Server-side error handling in NextJS - Step-by-Step Execution

Concept Flow - Server-side error handling

Request received

↓

Run server code

↓

Try block executes

↓

Success

↓

Return data

↓

Return error response

↓

Client receives response

The server tries to run code. If it works, it sends data. If an error happens, it catches it and sends an error response.

Execution Sample

NextJS

export async function GET() {
  try {
    const data = await fetchData();
    return new Response(JSON.stringify(data), { status: 200 });
  } catch (error) {
    return new Response('Error occurred', { status: 500 });
  }
}

This code tries to fetch data on a GET request and returns it. If fetching fails, it returns a 500 error response.

Execution Table

Step	Action	Try Block Result	Catch Block Triggered	Response Returned
1	Request received	No result yet	No	No
2	Try to fetch data	Success: data fetched	No	No
3	Return success response	Data returned	No	Response with status 200 and data
4	Request received	No result yet	No	No
5	Try to fetch data	Error thrown	Yes	No
6	Catch block runs	No	Yes	No
7	Return error response	No	Yes	Response with status 500 and error message

💡 Execution stops after returning a response to the client.

Variable Tracker

Variable	Start	After Step 2 (Success)	After Step 5 (Error)
data	undefined	fetched data object	undefined
error	undefined	undefined	Error object

Key Moments - 2 Insights

Why does the catch block run only when an error happens?

Because the catch block runs only if the try block throws an error, as shown in execution_table rows 5-7 where an error triggers the catch.

What response does the server send when data fetch is successful?

It sends a response with status 200 and the data, as shown in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the response status when the catch block runs?

A 200

B 500

C 404

D 302

💡 Hint

Check the Response Returned column in rows 6 and 7 where the catch block triggers.

At which step does the try block successfully return data?

A Step 3

B Step 2

C Step 5

D Step 7

💡 Hint

Look at the Try Block Result and Response Returned columns in the execution table.

If fetchData always throws an error, how many times does the catch block run per request?

A Zero times

B Twice

C Once

D Multiple times

💡 Hint

Refer to the flow where catch block runs only once per error thrown in the try block.

Concept Snapshot

Server-side error handling in Next.js uses try/catch.
Try block runs server code.
If error occurs, catch block sends error response.
Always return a Response object.
Use status codes like 200 for success, 500 for errors.

Full Transcript

In Next.js server-side code, when a request comes in, the server runs code inside a try block. If the code works without errors, it returns a success response with data and status 200. If an error happens, the catch block catches it and returns an error response with status 500. This ensures the client always gets a clear response. The execution table shows steps from receiving the request, trying to fetch data, and either returning success or catching errors and returning an error response. Variables like data and error change depending on success or failure. Understanding when the catch block runs and what response is sent helps beginners handle errors properly in server code.