Why middleware processes requests before handlers in NestJS - Performance Evidence

Performance: Why middleware processes requests before handlers

MEDIUM IMPACT

This affects the time it takes for the server to start processing a request and the overall responsiveness of the application.

Preprocessing requests before reaching route handlers

NestJS

app.use(async (req, res, next) => {
  // lightweight async preprocessing
  await Promise.resolve();
  next();
});
app.get('/data', (req, res) => {
  res.send('Data');
});

Async and minimal middleware avoids blocking, allowing faster handler execution.

📈 Performance GainReduces blocking time to near 0ms, improving INP and responsiveness.

Preprocessing requests before reaching route handlers

NestJS

app.use((req, res, next) => {
  // heavy synchronous processing
  for (let i = 0; i < 1e7; i++) {}
  next();
});
app.get('/data', (req, res) => {
  res.send('Data');
});

Heavy synchronous work in middleware blocks the event loop, delaying all request handling.

📉 Performance CostBlocks request processing for 100+ ms, increasing INP and slowing response.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy synchronous middleware	N/A (server-side)	N/A	N/A	[X] Bad
Lightweight async middleware	N/A (server-side)	N/A	N/A	[OK] Good

Rendering Pipeline

Middleware runs early in the request lifecycle, before route handlers, allowing preprocessing like authentication or logging. This stage affects how quickly the server can start generating a response.

→Request Parsing

→Middleware Execution

→Route Handler Execution

⚠️ BottleneckMiddleware Execution if it contains heavy synchronous code

Core Web Vital Affected

INP

This affects the time it takes for the server to start processing a request and the overall responsiveness of the application.

Optimization Tips

1Middleware runs before handlers to preprocess requests.

2Avoid heavy synchronous work in middleware to prevent blocking.

3Use async operations in middleware to keep the event loop free.

Performance Quiz - 3 Questions

Why does middleware run before route handlers in NestJS?

A To delay the response intentionally

B To preprocess requests and possibly modify them before handlers run

C To run after the response is sent

D To handle errors only

DevTools: Performance

How to check: Record a server-side profile or use Node.js profiling tools to measure middleware execution time before handlers.

What to look for: Look for long blocking tasks in middleware that delay route handler start times.

Practice

1. Why does middleware in NestJS process requests before the route handlers?

easy

A. To delay the request processing intentionally

B. To send the response after the handler finishes

C. To replace the route handler completely

D. To prepare or check the request before the handler uses it

Solution

1. Step 1: Understand middleware role

   Middleware runs before handlers to prepare or validate requests, like checking authentication or adding data.

2. Step 2: Compare with other options

   Middleware does not send responses after handlers or replace handlers; it only processes requests early.

3. Final Answer:

   To prepare or check the request before the handler uses it -> Option D

4. Quick Check:

   Middleware prepares requests = D [OK]

Hint: Middleware runs first to prep requests before handlers [OK]

Common Mistakes:

• Thinking middleware runs after handlers
• Believing middleware replaces handlers
• Assuming middleware delays requests

2. Which of the following is the correct way to continue request processing inside a NestJS middleware?

easy

A. Return the response directly

B. Call next() function

C. Throw an error to stop processing

D. Call the route handler manually

Solution

1. Step 1: Identify middleware flow control

   In NestJS, calling next() passes control to the next middleware or handler.

2. Step 2: Eliminate incorrect options

   Returning response or throwing error stops flow; calling handler manually is not standard.

3. Final Answer:

   Call next() function -> Option B

4. Quick Check:

   Use next() to continue middleware chain [OK]

Hint: Always call next() to move forward in middleware [OK]

Common Mistakes:

• Forgetting to call next(), causing request hang
• Trying to call handler directly inside middleware
• Returning response without next() when not ending request

3. Given this NestJS middleware code snippet, what will be logged first when a request arrives?

export function logger(req, res, next) {
  console.log('Middleware start');
  next();
  console.log('Middleware end');
}

medium

A. "Middleware start" then "Middleware end"

B. "Middleware end" then "Middleware start"

C. Only "Middleware start" is logged

D. No logs appear because next() stops execution

Solution

1. Step 1: Analyze middleware execution order

   Console logs before and after next() run synchronously; first logs "Middleware start".

2. Step 2: Understand next() behavior

   Calling next() passes control but does not pause middleware; "Middleware end" logs after next() call.

3. Final Answer:

   "Middleware start" then "Middleware end" -> Option A

4. Quick Check:

   Logs before and after next() run in order [OK]

Hint: Logs before and after next() run in sequence [OK]

Common Mistakes:

• Thinking next() pauses middleware execution
• Assuming logs after next() run later
• Believing next() stops middleware code

4. You wrote this NestJS middleware but requests never reach the handler. What is the problem?

export function authMiddleware(req, res, next) {
  if (!req.headers.authorization) {
    res.status(401).send('Unauthorized');
    return;
  }
  next();
}

medium

A. Middleware must not send responses directly

B. Authorization header check is incorrect syntax

C. next() is called even after sending response, causing errors

D. next() should be called before checking headers

Solution

1. Step 1: Review middleware flow control

   If authorization header is missing, response is sent but next() is still called, causing request to continue incorrectly.

2. Step 2: Fix flow by returning early

   To stop processing after sending response, return immediately without calling next().

3. Final Answer:

   next() is called even after sending response, causing errors -> Option C

4. Quick Check:

   Don't call next() after sending response [OK]

Hint: Return after sending response to avoid calling next() [OK]

Common Mistakes:

• Calling next() after response ends
• Not returning early to stop middleware
• Misunderstanding middleware flow control

5. You want to add a middleware that adds a timestamp to every request before handlers run. Which approach correctly ensures the timestamp is available in the handler?

hard

A. Add req.timestamp = Date.now() in middleware, then call next()

B. Call next() first, then add req.timestamp after

C. Add timestamp in the handler, not middleware

D. Send timestamp in response headers from middleware

Solution

1. Step 1: Understand middleware purpose

   Middleware runs before handlers, so adding data to req here makes it available later.

2. Step 2: Correct order of operations

   Assign req.timestamp before calling next() so handlers see the value.

3. Final Answer:

   Add req.timestamp = Date.now() in middleware, then call next() -> Option A

4. Quick Check:

   Add data before next() to share with handlers [OK]

Hint: Set request data before next() to share with handlers [OK]

Common Mistakes:

• Adding data after next(), so handlers miss it
• Trying to add data in handlers instead of middleware
• Sending data only in response headers