Third-party middleware (cors, helmet) in NestJS - Step-by-Step Execution

Concept Flow - Third-party middleware (cors, helmet)

Start NestJS App

↓

Import Middleware

↓

Apply Middleware Globally

↓

Incoming Request

↓

Middleware Processes Request

↓

Pass to Route Handler

↓

Send Response

The app starts, imports middleware like cors and helmet, applies them globally, then each incoming request passes through these middleware before reaching route handlers.

Execution Sample

NestJS

import helmet from 'helmet';
import cors from 'cors';

app.use(helmet());
app.use(cors());

This code applies helmet and cors middleware globally to secure HTTP headers and enable cross-origin requests.

Execution Table

Step	Action	Middleware Effect	Request State	Response State
1	App starts and imports helmet and cors	No effect yet	No request	No response
2	Apply helmet middleware	Sets security headers on response	Waiting for request	Headers ready to add
3	Apply cors middleware	Allows cross-origin requests	Waiting for request	Headers ready to add
4	Incoming request arrives	Middleware chain starts	Request received	No response yet
5	helmet processes request	Adds security headers	Request unchanged	Security headers added
6	cors processes request	Adds CORS headers	Request unchanged	Security + CORS headers added
7	Request passed to route handler	Middleware done	Request ready for handler	Response headers set
8	Route handler sends response	Response sent with headers	Request handled	Response sent to client
9	Request cycle ends	Ready for next request	No request	No response

💡 Request processed through helmet and cors middleware, response sent with security and CORS headers.

Variable Tracker

Variable	Start	After Step 5	After Step 6	Final
Request	No request	Received	Received	Handled
Response Headers	Empty	Security headers added	Security + CORS headers added	Sent to client

Key Moments - 2 Insights

Why do we apply helmet and cors before route handlers?

Because middleware runs in order before routes, so applying helmet and cors first ensures all requests get security and CORS headers before reaching routes, as shown in steps 5 and 6.

Does middleware change the request data?

No, in this example helmet and cors mainly add headers to the response, they do not modify the request object, as seen in the 'Request unchanged' state in steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution table, at which step does helmet add security headers?

A Step 7

B Step 3

C Step 5

D Step 2

💡 Hint

Check the 'Middleware Effect' column for helmet processing in the execution table.

According to the variable tracker, what is the state of Response Headers after Step 6?

A Empty

B Security + CORS headers added

C Security headers added

D Sent to client

💡 Hint

Look at the 'Response Headers' row in variable_tracker after Step 6.

If we remove cors middleware, what changes in the execution table?

A No CORS headers added in response

B Helmet will not add security headers

C Request will not reach route handler

D Response will not be sent

💡 Hint

Refer to the middleware effects in steps 5 and 6 in the execution table.

Concept Snapshot

Third-party middleware like helmet and cors are functions applied globally in NestJS.
They run before route handlers to modify requests or responses.
Helmet adds security headers; cors enables cross-origin requests.
Apply middleware with app.use() before routes.
Middleware order matters: earlier middleware runs first.
This setup improves app security and compatibility.

Full Transcript

In NestJS, third-party middleware such as helmet and cors are imported and applied globally using app.use(). When the app starts, these middleware are ready to process incoming requests. Each request passes through helmet first, which adds security headers to the response, then through cors, which adds headers to allow cross-origin requests. After middleware processing, the request reaches the route handler, which sends the response back to the client with the added headers. The variable tracker shows that the request object remains unchanged by these middleware, while the response headers accumulate security and CORS information. This flow ensures all responses are secure and accessible from other origins. Middleware order is important because it determines the sequence of processing. Removing cors middleware would mean no CORS headers are added, potentially blocking cross-origin requests.