Server-side error handling in NextJS

Introduction

Server-side error handling helps catch and manage problems that happen on the server. It keeps your app running smoothly and shows friendly messages instead of crashing.

When fetching data from a database or external API on the server

When processing user input on the server before sending a response

When a server function might fail and you want to show a custom error page

When you want to log errors for debugging without stopping the app

When you want to return specific HTTP status codes for errors

Syntax

NextJS

export async function GET(request) {
  try {
    // server logic that might fail
    return new Response('Success', { status: 200 })
  } catch (error) {
    return new Response('Error message', { status: 500 })
  }
}

Use try...catch blocks to catch errors in server functions.

Return a Response with an appropriate status code to inform the client.

Examples

This example fetches data from an API and handles fetch errors gracefully.

NextJS

export async function GET(request) {
  try {
    const data = await fetch('https://api.example.com/data')
    if (!data.ok) throw new Error('Failed to fetch')
    const json = await data.json()
    return new Response(JSON.stringify(json), { status: 200 })
  } catch (error) {
    return new Response('Could not load data', { status: 500 })
  }
}

This example checks user input and returns a 400 error if validation fails.

NextJS

export async function POST(request) {
  try {
    const body = await request.json()
    if (!body.name) throw new Error('Name is required')
    // process data
    return new Response('Data saved', { status: 201 })
  } catch (error) {
    return new Response(error.message, { status: 400 })
  }
}

Sample Program

This server function always throws an error, which is caught and returned as a 500 response with the error message.

NextJS

export async function GET(request) {
  try {
    // Simulate a server error
    throw new Error('Server failed to process request')
  } catch (error) {
    return new Response(`Error: ${error.message}`, { status: 500 })
  }
}

Important Notes

Always handle errors to avoid crashing your server functions.

Use meaningful status codes like 400 for client errors and 500 for server errors.

Logging errors on the server helps find and fix issues faster.

Summary

Server-side error handling keeps your app stable and user-friendly.

Use try...catch blocks in server functions to catch errors.

Return proper HTTP status codes and messages to inform clients about errors.

Practice

1. What is the main purpose of using try...catch blocks in Next.js server-side functions?

easy

A. To style components dynamically

B. To catch and handle errors gracefully during server-side execution

C. To improve client-side rendering speed

D. To manage user authentication on the client

Solution

1. Step 1: Understand server-side error handling

   Server-side functions can fail due to unexpected issues. Using try...catch helps catch these errors.

2. Step 2: Purpose of try...catch

   The try block runs code that might fail, and the catch block handles errors to prevent crashes and provide feedback.

3. Final Answer:

   To catch and handle errors gracefully during server-side execution -> Option B

4. Quick Check:

   Error handling = catch errors gracefully [OK]

Hint: Try-catch blocks catch errors on the server side [OK]

Common Mistakes:

• Confusing client-side and server-side error handling
• Thinking try-catch improves UI styling
• Assuming try-catch speeds up rendering

2. Which of the following is the correct syntax to catch errors in a Next.js server function?

easy

A. try { /* code */ } handle (error) { /* handle error */ }

B. catch { /* code */ } try (error) { /* handle error */ }

C. try { /* code */ } catch (error) { /* handle error */ }

D. try: { /* code */ } except (error) { /* handle error */ }

Solution

1. Step 1: Recall JavaScript error handling syntax

   The correct syntax uses try { ... } catch (error) { ... } to catch errors.

2. Step 2: Identify correct option

   try { /* code */ } catch (error) { /* handle error */ } matches the correct syntax. Other options use invalid keywords or order.

