Mastering Debugging & Testing in React Marketplaces: A Proven Guide to Fix Memory Leaks, Errors & Bugs

Unhandled errors in React applications often stem from:

• Asynchronous Operations: Marketplaces heavily rely on asynchronous operations like API calls to fetch product data, process payments, and update inventory. If promises are not properly handled with .catch() or try…catch blocks, errors during API requests or data processing can propagate up the component tree and crash the application if not caught.
• State Management Issues: Complex state management, especially in larger marketplaces, can lead to unexpected state transitions and errors if not meticulously managed. Incorrect state updates or accessing state at the wrong time can trigger runtime errors.
• Conditional Rendering Logic: Complex conditional rendering, while powerful, can introduce errors if conditions are not exhaustively checked. For example, attempting to access properties of an object that might be null or undefined based on rendering conditions can lead to errors.
• Third-Party Library Issues: Marketplaces often integrate with various third-party libraries for functionalities like payment gateways, analytics, or search. Errors originating from these external libraries, if not handled, can bubble up and cause application instability.

Let’s say you have a ProductListing component that fetches and displays a list of products:

import React, { useState, useEffect } from ‘react’;

import { fetchProducts } from ‘./api’; // Assume this fetches product data

function ProductListing() {

  const [products, setProducts] = useState([]);

  const [isLoading, setIsLoading] = useState(true);

  const [error, setError] = useState(null);

  useEffect(() => {

    setIsLoading(true);

    fetchProducts()

      .then(data => setProducts(data))

      .catch(err => setError(err))

      .finally(() => setIsLoading(false));

  }, []);

  if (isLoading) return <p>Loading products…</p>;

  if (error) return <p>Error loading products: {error.message}</p>;

  return (

    <ul>

      {products.map(product => (

        <li key={product.id}>{product.name}</li>

      ))}

    </ul>

  );

}

export default ProductListing;

Here’s a unit test using React Testing Library and Jest:   

import React from ‘react’;

import { render, screen, waitFor, act } from ‘@testing-library/react’;

import ProductListing from ‘./ProductListing’;

import { fetchProducts } from ‘./api’; // Import the API function

jest.mock(‘./api’); // Mock the API module

describe(‘ProductListing Component’, () => {

  it(‘renders loading state initially’, () => {

    render(<ProductListing />);

    expect(screen.getByText(‘Loading products…’)).toBeInTheDocument();

  });

  it(‘fetches and displays products on successful API call’, async () => {

    const mockProducts = [{ id: 1, name: ‘Product 1’ }, { id: 2, name: ‘Product 2’ }];

    fetchProducts.mockResolvedValue(mockProducts); // Mock successful API response

    await act(async () => { // Use act to wrap state updates

      render(<ProductListing />);

    });

    await waitFor(() => { // Wait for asynchronous operations to complete

      expect(screen.getByText(‘Product 1’)).toBeInTheDocument();

      expect(screen.getByText(‘Product 2’)).toBeInTheDocument();

      expect(screen.queryByText(‘Loading products…’)).not.toBeInTheDocument(); // Loading state removed

    });

  });

  it(‘displays error message on API call failure’, async () => {

    fetchProducts.mockRejectedValue(new Error(‘API Error’)); // Mock API error response

    await act(async () => {

      render(<ProductListing />);

    });

    await waitFor(() => {

      expect(screen.getByText(‘Error loading products:’)).toBeInTheDocument();

      expect(screen.getByText(‘API Error’)).toBeInTheDocument();

      expect(screen.queryByText(‘Loading products…’)).not.toBeInTheDocument(); // Loading state removed

    });

  });

});

 Cypress is a powerful E2E testing framework specifically designed for web applications. It allows you to write tests that:

• Visit Pages: Navigate to specific URLs within your marketplace.
• Interact with UI Elements: Find and interact with buttons, links, forms, and other UI elements using CSS selectors or other attributes.
• Assert on Content and Behavior: Verify that pages display the correct content, elements are visible or hidden, and user interactions trigger the expected behavior.
• Control Browser State: Manage cookies, local storage, and browser history during tests.