Request modification in NextJS - Performance & Optimization

Performance: Request modification

MEDIUM IMPACT

This affects the server response time and client perceived load speed by modifying requests before they reach the server or client.

Modifying HTTP requests in Next.js middleware

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const url = new URL(request.url);
  if (url.pathname === '/old-path') {
    url.pathname = '/new-path';
    return NextResponse.rewrite(url);
  }
  return NextResponse.next();
}

Only rewrites requests when necessary, reducing server processing and avoiding redundant rewrites.

📈 Performance Gainreduces average middleware processing time by 30-50%

Modifying HTTP requests in Next.js middleware

NextJS

import { NextResponse } from 'next/server';

export function middleware(request) {
  const url = new URL(request.url);
  url.pathname = '/old-path';
  return NextResponse.rewrite(url);
}

Rewriting requests without caching or conditional checks causes unnecessary rewrites on every request, increasing server processing time.

📉 Performance Costblocks rendering for 20-50ms per request depending on server load

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Unconditional rewrite in middleware	0 (server-side)	0	0	[X] Bad
Conditional rewrite with minimal logic	0 (server-side)	0	0	[OK] Good

Rendering Pipeline

Request modification happens before the server generates the response, affecting the critical rendering path by changing the resource requested or headers sent.

→Request Handling

→Server Response

→Critical Rendering Path

⚠️ BottleneckServer Response time increases if request modification is inefficient or excessive.

Core Web Vital Affected

LCP

This affects the server response time and client perceived load speed by modifying requests before they reach the server or client.

Optimization Tips

1Avoid unconditional rewrites in middleware to reduce server processing time.

2Use conditional logic to modify requests only when necessary.

3Cache or memoize results when possible to speed up repeated requests.

Performance Quiz - 3 Questions

What is a key performance risk when modifying requests unconditionally in Next.js middleware?

A Increased server processing time due to unnecessary rewrites

B Increased DOM nodes causing layout shifts

C Higher client-side JavaScript bundle size

D Slower CSS parsing on the client

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, and inspect the request URL and response headers to verify if rewrites or redirects occur.

What to look for: Look for extra redirects or rewrites causing delays; check timing waterfall for server response time.

Practice

1. In Next.js, what is the main purpose of modifying a request in middleware?

easy

A. To change request details like headers or body before the app processes it

B. To directly send a response to the client without processing

C. To update the database with request data

D. To log request details only without changing anything

Solution

1. Step 1: Understand middleware role in Next.js

   Middleware runs before the app processes a request, allowing changes to the request.

2. Step 2: Identify what request modification means

   Modifying means changing headers, body, or other request parts before the app sees it.

3. Final Answer:

   To change request details like headers or body before the app processes it -> Option A

4. Quick Check:

   Request modification = change request details [OK]

Hint: Middleware changes requests before app sees them [OK]

Common Mistakes:

• Confusing request modification with sending responses
• Thinking middleware updates databases directly
• Assuming middleware only logs requests

2. Which of the following is the correct way to return a modified request in Next.js middleware?

easy

A. return NextResponse.next(request);

B. return NextResponse.next(new Request(request));

C. return NextResponse.redirect('/new-path');

D. return NextResponse.next(new Request(request, { headers: { 'x-new': 'value' } }));

Solution

1. Step 1: Understand how to modify requests in Next.js middleware

   You must create a new Request object with changes (like new headers) and pass it to NextResponse.next().

2. Step 2: Check each option

   return NextResponse.next(request); passes original request without changes. return NextResponse.next(new Request(request)); creates a new Request but without changes. return NextResponse.next(new Request(request, { headers: { 'x-new': 'value' } })); creates a new Request with modified headers correctly. return NextResponse.redirect('/new-path'); redirects instead of modifying request.

3. Final Answer:

   return NextResponse.next(new Request(request, { headers: { 'x-new': 'value' } })); -> Option D

4. Quick Check:

   New Request with changes + NextResponse.next() = D [OK]

Hint: Wrap new Request with changes inside NextResponse.next() [OK]

Common Mistakes:

• Returning original request without changes
• Using redirect instead of next() for modification
• Not creating a new Request object for changes

3. Given this middleware code snippet, what will be the value of the header 'x-user' in the request seen by the app?

import { NextResponse } from 'next/server';
export function middleware(request) {
  const newHeaders = new Headers(request.headers);
  newHeaders.set('x-user', 'alice');
  const newRequest = new Request(request.url, { headers: newHeaders });
  return NextResponse.next(newRequest);
}

medium

A. null

B. undefined

C. alice

D. original header value if any

Solution

1. Step 1: Analyze header modification in middleware

   The code creates new headers from the original, sets 'x-user' to 'alice', then creates a new Request with these headers.