3. Final Answer:

   try { /* code */ } catch (error) { /* handle error */ } -> Option C

4. Quick Check:

   Correct try-catch syntax = try { /* code */ } catch (error) { /* handle error */ } [OK]

Hint: Remember: try then catch with parentheses [OK]

Common Mistakes:

• Using incorrect keywords like except or handle
• Swapping try and catch blocks
• Omitting parentheses after catch

3. Consider this Next.js server function snippet:

export async function GET() {
  try {
    throw new Error('Failed to fetch data');
  } catch (error) {
    return new Response(error.message, { status: 500 });
  }
}

What will be the HTTP status code returned when this function runs?

medium

A. 400

B. 404

C. 200

D. 500

Solution

1. Step 1: Analyze the thrown error

   The function throws an error with message 'Failed to fetch data'.

2. Step 2: Check the catch block response

   The catch block returns a Response with status 500, indicating a server error.

3. Final Answer:

   500 -> Option D

4. Quick Check:

   Server error status code = 500 [OK]

Hint: Error caught returns status 500 by default [OK]

Common Mistakes:

• Assuming status 200 despite error
• Confusing 404 (not found) with server error
• Ignoring the status property in Response

4. Identify the error in this Next.js server function code:

export async function GET() {
  try {
    const data = await fetch('https://api.example.com/data');
    return new Response(JSON.stringify(data));
  } catch (error) {
    return new Response('Error occurred', { status: 500 });
  }
}

medium

A. Not parsing fetch response to JSON before stringifying

B. Missing await on fetch call

C. Incorrect status code in catch block

D. No error handling implemented

Solution

1. Step 1: Understand fetch response handling

   The fetch call returns a Response object, not the actual data. We must call response.json() to parse it.

2. Step 2: Identify missing JSON parsing

   The code stringifies the Response object directly without parsing JSON, which is incorrect.

3. Final Answer:

   Not parsing fetch response to JSON before stringifying -> Option A

4. Quick Check:

   Parse response.json() before JSON.stringify() [OK]

Hint: Always parse fetch response with .json() before use [OK]

Common Mistakes:

• Stringifying the Response object directly
• Forgetting to await response.json()
• Assuming fetch returns JSON data directly

5. You want to create a Next.js server function that fetches user data and returns a 404 status if the user is not found, or a 500 status if the fetch fails. Which code snippet correctly implements this error handling?

hard

A. export async function GET() {
try {
const res = await fetch('https://api.example.com/user');
if (!res.ok) {
if (res.status === 404) return new Response('User not found', { status: 404 });
throw new Error('Fetch failed');
}
const user = await res.json();
return new Response(JSON.stringify(user));
} catch {
return new Response('Server error', { status: 500 });
}
}

B. export async function GET() {
const res = await fetch('https://api.example.com/user');
if (res.status === 404) return new Response('User not found', { status: 404 });
const user = await res.json();
return new Response(JSON.stringify(user));
}

C. export async function GET() {
try {
const user = await fetch('https://api.example.com/user').json();
return new Response(JSON.stringify(user));
} catch (error) {
return new Response('User not found', { status: 404 });
}
}

D. export async function GET() {
try {
const res = await fetch('https://api.example.com/user');
const user = await res.json();
return new Response(JSON.stringify(user));
} catch {
return new Response('User not found', { status: 404 });
}
}

Solution

1. Step 1: Check for HTTP errors explicitly

   export async function GET() { try { const res = await fetch('https://api.example.com/user'); if (!res.ok) { if (res.status === 404) return new Response('User not found', { status: 404 }); throw new Error('Fetch failed'); } const user = await res.json(); return new Response(JSON.stringify(user)); } catch { return new Response('Server error', { status: 500 }); } } checks res.ok and handles 404 with a specific response, throwing errors for other failures.

2. Step 2: Use try-catch for fetch failures

   The catch block returns a 500 status for server errors, correctly distinguishing error types.

3. Final Answer:

   Correctly handles 404 if the user is not found and 500 if the fetch fails -> Option A

4. Quick Check:

   Check res.ok and catch errors for 404 and 500 [OK]

Hint: Check res.ok and catch errors separately for 404 and 500 [OK]

Common Mistakes:

• Not checking res.ok before parsing JSON
• Returning 404 inside catch block incorrectly
• Assuming fetch throws on 404 automatically