Request context middleware in Express - Mini Project: Build & Apply

Request Context Middleware in Express

📖 Scenario: You are building a simple Express server that needs to keep track of some information about each request, like a unique request ID and the time the request started. This helps in logging and debugging.

🎯 Goal: Create a middleware in Express that adds a requestContext object to each incoming request. This object should contain a unique requestId and a startTime. Then, use this context in a route to send back the request ID and start time.

📋 What You'll Learn

Create an Express app with a middleware function

Middleware adds a requestContext object to req

requestContext must have requestId as a unique string

requestContext must have startTime as the current timestamp

Create a GET route /info that returns JSON with requestId and startTime

💡 Why This Matters

🌍 Real World

Middleware like this is used in real web servers to track requests uniquely for logging, debugging, and performance monitoring.

💼 Career

Understanding request context middleware is important for backend developers working with Express or similar frameworks to build maintainable and debuggable web applications.

1

Setup Express app and import required modules

Write code to import express and create an Express app called app. Also, import crypto from Node.js to generate unique IDs.

Express

const express = require('express');
// Your code here

Hint

Use require('express') to import Express and express() to create the app.

2

Create request context middleware

Create a middleware function called requestContextMiddleware that adds a requestContext object to req. This object should have a requestId generated by crypto.randomUUID() and startTime set to Date.now(). Use app.use(requestContextMiddleware) to apply it.

Express

const express = require('express');
const crypto = require('crypto');
const app = express();

// Create middleware to add requestContext
// Your code here

Hint

Middleware functions take req, res, next. Add requestContext to req and call next().

3

Create GET /info route using request context

Add a GET route /info to app that sends a JSON response with requestId and startTime from req.requestContext.

Express

const express = require('express');
const crypto = require('crypto');
const app = express();

function requestContextMiddleware(req, res, next) {
    req.requestContext = {
        requestId: crypto.randomUUID(),
        startTime: Date.now()
    };
    next();
}

app.use(requestContextMiddleware);

// Add GET /info route here
// Your code here

Hint

Use app.get with path /info. Send JSON with res.json() using values from req.requestContext.

4

Start the Express server

Add code to start the Express server on port 3000 using app.listen. Log a message Server running on port 3000 when it starts.

Express

const express = require('express');
const crypto = require('crypto');
const app = express();

function requestContextMiddleware(req, res, next) {
    req.requestContext = {
        requestId: crypto.randomUUID(),
        startTime: Date.now()
    };
    next();
}

app.use(requestContextMiddleware);

app.get('/info', (req, res) => {
    res.json({
        requestId: req.requestContext.requestId,
        startTime: req.requestContext.startTime
    });
});

// Start server on port 3000
// Your code here

Hint

Use app.listen(3000, () => { ... }) to start the server and log the message.

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