Handling runtime errors in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a runtime error in React?

A runtime error in React is a problem that happens while the app is running, causing it to stop working or behave unexpectedly.

beginner

What is an Error Boundary in React?

An Error Boundary is a React component that catches JavaScript errors anywhere in its child component tree and shows a fallback UI instead of crashing the whole app.

intermediate

How do you create an Error Boundary in React?

You create an Error Boundary by making a class component with the lifecycle methods <code>static getDerivedStateFromError()</code> and <code>componentDidCatch()</code> to catch errors and update the UI.

intermediate

Can functional components be Error Boundaries in React?

No, currently only class components can be Error Boundaries because they need lifecycle methods to catch errors.

beginner

What is a good user experience practice when handling runtime errors in React?

Show a friendly message or fallback UI that explains something went wrong and possibly offer a way to retry or go back, so users don’t see a broken app.

What does an Error Boundary do in React?

A Reloads the page when an error occurs

B Prevents errors from happening in the code

C Automatically fixes bugs in components

D Catches runtime errors in child components and shows fallback UI

Error Boundaries catch errors in their child components and display a fallback UI instead of crashing the whole app.

Which React component type can be an Error Boundary?

A Class components with lifecycle methods

B Pure components only

C Stateless components

D Functional components with hooks

Only class components with lifecycle methods like getDerivedStateFromError and componentDidCatch can be Error Boundaries.

What lifecycle method updates state when an error is caught in an Error Boundary?

A componentDidMount

B componentWillUnmount

C getDerivedStateFromError

D shouldComponentUpdate

getDerivedStateFromError updates the state to show fallback UI after an error is caught.

What should you show users when a runtime error happens in React?

A A detailed error stack trace

B A friendly fallback UI with a message

C A blank white screen

D Nothing, just reload silently

Showing a friendly fallback UI helps users understand something went wrong without confusing them.

Where do Error Boundaries catch errors?

A In any child components below them

B Anywhere in the app globally

C Only in the component itself

D Only in event handlers

Error Boundaries catch errors in their child components, not in themselves or globally.

Explain how Error Boundaries help handle runtime errors in React apps.

Think about how React stops the whole app from breaking when a part has an error.

Describe best practices for showing error messages to users when runtime errors occur in React.

Imagine you are explaining to a friend what happened without confusing them.

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