Error boundaries concept in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an error occurs inside a child component wrapped by an error boundary?

Consider a React app where a parent component uses an error boundary to wrap a child component. If the child component throws an error during rendering, what will be the visible result?

React

function ErrorBoundary({children}) {
  const [hasError, setHasError] = React.useState(false);
  React.useEffect(() => {
    // This is a simplified example, real error boundaries use componentDidCatch
  }, []);
  if (hasError) {
    return <div>Something went wrong.</div>;
  }
  return children;
}

function BuggyComponent() {
  throw new Error('Crash!');
  return <div>All good</div>;
}

function App() {
  return <ErrorBoundary><BuggyComponent /></ErrorBoundary>;
}

A The error is ignored and the buggy component renders normally.

B The error boundary catches the error and displays 'Something went wrong.' instead of the buggy component.

C The app crashes completely and shows a blank screen.

D The error boundary logs the error but still renders the buggy component's output.

💡 Hint

Think about what error boundaries are designed to do in React.

Error boundaries catch errors in their child components during rendering and lifecycle methods, then display a fallback UI instead of crashing the whole app.

❓ lifecycle

intermediate

Which lifecycle method is used in class components to catch errors for error boundaries?

In React class components, which lifecycle method allows an error boundary to catch errors from its child components?

A componentDidCatch(error, info)

B shouldComponentUpdate(nextProps, nextState)

C componentWillUnmount()

D getDerivedStateFromError(error)

💡 Hint

This method receives error details and is called after an error is thrown.

componentDidCatch(error, info) is the lifecycle method where error boundaries catch errors and can perform side effects like logging.

📝 Syntax

advanced

Identify the correct way to define an error boundary using a class component.

Which of the following class components correctly implements an error boundary in React?

A

class ErrorBoundary extends React.Component {
  state = { hasError: false };
  static getDerivedStateFromError(error) {
    return { hasError: true };
  }
  componentDidCatch(error, info) {
    console.log(error, info);
  }
  render() {
    if (this.state.hasError) {
      return <h1>Oops!</h1>;
    }
    return this.props.children;
  }
}

B

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }
  componentDidCatch(error) {
    this.setState({ hasError: true });
  }
  render() {
    if (this.state.hasError) {
      return <h1>Error!</h1>;
    }
    return this.props.children;
  }
}

C

class ErrorBoundary extends React.Component {
  state = { hasError: false };
  getDerivedStateFromError(error) {
    this.setState({ hasError: true });
  }
  render() {
    if (this.state.hasError) {
      return <h1>Error caught</h1>;
    }
    return this.props.children;
  }
}

D

class ErrorBoundary extends React.Component {
  state = { hasError: false };
  componentDidCatch(error, info) {
    this.state.hasError = true;
  }
  render() {
    if (this.state.hasError) {
      return <h1>Error!</h1>;
    }
    return this.props.children;
  }
}

💡 Hint

Remember that getDerivedStateFromError is static and returns new state, and state should not be mutated directly.

Option A correctly uses static getDerivedStateFromError to update state and componentDidCatch for side effects. Other options misuse lifecycle methods or mutate state directly.

🔧 Debug

advanced

Why does this functional component error boundary not catch errors?

Look at this React functional component intended as an error boundary. Why does it fail to catch errors from its children? function ErrorBoundary({children}) { try { return children; } catch (e) { return

Error caught

; } } function Buggy() { throw new Error('Oops'); return

Safe

; } function App() { return ; }

A The error boundary must use useEffect hook to catch errors in children.

B The try/catch block is missing a finally clause to handle errors properly.

C React error boundaries must be class components; try/catch in functional components does not catch render errors.

D The Buggy component does not throw an error during rendering, so no error is caught.

💡 Hint

Think about how React handles errors during rendering and the limitations of try/catch in JSX.

React only supports error boundaries as class components with lifecycle methods. Functional components cannot catch render errors with try/catch because rendering happens outside the function call.

🧠 Conceptual

expert

What is the main limitation of React error boundaries regarding error types they can catch?

Error boundaries in React catch errors in their child components. Which of the following best describes a limitation of error boundaries?

A They catch all JavaScript errors anywhere in the app, including event handlers and asynchronous code.

B They catch errors only in functional components but not in class components.

C They only catch errors in server-side rendering but not in client-side rendering.

D They catch errors during rendering, lifecycle methods, and constructors of child components but not errors in event handlers.

💡 Hint

Consider where React error boundaries are designed to catch errors and where they do not.

React error boundaries catch errors during rendering, lifecycle methods, and constructors of child components but do not catch errors in event handlers or asynchronous code.

Practice

1. What is the main purpose of an Error Boundary in React?

easy

A. To catch JavaScript errors in child components and display a fallback UI

B. To manage state across multiple components

C. To optimize rendering performance

D. To handle user input validation

Solution

1. Step 1: Understand Error Boundary role

   Error boundaries catch errors in the components below them in the tree to prevent the whole app from crashing.

2. Step 2: Identify the correct purpose

   They show a fallback UI when an error happens, improving user experience.