2. Step 2: Determine header value passed to app

   The app receives the new Request with 'x-user' header set to 'alice', so the value is 'alice'.

3. Final Answer:

   alice -> Option C

4. Quick Check:

   Header 'x-user' set to 'alice' in new Request [OK]

Hint: Headers set in new Request appear in app request [OK]

Common Mistakes:

• Assuming original headers remain unchanged
• Thinking header is removed or null
• Confusing middleware response headers with request headers

4. Identify the error in this Next.js middleware code that tries to add a custom header:

import { NextResponse } from 'next/server';
export function middleware(request) {
  request.headers.set('x-custom', '123');
  return NextResponse.next(request);
}

medium

A. Headers are immutable; cannot modify request.headers directly

B. NextResponse.next() cannot accept a Request object

C. Missing await keyword for asynchronous header setting

D. Middleware must return a Response, not NextResponse

Solution

1. Step 1: Check how headers can be modified in Next.js middleware

   Request headers are immutable; you cannot change them directly on the original request object.

2. Step 2: Identify correct way to modify headers

   You must create a new Headers object, modify it, then create a new Request with those headers.

3. Final Answer:

   Headers are immutable; cannot modify request.headers directly -> Option A

4. Quick Check:

   Request.headers immutable = B [OK]

Hint: Request.headers are read-only; create new Headers to modify [OK]

Common Mistakes:

• Trying to set headers directly on request.headers
• Assuming NextResponse.next() rejects Request objects
• Confusing async code requirement for headers

5. You want to add a custom header 'x-trace-id' with a unique value to every request in Next.js middleware, but only if the header is not already present. Which code snippet correctly implements this?

hard

A. import { NextResponse } from 'next/server';
export function middleware(request) {
const headers = request.headers;
if (!headers.has('x-trace-id')) {
headers.set('x-trace-id', crypto.randomUUID());
}
return NextResponse.next(request);
}

B. import { NextResponse } from 'next/server';
export function middleware(request) {
if (!request.headers.has('x-trace-id')) {
const headers = new Headers(request.headers);
headers.set('x-trace-id', crypto.randomUUID());
const newRequest = new Request(request.url, { headers });
return NextResponse.next(newRequest);
}
return NextResponse.next();
}

C. import { NextResponse } from 'next/server';
export function middleware(request) {
const newRequest = new Request(request.url, { headers: { 'x-trace-id': crypto.randomUUID() } });
return NextResponse.next(newRequest);
}

D. import { NextResponse } from 'next/server';
export function middleware(request) {
const headers = new Headers();
headers.set('x-trace-id', crypto.randomUUID());
const newRequest = new Request(request.url, { headers });
return NextResponse.next(newRequest);
}

Solution

1. Step 1: Identify the correct conditional logic and immutable handling

   Check !request.headers.has('x-trace-id'), then const headers = new Headers(request.headers); headers.set('x-trace-id', crypto.randomUUID()); const newRequest = new Request(request.url, { headers }); return NextResponse.next(newRequest); else return NextResponse.next().

2. Step 2: Why it works

   Clones headers immutably, adds conditionally if missing, preserves other headers, forwards new request or original.

3. Step 3: Why others fail

   Direct set on request.headers throws immutable error. new Request with { headers: { 'x-trace-id': ... } } replaces all headers. new Headers() starts empty, loses original headers.

4. Final Answer:

   import { NextResponse } from 'next/server'; export function middleware(request) { if (!request.headers.has('x-trace-id')) { const headers = new Headers(request.headers); headers.set('x-trace-id', crypto.randomUUID()); const newRequest = new Request(request.url, { headers }); return NextResponse.next(newRequest); } return NextResponse.next(); } -> Option B

5. Quick Check:

   Check header, clone headers, set new, return new Request = A [OK]

Hint: Clone headers, check presence, set if missing, return new Request [OK]

Common Mistakes:

• Modifying request.headers directly
• Overwriting all headers instead of cloning
• Not checking if header already exists