Request context middleware in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is request context middleware in Express?

It is a middleware that creates a shared space to store data related to a single HTTP request, allowing different parts of the app to access this data during that request.

beginner

How does request context middleware help in handling data during a request?

It keeps data like user info or request ID accessible across multiple middleware and route handlers without passing it explicitly as parameters.

beginner

Which Express object is commonly used to store request context data?

The req (request) object is often extended with custom properties to hold context data for the current request.

intermediate

Why should request context data not be stored globally in Express apps?

Because Express handles many requests at once, global data would mix information between requests, causing bugs and security issues.

beginner

Give an example of data you might store in request context middleware.

You might store a unique request ID, authenticated user details, or timing info to track how long the request takes.

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

A To store data globally for all requests

B To share data across different parts of the same request

C To handle static files

D To manage database connections

Request context middleware shares data only within the scope of a single request, not globally.

Where is request context data usually stored in Express?

A In the <code>res</code> object

B In a global variable

C In the <code>req</code> object

D In the <code>app</code> object

The req object is specific to each request and is the right place to store context data.

Why is it bad to store request data in global variables?

A It prevents bugs

B It improves performance

C It makes code easier to read

D It causes data to mix between requests

Global variables are shared across all requests, which can cause data leaks and bugs.

Which of these is NOT a typical use of request context middleware?

A Serving static images

B Tracking request duration

C Storing user authentication info

D Passing request ID

Serving static images is handled by static middleware, not request context middleware.

How can you add data to the request context in Express middleware?

A By adding properties to <code>req</code>, like <code>req.user = {...}</code>

B By modifying the <code>res</code> headers

C By changing global variables

D By editing the <code>app</code> object

Adding properties to req is the standard way to store request-specific data.

Explain what request context middleware is and why it is useful in Express apps.

Think about how to share data safely during one request.

Describe how you would implement simple request context middleware in Express.

Focus on modifying the req object inside middleware.

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