3. Final Answer:

   To catch JavaScript errors in child components and display a fallback UI -> Option A

4. Quick Check:

   Error boundaries catch errors = C [OK]

Hint: Error boundaries catch errors and show fallback UI [OK]

Common Mistakes:

• Confusing error boundaries with state management
• Thinking error boundaries optimize rendering
• Assuming error boundaries handle input validation

2. Which of the following is the correct way to define an Error Boundary component in React?

easy

A. A class component implementing componentDidCatch and getDerivedStateFromError

B. A functional component using useErrorBoundary hook

C. A class component implementing componentDidMount and shouldComponentUpdate

D. A functional component with useEffect to catch errors

Solution

1. Step 1: Recall Error Boundary requirements

   Error boundaries must be class components that implement componentDidCatch and getDerivedStateFromError lifecycle methods.

2. Step 2: Identify correct lifecycle methods

   Only A class component implementing componentDidCatch and getDerivedStateFromError lists these two required methods correctly.

3. Final Answer:

   A class component implementing componentDidCatch and getDerivedStateFromError -> Option A

4. Quick Check:

   Class + componentDidCatch + getDerivedStateFromError = A [OK]

Hint: Error boundaries are class components with special lifecycle methods [OK]

Common Mistakes:

• Thinking functional components can be error boundaries
• Confusing lifecycle methods needed for error boundaries
• Using hooks that don't exist for error boundaries

3. Given this Error Boundary class component, what will be rendered if ChildComponent throws an error?

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }
  static getDerivedStateFromError(error) {
    return { hasError: true };
  }
  componentDidCatch(error, info) {
    console.log('Error logged:', error);
  }
  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }
    return this.props.children;
  }
}

// Usage:
// <ErrorBoundary><ChildComponent /></ErrorBoundary>

medium

A. The original ChildComponent UI is shown

B. The app crashes with an uncaught error

C. Nothing is rendered (blank screen)

D. An error message Something went wrong. is shown

Solution

1. Step 1: Understand error handling in ErrorBoundary

   If ChildComponent throws an error, getDerivedStateFromError sets hasError to true.

2. Step 2: Check render output when error occurs

   When hasError is true, the component renders the fallback UI: <h1>Something went wrong.</h1>.

3. Final Answer:

   An error message Something went wrong. is shown -> Option D

4. Quick Check:

   Error caught shows fallback UI = D [OK]

Hint: Error sets state to show fallback UI, not original child [OK]

Common Mistakes:

• Expecting child UI to show despite error
• Thinking error causes app crash
• Assuming blank screen on error

4. What is wrong with this Error Boundary component code?

class MyErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { error: null };
  }
  componentDidCatch(error, info) {
    this.setState({ error });
  }
  render() {
    if (this.state.error) {
      return <h2>Error occurred</h2>;
    }
    return this.props.children;
  }
}

medium

A. componentDidCatch should not call setState

B. It lacks the static method getDerivedStateFromError to update state on error

C. The constructor should not initialize state

D. render method must not return children

Solution

1. Step 1: Check error state update method

   Error boundaries must implement getDerivedStateFromError to update state when an error happens.

2. Step 2: Identify missing static method

   This code only uses componentDidCatch to set state, which is incorrect because componentDidCatch is for side effects, not state updates.

3. Final Answer:

   It lacks the static method getDerivedStateFromError to update state on error -> Option B

4. Quick Check:

   Missing getDerivedStateFromError static method = A [OK]

Hint: State update on error must be in getDerivedStateFromError [OK]

Common Mistakes:

• Updating state inside componentDidCatch
• Forgetting static getDerivedStateFromError
• Misusing lifecycle methods for error handling

5. You want to create an Error Boundary that logs errors to an external service and shows a fallback UI. Which of the following is the best approach?

class LoggingErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }
  static getDerivedStateFromError(error) {
    return { hasError: true };
  }
  componentDidCatch(error, info) {
    // ???
  }
  render() {
    if (this.state.hasError) {
      return <div><h1>Oops!</h1><p>Try refreshing.</p></div>;
    }
    return this.props.children;
  }
}

What should replace // ??? to correctly log errors without causing issues?

hard

A. Call an async logging function directly without try/catch

B. Update state again inside componentDidCatch to trigger re-render

C. Use a try/catch inside componentDidCatch to call the logging service safely

D. Ignore logging and only show fallback UI

Solution

1. Step 1: Understand componentDidCatch role

   componentDidCatch is for side effects like logging errors but should handle errors safely to avoid crashing.

2. Step 2: Choose safe logging approach

   Using try/catch inside componentDidCatch when calling async logging prevents unhandled errors from breaking the app.

3. Final Answer:

   Use a try/catch inside componentDidCatch to call the logging service safely -> Option C

4. Quick Check:

   Safe error logging with try/catch in componentDidCatch = B [OK]

Hint: Wrap logging calls in try/catch inside componentDidCatch [OK]

Common Mistakes:

• Calling async logging without error handling
• Updating state inside componentDidCatch causing loops
• Skipping logging and losing error info