Error boundaries in React

What are Error Boundaries?

Error Boundaries are React components that catch JavaScript errors in their child component tree, log them, and display a fallback UI instead of crashing the entire application.

Why use Error Boundaries?

Before Error Boundaries, an error in one component would break the entire app. Error Boundaries allow you to:

Prevent the entire application from crashing
Show users a friendly error message
Log errors for analysis
Isolate problematic parts of the application

Creating an Error Boundary

An Error Boundary is a class component with getDerivedStateFromError and/or componentDidCatch methods.

javascript

import React from 'react';

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error) {
    // Update state to show fallback UI
    return { hasError: true };
  }

  componentDidCatch(error, errorInfo) {
    // Log error to reporting service
    console.error('Error caught by boundary:', error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }

    return this.props.children;
  }
}

Usage

Wrap components that might throw errors:

javascript

function App() {
  return (
    <div>
      <ErrorBoundary>
        <MyWidget />
      </ErrorBoundary>
      
      <ErrorBoundary>
        <AnotherWidget />
      </ErrorBoundary>
    </div>
  );
}

If MyWidget throws an error, only it will crash while AnotherWidget continues working.

Difference between methods

getDerivedStateFromError

Called during rendering phase. Used to update state and show fallback UI.

javascript

static getDerivedStateFromError(error) {
  // Must return an object to update state
  return { hasError: true, error };
}

componentDidCatch

Called after rendering. Used for logging errors.

javascript

componentDidCatch(error, errorInfo) {
  // errorInfo.componentStack contains the component call stack
  logErrorToService(error, errorInfo);
}

Advanced Example

javascript

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      hasError: false,
      error: null,
      errorInfo: null
    };
  }

  static getDerivedStateFromError(error) {
    return { hasError: true };
  }

  componentDidCatch(error, errorInfo) {
    this.setState({
      error,
      errorInfo
    });
    
    // Send to monitoring service
    logErrorToService(error, errorInfo);
  }

  handleReset = () => {
    this.setState({
      hasError: false,
      error: null,
      errorInfo: null
    });
  };

  render() {
    if (this.state.hasError) {
      return (
        <div className="error-container">
          <h2>An error occurred</h2>
          <details>
            <summary>Details</summary>
            <p>{this.state.error && this.state.error.toString()}</p>
            <pre>{this.state.errorInfo && this.state.errorInfo.componentStack}</pre>
          </details>
          <button onClick={this.handleReset}>
            Try again
          </button>
        </div>
      );
    }

    return this.props.children;
  }
}

What Error Boundaries DON'T catch

Error Boundaries catch errors in:

Render methods
Lifecycle methods
Constructors of child components

They DON'T catch:

Event handlers
Asynchronous code (setTimeout, Promise)
Server-side rendering errors
Errors in the Error Boundary itself

Event Handlers

Use regular try-catch for event handlers:

javascript

function MyComponent() {
  function handleClick() {
    try {
      // Code that might throw an error
      somethingDangerous();
    } catch (error) {
      console.error('Error in click handler:', error);
    }
  }

  return <button onClick={handleClick}>Click me</button>;
}

Asynchronous Errors

Use try-catch for asynchronous errors as well:

javascript

function MyComponent() {
  async function fetchData() {
    try {
      const response = await fetch('/api/data');
      const data = await response.json();
      // ...
    } catch (error) {
      console.error('Fetch error:', error);
    }
  }

  useEffect(() => {
    fetchData();
  }, []);

  return <div>...</div>;
}

Where to place Error Boundaries

Global Level

javascript

function App() {
  return (
    <ErrorBoundary>
      <Router>
        <Layout>
          <Routes />
        </Layout>
      </Router>
    </ErrorBoundary>
  );
}

Route Level

javascript

function App() {
  return (
    <Router>
      <ErrorBoundary>
        <Route path="/profile" element={<Profile />} />
      </ErrorBoundary>
      
      <ErrorBoundary>
        <Route path="/settings" element={<Settings />} />
      </ErrorBoundary>
    </Router>
  );
}

Component Level

javascript

function Dashboard() {
  return (
    <div>
      <ErrorBoundary>
        <ChartWidget />
      </ErrorBoundary>
      
