Request ID for tracing in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express middleware adding a request ID?

Consider this Express middleware that adds a unique request ID to each incoming request and logs it. What will be logged when a request is handled?

Express

import express from 'express';
import { v4 as uuidv4 } from 'uuid';

const app = express();

app.use((req, res, next) => {
  req.id = uuidv4();
  console.log(`Request ID: ${req.id}`);
  next();
});

app.get('/', (req, res) => {
  res.send(`Your request ID is ${req.id}`);
});

A Logs a unique UUID string for each request, e.g., 'Request ID: 123e4567-e89b-12d3-a456-426614174000'

B Logs 'Request ID: undefined' because req.id is not set

C Throws a ReferenceError because uuidv4 is not defined

D Logs the same fixed string 'Request ID: 1' for every request

💡 Hint

Look at how uuidv4() is called and assigned to req.id before logging.

The middleware assigns a new UUID to req.id for each request, then logs it. This ensures each request has a unique ID for tracing.

📝 Syntax

intermediate

Which option correctly adds a request ID middleware in Express?

You want to add a middleware that attaches a unique request ID to each request object. Which code snippet is syntactically correct and works as intended?

A app.use((req, res) => { req.id = uuidv4(); next(); });

B ;)} ;)(txen ;)(4vdiuu = di.qer { >= )txen ,ser ,qer((esu.ppa

C app.use((req, res, next) => { req.id = uuidv4(); return next; });

D app.use((req, res, next) => { req.id = uuidv4(); next(); });

💡 Hint

Middleware functions must have three parameters and call next() as a function.

Option D correctly defines middleware with (req, res, next) and calls next() properly. Option D misses next parameter, C misses semicolon causing syntax error, D returns next function instead of calling it.

🔧 Debug

advanced

Why does this Express app fail to log request IDs correctly?

This Express app tries to log request IDs but logs 'undefined' instead. What is the cause?

Express

import express from 'express';
import { v4 as uuidv4 } from 'uuid';

const app = express();

app.use((req, res, next) => {
  uuidv4();
  next();
});

app.use((req, res, next) => {
  console.log(`Request ID: ${req.id}`);
  next();
});

A uuidv4() is not imported correctly, causing a runtime error

B The UUID is generated but never assigned to req.id, so req.id is undefined

C Middleware order is wrong; logging happens before UUID generation

D next() is not called in the first middleware, blocking the chain

💡 Hint

Check if the generated UUID is saved anywhere on the request object.

The first middleware calls uuidv4() but does not assign its result to req.id. So req.id remains undefined when logged.

❓ state_output

advanced

What is the value of req.id after this middleware chain?

Given these two middlewares, what will be the final value of req.id when the request reaches the route handler?

Express

import express from 'express';
import { v4 as uuidv4 } from 'uuid';

const app = express();

app.use((req, res, next) => {
  req.id = 'fixed-id';
  next();
});

app.use((req, res, next) => {
  req.id = uuidv4();
  next();
});

app.get('/', (req, res) => {
  res.send(req.id);
});

A A unique UUID string generated by uuidv4()

B The string 'fixed-id' from the first middleware

C undefined because req.id is overwritten with undefined

D An error because req.id is assigned twice

💡 Hint

Later middleware can overwrite properties set by earlier middleware.

The second middleware overwrites req.id with a new UUID, so the final value is the UUID string.

🧠 Conceptual

expert

Why is adding a request ID important for tracing in Express apps?

Which of the following best explains why adding a unique request ID to each request is useful in Express applications?

A It automatically secures the app by encrypting request data with the ID

B It improves the speed of the server by caching requests with the same ID

C It helps track and correlate logs and errors for individual requests across asynchronous operations

D It replaces the need for session management by identifying users

💡 Hint

Think about how logs from different parts of the app can be connected.

A unique request ID allows developers to follow the flow of a single request through logs and error reports, especially when asynchronous code is involved.

Practice

1. What is the main purpose of adding a request ID in an Express app?

easy

A. To encrypt the request data for security

B. To speed up the request processing time

C. To uniquely identify each request for easier tracking and debugging

D. To limit the number of requests from a user

Solution

1. Step 1: Understand the role of request ID

   A request ID is used to uniquely mark each incoming request so it can be tracked through logs and debugging.

2. Step 2: Identify the main benefit

   This helps developers trace requests easily, especially when many requests happen simultaneously.

