Client-side error boundaries in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an error occurs inside a client-side error boundary in Next.js?

Consider a client-side error boundary component in Next.js wrapping a child component that throws an error during rendering. What will the user see?

NextJS

function ErrorBoundary({ children }) {
  const [hasError, setHasError] = React.useState(false);

  React.useEffect(() => {
    const handleError = () => setHasError(true);
    window.addEventListener('error', handleError);
    return () => window.removeEventListener('error', handleError);
  }, []);

  if (hasError) {
    return <div role="alert">Something went wrong.</div>;
  }

  return children;
}

function BuggyComponent() {
  throw new Error('Crash!');
}

export default function Page() {
  return (
    <ErrorBoundary>
      <BuggyComponent />
    </ErrorBoundary>
  );
}

A The page crashes and shows a blank screen with a console error.

B The error is ignored and the buggy component renders nothing silently.

C The error boundary catches the error and displays 'Something went wrong.' message.

D Next.js automatically reloads the page to recover from the error.

💡 Hint

Think about what an error boundary is designed to do in React and Next.js.

An error boundary catches errors in its child components during rendering and displays a fallback UI instead of crashing the whole app. Here, the ErrorBoundary sets hasError to true and renders the fallback message.

📝 Syntax

intermediate

Which code snippet correctly defines a client-side error boundary in Next.js?

Choose the code that correctly implements a client-side error boundary using React hooks in Next.js.

A

function ErrorBoundary({ children }) {
  const [hasError, setHasError] = React.useState(false);
  React.useEffect(() => {
    const handleError = () => setHasError(true);
    window.addEventListener('error', handleError);
    return () => window.removeEventListener('error', handleError);
  }, []);
  if (hasError) {
    return <div>Error occurred</div>;
  }
  return children;
}

B

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }
  static getDerivedStateFromError() {
    return { hasError: true };
  }
  render() {
    if (this.state.hasError) {
      return <div>Error occurred</div>;
    }
    return this.props.children;
  }
}

C

function ErrorBoundary({ children }) {
  const [hasError, setHasError] = React.useState(false);
  React.useEffect(() => {
    try {
      children;
    } catch {
      setHasError(true);
    }
  }, []);
  if (hasError) {
    return <div>Error occurred</div>;
  }
  return children;
}

D

function ErrorBoundary({ children }) {
  const [error, setError] = React.useState(null);
  if (error) {
    return <div>Error occurred</div>;
  }
  return children;
}

💡 Hint

Remember that React error boundaries cannot catch errors inside event handlers or async code, so a window error listener is sometimes used.

Option A uses a window error event listener to detect errors globally on the client side and updates state to show fallback UI. This pattern works in Next.js client components. Option A is a class component which Next.js discourages for new code. Option A tries to catch errors inside useEffect which won't catch render errors. Option A does not handle errors at all.

🔧 Debug

advanced

Why does this client-side error boundary fail to catch errors thrown in event handlers?

Review the code below. Why does the error boundary not catch errors thrown inside the button's onClick handler?

NextJS

function ErrorBoundary({ children }) {
  const [hasError, setHasError] = React.useState(false);
  React.useEffect(() => {
    const handleError = () => setHasError(true);
    window.addEventListener('error', handleError);
    return () => window.removeEventListener('error', handleError);
  }, []);
  if (hasError) {
    return <div>Something went wrong.</div>;
  }
  return children;
}

export default function Page() {
  return (
    <ErrorBoundary>
      <button onClick={() => { throw new Error('Click error'); }}>
        Click me
      </button>
    </ErrorBoundary>
  );
}

A Because errors in event handlers are not caught by React error boundaries or window error events by default.

B Because the error boundary is missing a try-catch block around the button's onClick handler.

C Because the window error event listener is not properly removed in the cleanup function.

D Because the button component is outside the error boundary's children.

💡 Hint

Think about how React handles errors in event handlers differently from render errors.

React error boundaries catch errors during rendering, lifecycle methods, and constructors, but not inside event handlers. Also, window error events do not catch errors thrown synchronously inside React event handlers. Therefore, the error boundary does not catch the error thrown in the button's onClick.

❓ state_output

advanced

What is the value of hasError state after an error is caught by this client-side error boundary?

Given the code below, what will be the value of hasError after an error event is triggered?

NextJS

function ErrorBoundary({ children }) {
  const [hasError, setHasError] = React.useState(false);

  React.useEffect(() => {
    const handleError = () => setHasError(true);
    window.addEventListener('error', handleError);
    return () => window.removeEventListener('error', handleError);
  }, []);

  return hasError ? <div>Error occurred</div> : children;
}

// Somewhere else in the app, an error is thrown causing a window error event.

A undefined

B false

C null

D true

💡 Hint

Consider what the event listener does when an error occurs.

The event listener calls setHasError(true) when an error event fires, so the hasError state becomes true.

🧠 Conceptual

expert

Why are client-side error boundaries important in Next.js applications?

Choose the best explanation for why client-side error boundaries are used in Next.js apps.

A They automatically fix bugs in the code by retrying failed components without user intervention.

B They prevent the entire React component tree from unmounting when a child component throws an error during rendering, improving user experience by showing fallback UI instead of a blank page.

C They enable server-side rendering to catch errors before sending HTML to the client.

D They replace the need for try-catch blocks in all asynchronous code throughout the app.

💡 Hint

Think about what happens when a React component crashes and how error boundaries help.

Client-side error boundaries catch rendering errors in their child components and display fallback UI, preventing the whole app from crashing and improving user experience. They do not fix bugs automatically or replace try-catch in async code. Server-side rendering error handling is separate.