Request context middleware in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Express middleware add to the request object?

Consider this middleware function in Express:

function addRequestId(req, res, next) {
  req.requestId = Date.now().toString();
  next();
}

What will req.requestId contain when accessed in a later route handler?

Express

function addRequestId(req, res, next) {
  req.requestId = Date.now().toString();
  next();
}

A A unique string representing the current timestamp when the request was received

B The IP address of the client making the request

C The HTTP method of the request (e.g., GET, POST)

D The URL path of the request

💡 Hint

Think about what Date.now() returns and how it is converted.

The middleware sets req.requestId to a string of the current timestamp, which is unique for each request.

❓ state_output

intermediate

What is the output of this middleware chain?

Given these two middlewares and a route handler:

app.use((req, res, next) => {
  req.user = { name: 'Alice' };
  next();
});

app.use((req, res, next) => {
  req.user.role = 'admin';
  next();
});

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

What will the client receive when requesting /?

Express

app.use((req, res, next) => {
  req.user = { name: 'Alice' };
  next();
});

app.use((req, res, next) => {
  req.user.role = 'admin';
  next();
});

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

A 'Alice'

B 'admin'

C undefined

D An error is thrown because req.user is undefined

💡 Hint

Consider how the middlewares modify req.user before the route handler runs.

The first middleware sets req.user to an object with name. The second adds role. The route sends req.user.role, which is 'admin'.

📝 Syntax

advanced

Which middleware code correctly attaches a unique request ID using async hooks?

Choose the correct Express middleware snippet that uses async_hooks to store a unique request ID accessible in async callbacks.

A

const asyncHooks = require('async_hooks');
const store = new Map();

function requestContext(req, res, next) {
  const id = Date.now().toString();
  store.set(asyncHooks.executionAsyncId(), id);
  next();
}

B

const asyncHooks = require('async_hooks');
const asyncLocalStorage = new asyncHooks.AsyncLocalStorage();

function requestContext(req, res, next) {
  const id = Date.now().toString();
  asyncLocalStorage.getStore().set('requestId', id);
  next();
}

C

const asyncHooks = require('async_hooks');
const asyncLocalStorage = new asyncHooks.AsyncLocalStorage();

function requestContext(req, res, next) {
  const id = Date.now().toString();
  asyncLocalStorage.enterWith(new Map([['requestId', id]]));
  next();
}

D

const asyncHooks = require('async_hooks');
const asyncLocalStorage = new asyncHooks.AsyncLocalStorage();

function requestContext(req, res, next) {
  const id = Date.now().toString();
  asyncLocalStorage.run(new Map(), () => {
    asyncLocalStorage.getStore().set('requestId', id);
    next();
  });
}

💡 Hint

Remember that enterWith sets the store for the current async context.

Option C correctly uses asyncLocalStorage.enterWith() to set the store with the request ID before calling next(). Option C calls next() inside run() callback which is valid but less common. Option C uses a Map but does not bind context properly. Option C calls getStore() before any store is set, causing an error.

🔧 Debug

advanced

Why does this middleware fail to preserve request context in async callbacks?

Examine this middleware:

const asyncLocalStorage = new AsyncLocalStorage();

app.use((req, res, next) => {
  asyncLocalStorage.run(new Map(), () => {
    asyncLocalStorage.getStore().set('user', req.user);
    setTimeout(() => {
      console.log(asyncLocalStorage.getStore().get('user'));
    }, 100);
    next();
  });
});

Why might console.log print undefined?

Express

const asyncLocalStorage = new AsyncLocalStorage();

app.use((req, res, next) => {
  asyncLocalStorage.run(new Map(), () => {
    asyncLocalStorage.getStore().set('user', req.user);
    setTimeout(() => {
      console.log(asyncLocalStorage.getStore().get('user'));
    }, 100);
    next();
  });
});

A Because <code>setTimeout</code> does not preserve the async context by default

B Because <code>next()</code> is called inside <code>run()</code>, the async context ends before <code>setTimeout</code> runs

C Because <code>asyncLocalStorage.getStore()</code> returns undefined inside <code>setTimeout</code> callback

D Because <code>req.user</code> is undefined when setting the store

💡 Hint

Think about how async context is preserved across timers.

AsyncLocalStorage does not automatically preserve context across native timers like setTimeout. The context is lost unless wrapped or patched. So inside setTimeout, getStore() returns undefined.

🧠 Conceptual

expert

What is the main benefit of using request context middleware with AsyncLocalStorage in Express?

Why would a developer use request context middleware based on AsyncLocalStorage in an Express app?

A To replace the need for session cookies and authentication

B To improve the performance of Express by caching request data globally

C To automatically retry failed requests without additional code

D To store and access data specific to each request across asynchronous operations without passing it manually

💡 Hint

Think about how data can be shared in async callbacks without changing function signatures.

AsyncLocalStorage allows storing data tied to a request context that is accessible in all async callbacks related to that request, avoiding manual passing of data through parameters.