3. Final Answer:

   To uniquely identify each request for easier tracking and debugging -> Option C

4. Quick Check:

   Request ID purpose = Unique tracking [OK]

Hint: Request ID helps track requests uniquely in logs [OK]

Common Mistakes:

• Thinking request ID speeds up processing
• Confusing request ID with encryption
• Assuming request ID limits requests

2. Which code snippet correctly adds a unique request ID to each request using the 'uuid' library in Express middleware?

easy

A. app.use((req, res, next) => { req.id = uuid.v4(); next(); });

B. app.use((req, res) => { req.id = uuid.v4(); });

C. app.use((req, res, next) => { res.id = uuid.v4(); next(); });

D. app.use((req, res, next) => { req.id = uuid(); next(); });

Solution

1. Step 1: Check middleware signature

   Express middleware must have three parameters: req, res, next to pass control.

2. Step 2: Verify correct uuid usage

   uuid.v4() generates a unique ID; assigning it to req.id is correct.

3. Final Answer:

   app.use((req, res, next) => { req.id = uuid.v4(); next(); }); -> Option A

4. Quick Check:

   Correct middleware and uuid call = app.use((req, res, next) => { req.id = uuid.v4(); next(); }); [OK]

Hint: Middleware needs (req, res, next) and uuid.v4() for ID [OK]

Common Mistakes:

• Omitting next() call in middleware
• Assigning ID to res instead of req
• Calling uuid() instead of uuid.v4()

3. Given this middleware code, what will be logged when a request is made?

const { v4: uuidv4 } = require('uuid');
app.use((req, res, next) => {
  req.id = uuidv4();
  console.log(`Request ID: ${req.id}`);
  next();
});
app.get('/', (req, res) => {
  res.send(`Your request ID is ${req.id}`);
});

medium

A. Logs 'undefined' and responds with 'Your request ID is undefined'

B. Logs a unique UUID string and responds with the same ID

C. Throws an error because req.id is not set

D. Logs a fixed string 'Request ID: 1234' every time

Solution

1. Step 1: Analyze middleware behavior

   The middleware assigns a new UUID to req.id and logs it before calling next().

2. Step 2: Check route handler response

   The route sends back the same req.id value, so the response matches the logged ID.

3. Final Answer:

   Logs a unique UUID string and responds with the same ID -> Option B

4. Quick Check:

   UUID assigned and logged = Logs a unique UUID string and responds with the same ID [OK]

Hint: Middleware sets req.id before route uses it [OK]

Common Mistakes:

• Assuming req.id is undefined in route
• Thinking the ID is fixed or static
• Expecting an error due to missing ID

4. Identify the error in this Express middleware that tries to add a request ID:

app.use((req, res, next) => {
  req.id = uuidv4;
  next();
});

medium

A. req.id should be assigned to res.id

B. Middleware is missing the next parameter

C. next() is called before assigning req.id

D. uuidv4 is assigned without calling it as a function

Solution

1. Step 1: Check uuidv4 usage

   uuidv4 is a function and must be called with parentheses to generate an ID.

2. Step 2: Confirm middleware parameters and flow

   Middleware has correct parameters and calls next() after assignment, so no issue there.

3. Final Answer:

   uuidv4 is assigned without calling it as a function -> Option D

4. Quick Check:

   Function call missing parentheses = uuidv4 is assigned without calling it as a function [OK]

Hint: Call uuidv4() with () to get ID, not just uuidv4 [OK]

Common Mistakes:

• Forgetting parentheses on function calls
• Mixing req and res objects
• Calling next() too early

5. You want to add a request ID to each request and include it in all logs and the response header. Which approach correctly achieves this in Express?

hard

A. Use middleware to assign req.id = uuidv4(), add res.set('X-Request-ID', req.id), then log req.id in all handlers

B. Assign req.id in each route handler separately without middleware

C. Generate a random number in middleware but do not pass it to response headers

D. Use middleware to assign req.id but never call next() to stop processing

Solution

1. Step 1: Assign request ID in middleware

   Middleware is the right place to assign req.id once per request using uuidv4().

2. Step 2: Add request ID to response headers and logs

   Setting res.set('X-Request-ID', req.id) sends the ID to clients; logging req.id in handlers tracks requests.

3. Final Answer:

   Use middleware to assign req.id = uuidv4(), add res.set('X-Request-ID', req.id), then log req.id in all handlers -> Option A

4. Quick Check:

   Middleware + header + logging = Use middleware to assign req.id = uuidv4(), add res.set('X-Request-ID', req.id), then log req.id in all handlers [OK]

Hint: Assign ID once in middleware, add header, log everywhere [OK]

Common Mistakes:

• Assigning ID in every route instead of middleware
• Not adding request ID to response headers
• Forgetting to call next() in middleware