Why middleware processes requests before handlers in NestJS - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to apply middleware that logs requests before reaching the handler.

NestJS

app.use([1]);

Drag options to blanks, or click blank then click option'

AerrorHandler

BhandleRequest

CresponseMiddleware

DloggerMiddleware

Attempts:

3 left

💡 Hint

Common Mistakes

Using the handler function instead of middleware.

Placing response middleware before request logging.

The middleware function loggerMiddleware processes requests before handlers to log details.

2fill in blank

medium

Complete the middleware function to call the next handler in the chain.

NestJS

function logger(req, res, [1]) {
  console.log('Request received');
  [1]();
}

Drag options to blanks, or click blank then click option'

Ahandle

Bprocess

Cnext

Dcontinue

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling the next function, causing the request to hang.

Using incorrect parameter names.

The next function passes control to the next middleware or handler.

3fill in blank

hard

Fix the error in the middleware to ensure it processes requests before handlers.

NestJS

function authMiddleware(req, res, next) {
  if (!req.user) {
    res.status(401).send('Unauthorized');
  } else {
    [1]();
  }
}

Drag options to blanks, or click blank then click option'

Anext

Bhandle

Cprocess

Dcontinue

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to call next() after checks.

Calling a wrong function name.

Calling next() passes control to the next middleware or handler after authentication check.

4fill in blank

hard

Fill both blanks to create middleware that modifies request data before handler.

NestJS

function modifyMiddleware(req, res, [1]) {
  req.customData = 'data';
  [2]();
}

Drag options to blanks, or click blank then click option'

Anext

Bhandle

Cprocess

Dcontinue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and call.

Not calling next() after modification.

The next function is used both as parameter and called to continue processing.

5fill in blank

hard

Fill all three blanks to create middleware that logs, modifies, and passes request to handler.

NestJS

function combinedMiddleware(req, res, [1]) {
  console.log('Start');
  req.modified = true;
  [2]();
  console.log([3]);
}

Drag options to blanks, or click blank then click option'

Anext

Bhandle

C'End'

Dcontinue

Attempts:

3 left

💡 Hint

Common Mistakes

Not calling next() before logging 'End'.

Using wrong variable names.

The middleware uses next to continue and logs 'End' after calling it.

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