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.