RTL

npm install --save-dev @testing-library/react @testing-library/jest-dom

Example: Component Test


// components/Greeting.js
export default function Greeting({ name }) {
  return <h1>Hello, {name}!</h1>;
}

// __tests__/Greeting.test.js
import { render, screen } from '@testing-library/react';
import Greeting from '../components/Greeting';

test('renders greeting message', () => {
  render(<Greeting name="Alice" />);
  expect(screen.getByText(/hello, alice/i)).toBeInTheDocument();
});
  

Simulating User Events

RTL can simulate user interactions using @testing-library/user-event.

npm install --save-dev @testing-library/user-event

// components/Counter.js
import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

// __tests__/Counter.test.js
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import Counter from '../components/Counter';

test('increments count when button is clicked', async () => {
  render(<Counter />);
  const button = screen.getByText(/increment/i);
  await userEvent.click(button);
  expect(screen.getByText(/count: 1/i)).toBeInTheDocument();
});
  

Conclusion

Testing ensures your React components behave as expected and helps catch bugs early in the development process. Jest and React Testing Library provide a powerful combination for unit and component testing. In the next post, we’ll cover how to deploy your React application to platforms like Vercel, Netlify, and AWS.

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