Client-side error boundaries in NextJS - Step-by-Step Execution

Concept Flow - Client-side error boundaries

Component Renders

↓

Error Occurs?

No→Show Normal UI

Yes↓

Error Boundary Catches

↓

Render Fallback UI

↓

User Interaction

↓

Reset Error Boundary?

Yes→Retry Render

No↓

Stay on Fallback UI

The component tries to render. If an error happens, the error boundary catches it and shows fallback UI. User can retry to reset the error and render again.

Execution Sample

NextJS

'use client';

import { Component } from 'react';

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

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

  resetError = () => {
    this.setState({ hasError: false });
  };

  render() {
    if (this.state.hasError) {
      return (
        <div>
          Something went wrong.
          <button onClick={this.resetError}>Retry</button>
        </div>
      );
    }

    return this.props.children;
  }
}

A simple error boundary component that catches errors during rendering and shows fallback UI.

Execution Table

Step	Action	State Before	Error Occurs?	State After	UI Rendered
1	Render child component	hasError = false	No	hasError = false	Child UI
2	Render child component	hasError = false	Yes (error thrown)	hasError = true	Fallback UI: Something went wrong.
3	User clicks retry (reset)	hasError = true	No	hasError = false	Child UI
4	Render child component	hasError = false	No	hasError = false	Child UI
5	Render child component	hasError = false	No	hasError = false	Child UI

💡 Execution stops when UI renders without error or fallback UI is shown after error.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
hasError	false	false	true	false	false	false

Key Moments - 2 Insights

Why does the UI switch to fallback after an error?

Because the error boundary sets hasError to true when catching an error (see execution_table step 2), causing fallback UI to render.

How can the error boundary try rendering the child component again?

By resetting hasError to false (step 3), usually triggered by user action, allowing normal UI to render again.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of hasError after step 2?

A true

B false

C undefined

D null

💡 Hint

Check the 'State After' column in row for step 2 in execution_table.

At which step does the UI show the fallback message?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'UI Rendered' column in execution_table for fallback UI.

If the user never resets the error, what UI will continue to show?

A Child UI

B Blank screen

C Fallback UI

D Error message in console only

💡 Hint

Refer to variable_tracker and execution_table steps after error occurs.

Concept Snapshot

Client-side error boundaries catch errors during rendering.
They show fallback UI instead of crashing the app.
Use state to track error occurrence.
Provide a way to reset and retry rendering.
In Next.js, use React error boundaries in client components.

Full Transcript

Client-side error boundaries in Next.js catch errors that happen during rendering of components. When an error occurs, the boundary sets a state variable to true and shows fallback UI instead of the broken component. The user can trigger a reset to clear the error state and try rendering the component again. This prevents the whole app from crashing and improves user experience by showing a friendly message. The execution table shows how the error state changes step-by-step and how the UI switches between normal and fallback views.

Practice

1. What is the main purpose of client-side error boundaries in Next.js?

easy

A. To catch errors in UI components and show a fallback UI instead of crashing the whole app

B. To improve server-side rendering speed

C. To handle database connection errors

D. To optimize image loading performance

Solution

1. Step 1: Understand error boundaries role

   Error boundaries catch errors in parts of the UI to prevent the entire app from crashing.

2. Step 2: Identify their main effect

   They show a fallback UI so users see a friendly message instead of a broken screen.

3. Final Answer:

   To catch errors in UI components and show a fallback UI instead of crashing the whole app -> Option A

4. Quick Check:

   Error boundaries catch UI errors = C [OK]

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

Common Mistakes:

• Confusing error boundaries with server-side features
• Thinking they handle backend or database errors
• Assuming they improve performance directly

2. Which of the following is the correct way to define a client-side error boundary component in Next.js using React hooks?

easy

A. function ErrorBoundary({ children }) { try { return children; } catch { return ; } }

B. class ErrorBoundary extends React.Component { render() { return this.props.children; } }

C. function ErrorBoundary() { return Error; }

D. const ErrorBoundary = () => { return children; }

Solution

1. Step 1: Identify hook-based error boundary pattern

   Client-side error boundaries in Next.js use try/catch inside functional components to catch errors during rendering.

