Request context middleware in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a middleware function that adds a context object to the request.

Express

function contextMiddleware(req, res, next) {
  req.[1] = {};
  next();
}

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

Acontext

Bdata

Cinfo

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Using a property name that conflicts with existing Express properties.

Forgetting to call next() to continue the middleware chain.

The middleware adds a context object to the req so other handlers can store data there.

2fill in blank

medium

Complete the code to attach a user ID to the request context inside the middleware.

Express

function contextMiddleware(req, res, next) {
  req.context = {};
  req.context.[1] = req.headers['x-user-id'];
  next();
}

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

Auid

Buser

Cid

DuserId

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic name like id which may be ambiguous.

Not reading the user ID from the correct header.

We store the user ID under userId in the context for clarity.

3fill in blank

hard

Fix the error in the middleware to ensure the context is initialized only once per request.

Express

function contextMiddleware(req, res, next) {
  if (!req.[1]) {
    req.context = {};
  }
  next();
}

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

Acontext

Bctx

Cstate

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Checking a different property name than the one used to store context.

Not guarding initialization causing data loss.

We check if req.context exists before initializing to avoid overwriting.

4fill in blank

hard

Fill both blanks to create a middleware that adds a timestamp and a request ID to the context.

Express

function contextMiddleware(req, res, next) {
  req.context = {
    [1]: Date.now(),
    [2]: req.headers['x-request-id']
  };
  next();
}

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

Atimestamp

BrequestId

CreqId

Dtime

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent or unclear property names.

Not reading the request ID from the correct header.

We use timestamp for the current time and requestId for the unique request identifier.

5fill in blank

hard

Fill all three blanks to create a middleware that initializes context, sets a user role, and logs the context.

Express

function contextMiddleware(req, res, next) {
  req.[1] = {};
  req.context.[2] = req.headers['x-user-role'] || 'guest';
  console.log('Context:', req.[3]);
  next();
}

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

Acontext

Brole

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Using different property names for context initialization and logging.

Forgetting to provide a default user role.

The context is initialized on req.context. The user role is stored as role. The console logs the context property.

Practice

1. What is the main purpose of request context middleware in Express?

easy

A. To serve static files like images and CSS

B. To handle database connections globally for all requests

C. To store and share data specific to each request across middleware and routes

D. To manage user authentication tokens only

Solution

1. Step 1: Understand request context middleware role

   Request context middleware stores data that belongs to a single request, making it accessible throughout that request's lifecycle.

2. Step 2: Compare with other options

   The other options describe different middleware roles but not the core purpose of request context middleware.

3. Final Answer:

   To store and share data specific to each request across middleware and routes -> Option C

4. Quick Check:

   Request context middleware = store per-request data [OK]

Hint: Remember: context middleware holds data per request only [OK]

Common Mistakes:

• Confusing request context with global app state
• Thinking it manages static files
• Assuming it only handles authentication

2. Which of the following is the correct way to add a property to the request context in Express middleware?

easy

A. req.context = { userId: 123 }; next();

B. res.context.userId = 123; next();

C. app.context.userId = 123; next();

D. context.userId = 123; next();

Solution

1. Step 1: Identify where to store request-specific data

   In Express, the request object (req) is unique per request, so adding a property to req is correct.

2. Step 2: Check other options for correctness

   res is for response, app is global, and context alone is undefined, so only req.context works properly.

3. Final Answer:

   req.context = { userId: 123 }; next(); -> Option A

4. Quick Check:

   Store data on req object for request context [OK]

Hint: Attach context data to req, not res or app [OK]

Common Mistakes:

• Using res or app instead of req for context
• Forgetting to call next() after middleware
• Assigning context to undefined variable

3. Given this middleware and route, what will be the output when a GET request is made to /user?

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

app.get('/user', (req, res) => {
  res.send(`User: ${req.context.user}`);
});

medium

A. User: Alice

B. User: undefined

C. Error: Cannot read property 'user' of undefined

D. User:

Solution

1. Step 1: Analyze middleware setting req.context

   The middleware sets req.context to an object with user property 'Alice' before passing control.

2. Step 2: Check route handler usage

   The route reads req.context.user and sends it in the response, so it will output 'User: Alice'.

3. Final Answer:

   User: Alice -> Option A

4. Quick Check:

   Middleware sets req.context.user = 'Alice' [OK]

Hint: Middleware runs before route, so context is set [OK]

Common Mistakes:

• Assuming req.context is undefined in route
• Forgetting middleware runs before route
• Confusing res.context with req.context

4. Identify the error in this request context middleware code:

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

medium

A. Middleware should not modify req object

B. req.context is undefined, so assigning user property causes an error

C. res object is used incorrectly

D. next() is missing, so request hangs

Solution

1. Step 1: Check initialization of req.context

   The code tries to assign user to req.context.user, but req.context was never created, so it's undefined.

2. Step 2: Understand the error cause

   Accessing a property on undefined causes a runtime error. next() is called, so no hanging issue.

3. Final Answer:

   req.context is undefined, so assigning user property causes an error -> Option B

4. Quick Check:

   Initialize req.context before adding properties [OK]

Hint: Always initialize req.context before use [OK]

Common Mistakes:

• Not initializing req.context as an object
• Forgetting to call next()
• Misusing res instead of req

5. You want to store a request ID in the context for every request and access it in multiple routes. Which is the best way to implement this in Express?

hard

A. Pass requestId as a query parameter in every route URL

B. Store requestId in a global variable and access it in routes

C. Attach requestId to res.locals and access it in routes

D. Create middleware that sets req.context = { requestId: uniqueId } and call next(), then read req.context.requestId in routes

Solution

1. Step 1: Understand request-specific data storage

   Request ID is unique per request, so it must be stored in a per-request object like req.context.

2. Step 2: Evaluate options for sharing data

   Global variables cause conflicts between concurrent requests, query parameters are not suitable for internal data sharing, and res.locals is primarily for template data rather than request processing.

3. Final Answer:

   Create middleware that sets req.context = { requestId: uniqueId } and call next(), then read req.context.requestId in routes -> Option D

4. Quick Check:

   Use req.context for per-request data sharing [OK]

Hint: Use middleware to set req.context with request ID [OK]

Common Mistakes:

• Using global variables for request data
• Relying on query parameters for context
• Confusing res.locals with request context