Separation of concerns in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does separation of concerns improve React component design?

Consider a React app where UI, data fetching, and state management are all mixed in one component. What is the main benefit of separating these concerns into different components or hooks?

A It reduces the number of components, making the app faster to load.

B It allows mixing UI and data logic freely without any structure.

C It makes the code easier to read, test, and maintain by isolating responsibilities.

D It forces all logic to be written inside the main App component for simplicity.

💡 Hint

Think about how breaking tasks into smaller parts helps in real life, like organizing a kitchen by utensils, ingredients, and appliances.

Separating concerns means each part of the app handles one job well. This makes the code cleaner and easier to fix or update.

❓ state_output

intermediate

What is the output of this React component with separated concerns?

Given the following React components, what will be rendered on the screen?

React

function useCounter() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    const timer = setInterval(() => setCount(c => c + 1), 1000);
    return () => clearInterval(timer);
  }, []);
  return count;
}

function CounterDisplay() {
  const count = useCounter();
  return <div>Count: {count}</div>;
}

A A div showing 'Count: 0' initially, then increasing by 1 every second.

B A div showing 'Count: 0' that never changes.

C A div showing 'Count: NaN' due to incorrect state initialization.

D Nothing renders because useCounter is not a valid hook.

💡 Hint

Look at how useCounter updates state every second and returns the count.

The custom hook useCounter manages the count state and updates it every second. CounterDisplay uses this hook and shows the current count.

📝 Syntax

advanced

Which option correctly separates data fetching from UI in React?

Choose the code snippet that properly separates data fetching logic from UI rendering using hooks.

A

function DataComponent() {
  const [data, setData] = React.useState(null);
  fetch('/api/data').then(res => res.json()).then(setData);
  if (!data) return <div>Loading...</div>;
  return <div>{data.title}</div>;
}

B

function useData() {
  const [data, setData] = React.useState(null);
  React.useEffect(() => {
    fetch('/api/data').then(res => res.json()).then(setData);
  }, []);
  return data;
}

function DataComponent() {
  const data = useData();
  if (!data) return <div>Loading...</div>;
  return <div>{data.title}</div>;
}

C

function DataComponent() {
  const data = fetch('/api/data').then(res => res.json());
  return <div>{data.title}</div>;
}

D

function useData() {
  const data = fetch('/api/data').then(res => res.json());
  return data;
}

function DataComponent() {
  const data = useData();
  return <div>{data.title}</div>;
}

💡 Hint

Remember that data fetching should happen inside useEffect to avoid infinite loops and to handle async properly.

Option B uses a custom hook with useEffect to fetch data once and returns it. The component uses this hook and renders UI accordingly.

🔧 Debug

advanced

Why does this React component violate separation of concerns?

Examine the code below. What is the main problem related to separation of concerns?

React

function UserProfile() {
  const [user, setUser] = React.useState(null);
  React.useEffect(() => {
    fetch('/api/user').then(res => res.json()).then(setUser);
  }, []);

  function handleClick() {
    alert(`User: ${user.name}`);
  }

  return (
    <div>
      <h1>{user ? user.name : 'Loading...'}</h1>
      <button onClick={handleClick}>Show Name</button>
    </div>
  );
}

A The component mixes data fetching, UI rendering, and event handling all in one place, making it harder to maintain.

B The alert inside handleClick causes a runtime error if user is null.

C The component does not fetch data correctly because useEffect is missing dependencies.

D The component should not use useState for user data.

💡 Hint

Think about how separating data fetching, UI, and event logic into different parts helps clarity.

While the code works, combining fetching, UI, and event logic in one component violates separation of concerns, making future changes harder.

🧠 Conceptual

expert

What is the best practice for separation of concerns in a large React app?

In a large React application, which approach best follows separation of concerns principles?

A Use only class components to keep state and UI tightly coupled.

B Put all logic and UI in one component to reduce the number of files and simplify imports.

C Write all data fetching in the root App component and pass data down as props everywhere.

D Use custom hooks for data and state logic, presentational components for UI, and container components to connect them.

💡 Hint

Think about dividing tasks like a team where each member has a clear role.

Custom hooks handle logic, presentational components focus on UI, and container components manage data flow. This clear separation improves maintainability and testing.