2. Step 2: Check each option

   function ErrorBoundary({ children }) { try { return children; } catch { return ; } } uses try/catch inside a function component returning children or fallback UI, which is correct.

3. Final Answer:

   function ErrorBoundary({ children }) { try { return children; } catch { return <Fallback />; } } -> Option A

4. Quick Check:

   Try/catch in function component = A [OK]

Hint: Use try/catch inside functional component for client error boundaries [OK]

Common Mistakes:

• Using class components instead of functional components
• Not using try/catch to catch errors
• Returning children without error handling

3. Given this client-side error boundary component in Next.js:

function ErrorBoundary({ children }) {
  try {
    return children;
  } catch {
    return <div>Error occurred</div>;
  }
}

function BuggyComponent() {
  throw new Error('Bug!');
}

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

What will be rendered on the page?

medium

A. output

B. Error occurred

C. Nothing is rendered

D. The app crashes with an uncaught error

Solution

1. Step 1: Understand error throwing in BuggyComponent

   BuggyComponent throws an error immediately when rendered.

2. Step 2: Check ErrorBoundary behavior

   ErrorBoundary tries to render children inside try block; error triggers catch block returning fallback UI <div>Error occurred</div>.

3. Final Answer:

   <div>Error occurred</div> -> Option B

4. Quick Check:

   Error caught, fallback shown = D [OK]

Hint: Error in child triggers catch, fallback UI renders [OK]

Common Mistakes:

• Expecting the app to crash instead of showing fallback
• Thinking children render despite error
• Confusing output with component name

4. Identify the problem in this client-side error boundary code snippet:

function ErrorBoundary({ children }) {
  try {
    return children;
  } catch (error) {
    console.log(error);
  }
}

export default function App() {
  return (
    <ErrorBoundary>
      <div>Hello</div>
    </ErrorBoundary>
  );
}

What issue will occur when an error happens inside children?

medium

A. The error is caught and fallback UI is shown

B. The error is logged and children still render

C. The app crashes due to syntax error

D. No fallback UI is returned, so the component renders nothing

Solution

1. Step 1: Analyze catch block behavior

   The catch block logs the error but does not return any UI.

2. Step 2: Understand React rendering rules

   Without a return in catch, the component returns undefined, rendering nothing on error.

3. Final Answer:

   No fallback UI is returned, so the component renders nothing -> Option D

4. Quick Check:

   Missing return in catch = renders nothing [OK]

Hint: Always return fallback UI in catch block [OK]

Common Mistakes:

• Forgetting to return fallback UI in catch
• Assuming logging error is enough
• Expecting children to render after error

5. You want to create a client-side error boundary in Next.js that catches errors in nested components and logs the error before showing fallback UI. Which approach correctly combines error catching, logging, and fallback rendering?

hard

A. function ErrorBoundary({ children }) {
try {
return children;
} catch (error) {
return Something went wrong.;
}
}

B. function ErrorBoundary({ children }) {
try {
return children;
} catch {
return Something went wrong.;
}
}

C. function ErrorBoundary({ children }) {
try {
return children;
} catch (error) {
console.error(error);
return Something went wrong.;
}
}

D. function ErrorBoundary({ children }) {
try {
return children;
} catch (error) {
console.log('Error caught');
}
}

Solution

1. Step 1: Check error catching and logging

   function ErrorBoundary({ children }) { try { return children; } catch (error) { console.error(error); return
   Something went wrong.; } } catches error, logs it with console.error, then returns fallback UI.

2. Step 2: Verify fallback UI return

   function ErrorBoundary({ children }) { try { return children; } catch (error) { console.error(error); return
   Something went wrong.
   ; } } returns fallback UI after logging, ensuring user sees message and error is logged.

3. Final Answer:

   function ErrorBoundary({ children }) { try { return children; } catch (error) { console.error(error); return <div>Something went wrong.</div>; } } -> Option C

4. Quick Check:

   Error caught, logged, fallback returned = A [OK]

Hint: Catch error, log it, then return fallback UI [OK]

Common Mistakes:

• Not logging the error before fallback
• Not returning fallback UI after catching error
• Logging without returning fallback UI