Global middleware in NestJS - Step-by-Step Execution

Concept Flow - Global middleware

Start Application

↓

Register Global Middleware

↓

Incoming Request

↓

Global Middleware Executes

↓

Pass to Route Handler

↓

Send Response

↓

End Request

The app starts, global middleware is registered, then for every request the middleware runs before the route handler, finally the response is sent.

Execution Sample

NestJS

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

@Injectable()
export class LoggerMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    console.log(`Request to: ${req.url}`);
    next();
  }
}

// In main.ts
app.use(new LoggerMiddleware().use.bind(new LoggerMiddleware()));

This code defines a global middleware that logs each request URL before passing control to the next handler.

Execution Table

Step	Action	Request URL	Middleware Log	Next Called	Route Handler Called
1	Incoming request received	/home		No	No
2	Middleware runs	/home	Request to: /home	Yes	No
3	Route handler runs	/home	Request to: /home	Yes	Yes
4	Response sent	/home	Request to: /home	Yes	Yes
5	Request ends	/home	Request to: /home	Yes	Yes

💡 Request completes after middleware and route handler finish processing.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
req.url	undefined	/home	/home	/home	/home
Middleware Log	empty	empty	Request to: /home	Request to: /home	Request to: /home
next() called	false	false	true	true	true
Route Handler Called	false	false	false	true	true

Key Moments - 2 Insights

Why does the middleware need to call next()?

Calling next() tells NestJS to continue to the next step, usually the route handler. Without next(), the request would hang and never reach the handler, as shown in step 2 of the execution_table.

Is the middleware run for every request?

Yes, global middleware runs for every incoming request before any route handler, as seen in the concept_flow and execution_table steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of Middleware Log at step 2?

A "" (empty string)

B "Request to: /home"

C undefined

D null

💡 Hint

Check the Middleware Log column at step 2 in the execution_table.

At which step does the route handler start executing?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Route Handler Called column in the execution_table.

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

A Request would hang and not reach route handler

B Route handler would still run

C Middleware would run twice

D Response would be sent immediately

💡 Hint

Refer to the key_moments explanation about next() and step 2 in execution_table.

Concept Snapshot

Global middleware runs for every request before route handlers.
Define middleware class with use(req, res, next).
Call next() to continue processing.
Register globally with app.use(middleware).
Useful for logging, auth, or modifying requests.

Full Transcript

Global middleware in NestJS runs for every incoming request before the route handler. It is defined as a class implementing NestMiddleware with a use method that receives request, response, and next function. The middleware can perform actions like logging the request URL, then calls next() to pass control to the next handler. If next() is not called, the request will hang and not proceed. Global middleware is registered in main.ts using app.use(). This ensures consistent behavior for all routes, such as logging or authentication checks.