Building custom middleware in Node.js - Step-by-Step Execution

Concept Flow - Building custom middleware

Request received

↓

Middleware function called

↓

Middleware processes request

↓

Modify req/res objects

↓

Perform logic (e.g., logging, auth)

↓

Call next() to pass control

↓

Next middleware or route handler runs

↓

Response sent back

When a request comes in, the middleware runs, can change request or response, then calls next() to continue the flow.

Execution Sample

Node.js

function logger(req, res, next) {
  console.log(`Request: ${req.method} ${req.url}`);
  next();
}

This middleware logs the request method and URL, then passes control to the next handler.

Execution Table

Step	Action	req.method	req.url	Console Output	next() called	Next Handler
1	Request received	GET	/home		No	No
2	Middleware logger runs	GET	/home	Request: GET /home	Yes	No
3	next() passes control	GET	/home		Yes	Yes
4	Route handler runs	GET	/home		Yes	Yes
5	Response sent	GET	/home		Yes	Yes

💡 Middleware calls next(), so control moves to next handler and response is sent.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
req.method	GET	GET	GET	GET
req.url	/home	/home	/home	/home
next() called	false	true	true	true

Key Moments - 2 Insights

Why do we need to call next() inside middleware?

Calling next() tells Node.js to continue to the next middleware or route handler. Without it, the request will hang and never complete, as shown in execution_table step 2 where next() is called to proceed.

Can middleware modify the request or response objects?

Yes, middleware can change req or res objects before passing control. This is useful for adding data or headers. The execution_table shows req.method and req.url unchanged here, but they could be modified.

Visual Quiz - 3 Questions

Look at the execution_table, what is logged to the console at step 2?

A "Request: GET /home"

B "Request: POST /login"

C "Error: no next() called"

D "Response sent"

💡 Hint

Check the 'Console Output' column at step 2 in execution_table.

At which step is next() first called according to the execution_table?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'next() called' column to see when it changes from No to Yes.

If next() was not called in the middleware, what would happen?

A Request would continue to next handler

B Request would hang and never complete

C Response would be sent immediately

D Middleware would run twice

💡 Hint

Refer to key_moments explanation about the importance of calling next().

Concept Snapshot

Middleware is a function with (req, res, next).
It runs on each request.
It can modify req or res.
Must call next() to continue.
Without next(), request hangs.
Used for logging, auth, etc.

Full Transcript

When a request arrives, Node.js calls middleware functions in order. Each middleware receives request and response objects and a next function. Middleware can log info, check authentication, or modify data. After its work, middleware must call next() to pass control to the next middleware or route handler. If next() is not called, the request will hang and no response will be sent. This flow ensures modular and reusable processing steps for requests.