Handling runtime errors in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch runtime errors in a React component using try-catch.

React

function Example() {
  try {
    const result = riskyOperation();
    return <div>{result}</div>;
  } catch ([1]) {
    return <div>Error occurred</div>;
  }
}

Drag options to blanks, or click blank then click option'

Ae.preventDefault()

Bevent

Cerror

Dexception

Attempts:

3 left

💡 Hint

Common Mistakes

Using event or exception as the catch variable without declaration

Calling methods like e.preventDefault() inside catch parameter

The catch block receives the error object, commonly named error or e. Here, error is the correct variable name.

2fill in blank

medium

Complete the code to display an error message when a runtime error occurs in a React component.

React

function ErrorMessage({ error }) {
  return <div role="alert">{ [1] }</div>;
}

Drag options to blanks, or click blank then click option'

Aerror.message

B"An error happened"

Cerror.stack

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Displaying the whole error object instead of its message

Using null or static text instead of dynamic error message

To show the error message, use error.message which contains the error description.

3fill in blank

hard

Fix the error in the React error boundary component by completing the missing lifecycle method name.

React

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

  static [1](error) {
    return { hasError: true };
  }

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

Drag options to blanks, or click blank then click option'

AgetDerivedStateFromError

BshouldComponentUpdate

CcomponentWillUnmount

DcomponentDidCatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using componentDidCatch instead of getDerivedStateFromError for state update

Using lifecycle methods unrelated to error handling

getDerivedStateFromError is the static lifecycle method used in error boundaries to update state when an error occurs.

4fill in blank

hard

Fill both blanks to correctly log error details in the error boundary's lifecycle method.

React

class ErrorBoundary extends React.Component {
  componentDidCatch([1], [2]) {
    console.error([1], [2]);
  }
  render() {
    return this.props.children;
  }
}

Drag options to blanks, or click blank then click option'

Aerror

Binfo

Cevent

Dprops

Attempts:

3 left

💡 Hint

Common Mistakes

Using event or props instead of info for the second parameter

Swapping the order of parameters

The componentDidCatch method receives two parameters: error and info. These provide error details and component stack info.

5fill in blank

hard

Fill all three blanks to create a functional component that catches errors using React hooks and displays a fallback UI.

React

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

function ErrorCatcher({ children }) {
  const [hasError, [1]] = useState(false);

  useEffect(() => {
    const handleError = () => [2](true);
    window.addEventListener('error', handleError);
    return () => window.removeEventListener('error', handleError);
  }, []);

  if ([3]) {
    return <div role="alert">An unexpected error occurred.</div>;
  }

  return children;
}

Drag options to blanks, or click blank then click option'

AsetHasError

ChasError

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using the state variable name instead of setter to update state

Checking the setter function in the if condition instead of the state variable

We use useState to track error state with hasError and its setter setHasError. The effect listens for errors and sets the error state. The component renders fallback UI if hasError is true.

Practice

1. What is the main purpose of an error boundary in React?

easy

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

B. To improve the performance of React components

C. To handle user input validation

D. To manage component state more efficiently

Solution

1. Step 1: Understand error boundaries role

   Error boundaries catch errors during rendering, lifecycle methods, and constructors of child components.

2. Step 2: Identify their main use

   They prevent the entire app from crashing by showing a fallback UI instead of the broken component.

3. Final Answer:

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

4. Quick Check:

   Error boundaries catch errors = D [OK]

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

Common Mistakes:

• Thinking error boundaries improve performance
• Confusing error boundaries with input validation
• Assuming error boundaries manage state

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 functional component with try/catch inside render

D. A class component using componentWillCatch lifecycle method

Solution

1. Step 1: Recall error boundary requirements

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

2. Step 2: Check options for correct lifecycle methods

   Only A class component implementing componentDidCatch and getDerivedStateFromError correctly names these methods; componentWillCatch does not exist and hooks for error boundaries are not standard.

3. Final Answer:

   A class component implementing componentDidCatch and getDerivedStateFromError -> Option A

4. Quick Check:

   Error boundaries require class + lifecycle methods = A [OK]

Hint: Error boundaries are class components with componentDidCatch [OK]

Common Mistakes:

• Using functional components for error boundaries
• Using non-existent lifecycle methods
• Trying to catch errors inside render with try/catch

3. Given this error boundary component, what will be rendered if a child component throws an error during rendering?

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 caught:', error);
  }
  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }
    return this.props.children;
  }
}

medium

A. The child component will render normally

B. An error message 'Something went wrong.' will be displayed

C. The app will crash with an uncaught error

D. Nothing will render because of infinite loop

Solution

1. Step 1: Understand error boundary state update

   When an error occurs, getDerivedStateFromError sets hasError to true, triggering fallback UI.

2. Step 2: Check render output on error

   If hasError is true, render returns <h1>Something went wrong.</h1> instead of children.

3. Final Answer:

   An error message 'Something went wrong.' will be displayed -> Option B

4. Quick Check:

   Error caught triggers fallback UI = C [OK]

Hint: Error sets state to show fallback UI message [OK]

Common Mistakes:

• Expecting child to render despite error
• Thinking app crashes without error boundary
• Assuming infinite loop occurs

4. Identify the error in this error boundary implementation:

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

medium

A. State should be initialized in constructor, not as class field

B. Using setState inside componentDidCatch causes infinite loop

C. Missing static getDerivedStateFromError method

D. Error boundary must be a functional component

Solution

1. Step 1: Check error boundary lifecycle methods

   To update state on error, static getDerivedStateFromError must be implemented to update state before render.

2. Step 2: Analyze componentDidCatch usage

   componentDidCatch is for side effects like logging, not for state updates; setState here can cause issues.

3. Final Answer:

   Missing static getDerivedStateFromError method -> Option C

4. Quick Check:

   State update on error requires getDerivedStateFromError = A [OK]

Hint: Use getDerivedStateFromError to update state on error [OK]

Common Mistakes:

• Updating state only in componentDidCatch
• Thinking class fields can't hold state
• Believing error boundaries can be functional components

5. You want to catch errors in multiple nested components but only show one fallback UI at the top level. Which approach is best?

hard

A. Add error boundaries around each nested component separately

B. Use a functional component with useErrorBoundary hook at each level

C. Use try/catch blocks inside each component's render method

D. Wrap the entire app in a single error boundary component

Solution

1. Step 1: Understand error boundary scope

   An error boundary catches errors in its child components and their descendants.

2. Step 2: Choose approach for single fallback UI

   Wrapping the entire app in one error boundary ensures all errors show one fallback UI at top level.

3. Final Answer:

   Wrap the entire app in a single error boundary component -> Option D

4. Quick Check:

   One top-level error boundary = B [OK]

Hint: One top-level error boundary catches all nested errors [OK]

Common Mistakes:

• Adding many error boundaries causing multiple fallback UIs
• Trying to catch errors with try/catch in render
• Using non-existent hooks for error boundaries