Handling runtime errors in React

Introduction

Handling runtime errors helps your app stay working even if something goes wrong. It shows friendly messages instead of breaking.

When fetching data from the internet that might fail

When a user inputs unexpected or wrong data

When a part of your app might crash due to bugs

When you want to show a fallback UI instead of a broken screen

Syntax

React

import React, { Component } from 'react';

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

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

  componentDidCatch(error, errorInfo) {
    // You can log the error here
  }

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

Error boundaries must be class components in React.

They catch errors in their child components during rendering.

Examples

This component throws an error when rendered.

React

function BuggyComponent() {
  throw new Error('Bug!');
  return <div>Won't render</div>;
}

A simple fallback UI to show when an error happens.

React

import React from 'react';

function ErrorFallback() {
  return <h2>Oops! Something went wrong.</h2>;
}

Basic error boundary that shows a message if a child throws an error.

React

import React, { Component } from 'react';

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

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

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

Sample Program

This example shows an error boundary catching an error from a child component. Instead of crashing, it shows a friendly message.

React

import React, { Component } from 'react';
import { createRoot } from 'react-dom/client';

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

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

  componentDidCatch(error, errorInfo) {
    console.log('Error caught:', error.message);
  }

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

function BuggyComponent() {
  throw new Error('Bug!');
  return <div>This will not render</div>;
}

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

const container = document.getElementById('root');
const root = createRoot(container);
root.render(<App />);

Important Notes

Error boundaries do not catch errors inside event handlers.

Use error boundaries to improve user experience by showing fallback UI.

Summary

Error boundaries catch errors in child components during rendering.

They help keep your app running and show friendly messages.

They must be class components in React.

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