Request context middleware in Express

Introduction

Request context middleware helps keep data related to a single request in one place. It makes sharing information between parts of your app easier without passing many variables around.

You want to store user info after login and access it in different routes.

You need to track request-specific data like request ID for logging.

You want to share database connections or settings during a request.

You want to avoid passing many parameters through multiple functions.

You want to add timing or performance info for each request.

Syntax

Express

function requestContextMiddleware(req, res, next) {
  req.context = { /* your data here */ };
  next();
}

Middleware is a function that runs during the request before your route handlers.

You add a new property (like req.context) to store data for that request.

Examples

This example adds a unique requestId to each request using the current time.

Express

function requestContextMiddleware(req, res, next) {
  req.context = { requestId: Date.now() };
  next();
}

This example prepares a user property to store logged-in user info later.

Express

function requestContextMiddleware(req, res, next) {
  req.context = { user: null };
  next();
}

Sample Program

This Express app uses request context middleware to add a unique requestId to each request. The route then sends this ID back in the response.

Express

import express from 'express';

const app = express();

// Request context middleware
function requestContextMiddleware(req, res, next) {
  req.context = { requestId: Date.now() };
  next();
}

app.use(requestContextMiddleware);

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

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Each request gets its own context object, so data does not mix between users.

Always call next() to continue to the next middleware or route.

Use context to keep your code clean and avoid global variables.

Summary

Request context middleware stores data for each request in one place.

It helps share info between middleware and routes easily.

Use it to keep your app organized and avoid passing many parameters.

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