Request context middleware in Express - Step-by-Step Execution

Concept Flow - Request context middleware

Incoming HTTP Request

↓

Middleware 1: Setup Context

↓

Attach Context to req

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Next Middleware or Route Handler

↓

Use Context in Handlers

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Send Response

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Request Ends

The middleware intercepts each request, creates a context object, attaches it to the request, then passes control to the next handler which can use this context.

Execution Sample

Express

app.use((req, res, next) => {
  req.context = { requestId: Date.now() };
  next();
});

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

This middleware adds a unique requestId to each request's context, which the route handler then uses in the response.

Execution Table

Step	Action	req.context Before	Action Result	req.context After	Next Called
1	Incoming request arrives	undefined	Middleware runs	undefined	No
2	Middleware sets req.context	undefined	req.context = {requestId: timestamp}	{requestId: 1680000000000}	No
3	Middleware calls next()	{requestId: 1680000000000}	Pass control to next handler	{requestId: 1680000000000}	Yes
4	Route handler reads req.context	{requestId: 1680000000000}	Reads requestId	{requestId: 1680000000000}	N/A
5	Route handler sends response	{requestId: 1680000000000}	Response sent with ID	{requestId: 1680000000000}	N/A
6	Request ends	{requestId: 1680000000000}	Connection closed	{requestId: 1680000000000}	N/A

💡 Request ends after response is sent and connection closes.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
req.context	undefined	{requestId: 1680000000000}	{requestId: 1680000000000}	{requestId: 1680000000000}

Key Moments - 3 Insights

Why is req.context undefined before the middleware runs?

Because req.context is not a built-in property; the middleware creates and attaches it at step 2 as shown in the execution_table.

What happens if next() is not called in the middleware?

The request will hang and never reach the route handler, because next() passes control forward, as seen between steps 2 and 3.

Can the route handler modify req.context?

Yes, since req.context is attached to the request object, the handler can read or update it after middleware sets it, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is req.context after step 2?

A null

B undefined

C {requestId: 1680000000000}

D {}

💡 Hint

Check the 'req.context After' column in row for step 2.

At which step does the middleware pass control to the next handler?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look for 'Next Called' column showing 'Yes' in the execution_table.

If the middleware did not set req.context, what would the route handler see?

A req.context would be undefined

B req.context would be an empty object

C req.context would be null

D req.context would have a default requestId

💡 Hint

Refer to the variable_tracker start value and step 2 in execution_table.

Concept Snapshot

Request context middleware in Express:
- Middleware runs on every request
- Creates a context object (e.g., req.context)
- Attaches it to req for downstream use
- Calls next() to continue processing
- Route handlers access req.context for request-specific data

Full Transcript

In Express, request context middleware runs on each incoming HTTP request. It creates a context object, such as a unique requestId, and attaches it to the request object as req.context. This allows later middleware or route handlers to access and use this context data. The middleware must call next() to pass control forward. The route handler then reads req.context and can use it to customize the response. The request ends after the response is sent and the connection closes.