      <ErrorBoundary>
        <StatsWidget />
      </ErrorBoundary>
    </div>
  );
}

Integration with monitoring services

Sentry

javascript

import * as Sentry from '@sentry/react';

class ErrorBoundary extends React.Component {
  componentDidCatch(error, errorInfo) {
    Sentry.captureException(error, { contexts: { react: errorInfo } });
  }

  // ...
}

// Or use Sentry's built-in ErrorBoundary
import { ErrorBoundary } from '@sentry/react';

function App() {
  return (
    <ErrorBoundary fallback={<ErrorFallback />}>
      <MyApp />
    </ErrorBoundary>
  );
}

React 18 and useErrorBoundary

React doesn't have a hook for Error Boundaries yet, but libraries offer solutions:

react-error-boundary

javascript

import { ErrorBoundary, useErrorHandler } from 'react-error-boundary';

function ErrorFallback({ error, resetErrorBoundary }) {
  return (
    <div role="alert">
      <p>Something went wrong:</p>
      <pre>{error.message}</pre>
      <button onClick={resetErrorBoundary}>Try again</button>
    </div>
  );
}

function App() {
  return (
    <ErrorBoundary
      FallbackComponent={ErrorFallback}
      onReset={() => {
        // Reset app state
      }}
      onError={(error, errorInfo) => {
        // Log error
      }}
    >
      <MyApp />
    </ErrorBoundary>
  );
}

// Use in functional components
function MyComponent() {
  const handleError = useErrorHandler();

  async function fetchData() {
    try {
      const response = await fetch('/api/data');
      if (!response.ok) throw new Error('Failed to fetch');
      return response.json();
    } catch (error) {
      handleError(error);
    }
  }

  // ...
}

Best Practices

Multiple Error Boundaries

Don't use a single global Error Boundary. Use multiple at different levels:

javascript

function App() {
  return (
    <ErrorBoundary fallback={<GlobalError />}>
      <Header />
      
      <ErrorBoundary fallback={<SidebarError />}>
        <Sidebar />
      </ErrorBoundary>
      
      <ErrorBoundary fallback={<ContentError />}>
        <Content />
      </ErrorBoundary>
      
      <Footer />
    </ErrorBoundary>
  );
}

Informative Messages

Show users helpful information:

javascript

function ErrorFallback({ error }) {
  return (
    <div className="error-page">
      <h1>Oops! Something went wrong</h1>
      <p>We're working on fixing this.</p>
      <p>Try:</p>
      <ul>
        <li>Refreshing the page</li>
        <li>Going back to home</li>
        <li>Reporting the issue to us</li>
      </ul>
      {process.env.NODE_ENV === 'development' && (
        <details>
          <summary>Technical details</summary>
          <pre>{error.message}</pre>
        </details>
      )}
    </div>
  );
}

Recovery Options

Give users a way to try again:

javascript

class ErrorBoundary extends React.Component {
  state = { hasError: false, errorCount: 0 };

  static getDerivedStateFromError(error) {
    return { hasError: true };
  }

  componentDidUpdate(prevProps) {
    // Reset error when children change
    if (this.state.hasError && prevProps.children !== this.props.children) {
      this.setState({ hasError: false, errorCount: 0 });
    }
  }

  handleReset = () => {
    this.setState(state => ({
      hasError: false,
      errorCount: state.errorCount + 1
    }));
  };

  render() {
    if (this.state.hasError) {
      return (
        <div>
          <h1>Error</h1>
          <button onClick={this.handleReset}>Try again</button>
          {this.state.errorCount > 2 && (
            <p>If the problem persists, please contact support.</p>
          )}
        </div>
      );
    }

    return this.props.children;
  }
}

Conclusion

Error Boundaries:

Catch errors in React components
Must be class components
Use getDerivedStateFromError and componentDidCatch
Don't catch errors in event handlers and async code
Best to use multiple at different levels
Allow showing fallback UI instead of blank screen
Essential for production applications

In interviews:

Important to be able to:

Explain what Error Boundaries are
Show how to create an Error Boundary
Describe which errors they don't catch
Explain the difference between getDerivedStateFromError and componentDidCatch
Give examples of proper Error Boundary placement

Short Answer

Interview ready

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A concise answer to help you respond confidently on this topic during an interview.

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