Handling runtime errors in React - Mini Project: Build & Apply

Handling runtime errors

📖 Scenario: You are building a simple React app that fetches user data from an API and displays it. Sometimes the API might fail or return an error. You want to handle these runtime errors gracefully so the app does not crash and shows a friendly message instead.

🎯 Goal: Create a React functional component that fetches user data and handles runtime errors using error boundaries and state. The app should show user names when data loads successfully, and show an error message if something goes wrong.

📋 What You'll Learn

Create a React functional component called UserList.

Use useState to hold user data and error state.

Use useEffect to fetch data from https://jsonplaceholder.typicode.com/users.

Handle fetch errors by setting an error state.

Render user names if data loads, or an error message if there is an error.

💡 Why This Matters

🌍 Real World

Handling runtime errors gracefully is important in real apps to avoid crashes and provide good user experience when things go wrong.

💼 Career

React developers must know how to manage errors and loading states to build robust, user-friendly interfaces.

1

Set up initial state variables

In the UserList component, create two state variables using useState: users initialized to an empty array, and error initialized to null.

React

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

export default function UserList() {
  // Create state variables users and error
  
}

Hint

Use const [users, setUsers] = useState([]) and const [error, setError] = useState(null) inside the component.

2

Add data fetching with error handling

Inside UserList, use useEffect to fetch data from https://jsonplaceholder.typicode.com/users. If the fetch is successful, update users state with the JSON data. If an error occurs, catch it and update error state with the error message.

React

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

export default function UserList() {
  const [users, setUsers] = useState([]);
  const [error, setError] = useState(null);

  // Add useEffect to fetch user data and handle errors
}

Hint

Use fetch inside useEffect. Check response.ok and throw an error if false. Use setUsers for data and setError for errors.

3

Render user list or error message

In the UserList component's return statement, render a <ul> with each user's name inside <li> if error is null. If error is not null, render a <p> element with the text Error: followed by the error message.

React

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

export default function UserList() {
  const [users, setUsers] = useState([]);
  const [error, setError] = useState(null);

  useEffect(() => {
    fetch('https://jsonplaceholder.typicode.com/users')
      .then(response => {
        if (!response.ok) {
          throw new Error('Network response was not ok');
        }
        return response.json();
      })
      .then(data => setUsers(data))
      .catch(err => setError(err.message));
  }, []);

  // Render user list or error message
}

Hint

Use a conditional if (error) to return an error message. Otherwise, map over users to create list items.

4

Add ARIA role and accessibility improvements

Add an aria-live="polite" attribute to the error <p> element to announce error messages to screen readers. Also, add a role="list" attribute to the <ul> element for better accessibility.

React

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

export default function UserList() {
  const [users, setUsers] = useState([]);
  const [error, setError] = useState(null);

  useEffect(() => {
    fetch('https://jsonplaceholder.typicode.com/users')
      .then(response => {
        if (!response.ok) {
          throw new Error('Network response was not ok');
        }
        return response.json();
      })
      .then(data => setUsers(data))
      .catch(err => setError(err.message));
  }, []);

  // Add aria-live to error message and role to ul
}

Hint

Add aria-live="polite" to the error paragraph and role="list" to the unordered list.

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