🚀 React Interview Experience(August 2025): SimForm

Overview

This document outlines a React developer interview experience at SimForm in August 2025. The interview process assessed the candidate's knowledge of React concepts, asynchronous JavaScript, and testing methodologies. The following sections detail the specific questions and expected answers, providing a valuable resource for individuals preparing for similar frontend engineering roles.

Interview Rounds

The interview covered a range of topics, including:

• Asynchronous Data Loading: The candidate was questioned on how to handle asynchronous data loading in React applications using useEffect and useState hooks, along with libraries such as axios. The interviewer expected the candidate to demonstrate understanding of managing loading and error states.

import { useState, useEffect } from 'react';
import axios from 'axios';

function DataFetcher() {
  const [data, setData] = useState([]);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);

  useEffect(() => {
    axios.get('https://jsonplaceholder.typicode.com/posts')
      .then(response => setData(response.data))
      .catch(err => setError(err))
      .finally(() => setLoading(false));
  }, []);

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

  return (
    <ul>
      {data.map(post => <li key={post.id}>{post.title}</li>)}
    </ul>
  );
}

• Programmatic Navigation with React Router: The candidate was asked about navigating programmatically in React Router, with emphasis on the useNavigate hook (v6+). The expected answer involved using the hook to navigate to specific routes in response to events or API calls.

import { useNavigate } from 'react-router-dom';

function MyComponent() {
  const navigate = useNavigate();

  const handleClick = () => {
    // Navigate to /dashboard programmatically
    navigate('/dashboard');
  };

  return <button onClick={handleClick}>Go to Dashboard</button>;
}

• Localization in React Applications: Questions were posed regarding localizing React applications using i18n libraries like react-i18next. The candidate needed to describe how to store translations, detect user locales, and use translation hooks or components.

import { useTranslation } from 'react-i18next';

function App() {
  const { t, i18n } = useTranslation();

  const changeLanguage = (lang) => i18n.changeLanguage(lang);

  return (
    <div>
      <p>{t('welcome_message')}</p>
      <button onClick={() => changeLanguage('en')}>English</button>
      <button onClick={() => changeLanguage('fr')}>French</button>
    </div>
  );
}

• Automatic Redirect After Login: The interviewer inquired about performing automatic redirects after login using React Router. The expected answer included using useNavigate or <Navigate> after a successful login API call.

import { useNavigate } from 'react-router-dom';

function Login() {
  const navigate = useNavigate();

  const handleLogin = async () => {
    const success = await loginApiCall(); // assume this returns true on success
    if (success) {
      navigate('/dashboard'); // redirect to dashboard
    }
  };

  return <button onClick={handleLogin}>Login</button>;
}

• Testing React Applications: The discussion covered different approaches to testing React applications, including unit, component/UI, integration, and end-to-end testing.

• Testing Asynchronous Code: The candidate was asked about testing asynchronous code in React components, focusing on the use of async/await and utilities from React Testing Library like waitFor.

import { render, screen, waitFor } from '@testing-library/react';
import MyComponent from './MyComponent';

test('fetches data and renders it', async () => {
  render(<MyComponent />);
  await waitFor(() => {
    expect(screen.getByText('Data loaded')).toBeInTheDocument();
  });
});

• Mocking API Calls: Questions were raised regarding mocking API calls in React component tests using Jest mocks, msw (Mock Service Worker), or mocking libraries like axios.

import { render, screen, waitFor } from '@testing-library/react';
import axios from 'axios';
import UserList from './UserList';

jest.mock('axios');

test('renders users from API', async () => {
  axios.get.mockResolvedValue({ data: [{ id: 1, name: 'John' }] });
  
  render(<UserList />);

  const user = await screen.findByText('John');
  expect(user).toBeInTheDocument();
});

• Testing React Hooks: The candidate was asked about testing React hooks, including testing through components and using @testing-library/react-hooks.

import { renderHook, act } from '@testing-library/react';
import useCounter from './useCounter';

test('should increment counter', () => {
  const { result } = renderHook(() => useCounter());

  act(() => {
    result.current.increment();
  });

  expect(result.current.count).toBe(1);
});

• createElement vs. cloneElement: The interviewer explored the difference between React.createElement and React.cloneElement.

const element = React.createElement('button', { className: 'btn' }, 'Click Me');

const oldElement = <button>Click</button>;
const newElement = React.cloneElement(oldElement, { className: 'btn' });

• Rules of React Hooks: The candidate was questioned on the rules of React hooks, including calling hooks at the top level, only from React functions, following the naming convention, and keeping hook dependencies correct.

Key Takeaways

This interview experience highlights the importance of a strong understanding of React fundamentals, asynchronous JavaScript, React Router, localization techniques, and testing methodologies. Candidates should be prepared to discuss and demonstrate their knowledge of these concepts with code examples. Familiarity with testing libraries such as React Testing Library and mocking techniques is also crucial for success in frontend engineering interviews.