Functional middleware in NestJS - Step-by-Step Execution

Concept Flow - Functional middleware

Request Received

↓

Middleware Function Called

↓

Middleware Logic Executes

↓

Call next() to pass control

↓

Next Middleware or Handler Executes

↓

Response Sent

The request passes through the functional middleware which runs its logic and calls next() to continue the flow.

Execution Sample

NestJS

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

Logs the request URL and passes control to the next middleware or handler.

Execution Table

Step	Action	Request URL	Console Output	next() Called	Next Step
1	Middleware function invoked	/api/data		No	Middleware logic runs
2	Log request URL	/api/data	Request to /api/data	No	Middleware logic continues
3	Call next()	/api/data	Request to /api/data	Yes	Next middleware or handler runs
4	Next middleware or handler executes	/api/data		Yes	Response sent
5	Response sent to client	/api/data		Yes	End

💡 Middleware calls next(), passing control to next middleware or handler, ending middleware execution.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
req.url		/api/data	/api/data	/api/data	/api/data
console output			Request to /api/data	Request to /api/data	Request to /api/data
next() called	No	No	No	Yes	Yes

Key Moments - 2 Insights

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

Calling next() passes control to the next middleware or route handler. Without it, the request would hang and never reach the handler, as shown in execution_table step 3.

What happens if we don't call next()?

The request stops at the middleware and the client waits forever. The execution_table shows next() called as 'No' means no further processing.

Visual Quiz - 3 Questions

Look at the execution_table, what is the console output at step 2?

A "Request received"

B "Middleware started"

C "Request to /api/data"

D "No output"

💡 Hint

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

At which step is next() called to continue processing?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'next() Called' column in the execution_table.

If next() was not called, what would happen to the request?

A It would hang and never reach the handler

B It would immediately send a response

C It would continue to the next middleware

D It would throw an error automatically

💡 Hint

Refer to the key_moments section and execution_table exit_note.

Concept Snapshot

Functional middleware in NestJS is a function with (req, res, next).
It runs code, then calls next() to pass control.
Without next(), request processing stops.
Use middleware to log, modify requests, or check auth.
Always call next() unless sending a response directly.

Full Transcript

Functional middleware in NestJS is a simple function that takes three arguments: request, response, and next. When a request comes in, NestJS calls this middleware function. Inside, you can run any code you want, like logging the request URL. After your code runs, you must call next() to pass control to the next middleware or the route handler. If you forget to call next(), the request will hang and never reach the handler. This flow ensures middleware can process requests step-by-step before the final response is sent.