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.

Practice

1. What is the main purpose of global middleware in a NestJS application?

easy

A. To define database models for the application

B. To run code on every incoming request before reaching route handlers

C. To create new routes dynamically

D. To handle errors after the response is sent

Solution

1. Step 1: Understand middleware role

   Middleware runs before route handlers to process requests or add features like logging.

2. Step 2: Identify global middleware scope

   Global middleware runs on every request, so it applies common logic everywhere.

3. Final Answer:

   To run code on every incoming request before reaching route handlers -> Option B

4. Quick Check:

   Global middleware = runs on every request [OK]

Hint: Global middleware runs before routes on all requests [OK]

Common Mistakes:

• Confusing middleware with route handlers
• Thinking middleware only runs on some routes
• Mixing middleware with error handling

2. Which of the following is the correct way to apply a middleware globally in NestJS?

easy

A. app.useGlobalMiddleware(MyMiddleware)

B. consumer.apply(MyMiddleware).forRoutes('users')

C. consumer.apply(MyMiddleware).forRoutes('*')

D. app.applyMiddlewareGlobally(MyMiddleware)

Solution

1. Step 1: Recall global middleware syntax

   In NestJS, the recommended way to apply middleware globally is using app.useGlobalMiddleware(MyMiddleware).

2. Step 2: Compare options

   consumer.apply(MyMiddleware).forRoutes('*') applies middleware to all routes but is not truly global middleware; app.useGlobalMiddleware applies middleware globally.

3. Final Answer:

   app.useGlobalMiddleware(MyMiddleware) -> Option A

4. Quick Check:

   Global middleware uses app.useGlobalMiddleware [OK]

Hint: Use app.useGlobalMiddleware to apply middleware globally [OK]

Common Mistakes:

• Using consumer.apply(...).forRoutes('*') for global middleware
• Trying to call non-existent app methods
• Confusing middleware registration with guards

3. Given this middleware code, what will be logged for every request?

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

// Applied globally with consumer.apply(LoggerMiddleware).forRoutes('*')

medium

A. Logs the HTTP method only

B. Logs the response status code

C. Logs nothing because next() is missing

D. Logs the URL of every incoming request

Solution

1. Step 1: Analyze middleware code

   The middleware logs the string 'Request to:' plus the request URL from req.url.

2. Step 2: Understand global application

   Since applied globally, this log happens on every request URL received by the app.

3. Final Answer:

   Logs the URL of every incoming request -> Option D

4. Quick Check:

   LoggerMiddleware logs req.url every request [OK]

Hint: Look for console.log inside middleware use() method [OK]

Common Mistakes:

• Assuming HTTP method is logged instead of URL
• Forgetting next() causes no further processing
• Thinking response status is logged

4. Identify the error in this global middleware setup:

export class AuthMiddleware implements NestMiddleware {
  use(req: Request, res: Response) {
    if (!req.headers.authorization) {
      res.status(401).send('Unauthorized');
    }
  }
}

// Applied with consumer.apply(AuthMiddleware).forRoutes('*')

medium

A. Missing call to next() to continue request processing

B. Middleware should not check headers

C. forRoutes('*') cannot be used for global middleware

D. res.status(401) is invalid syntax

Solution

1. Step 1: Check middleware method signature

   The use method lacks the next parameter and does not call next(), so request stops here.

2. Step 2: Understand middleware flow

   Without calling next(), the request will hang or not proceed to route handlers after middleware.

3. Final Answer:

   Missing call to next() to continue request processing -> Option A

4. Quick Check:

   Middleware must call next() to proceed [OK]

Hint: Always call next() in middleware to continue [OK]

Common Mistakes:

• Omitting next() call in middleware
• Misunderstanding middleware flow control
• Thinking forRoutes('*') is invalid

5. You want to apply a logging middleware globally but only for HTTP POST requests. Which approach correctly achieves this in NestJS?

hard

A. Use consumer.apply(LoggerMiddleware).forRoutes('*') and check req.method inside middleware

B. Apply middleware only on POST routes manually, no global option

C. Use consumer.apply(LoggerMiddleware).forRoutes({ path: '*', method: RequestMethod.POST })

D. Use app.useGlobalMiddleware(LoggerMiddleware) with method filter

Solution

1. Step 1: Understand route filtering in middleware

   NestJS allows filtering middleware by path and HTTP method using forRoutes with an object.

2. Step 2: Apply middleware globally but only for POST

   Using forRoutes({ path: '*', method: RequestMethod.POST }) applies middleware to all paths but only POST requests.

3. Step 3: Evaluate other options

   Use consumer.apply(LoggerMiddleware).forRoutes('*') and check req.method inside middleware applies middleware on all methods, not just POST. Apply middleware only on POST routes manually, no global option is manual, not global. Use app.useGlobalMiddleware(LoggerMiddleware) with method filter is invalid syntax.

4. Final Answer:

   Use consumer.apply(LoggerMiddleware).forRoutes({ path: '*', method: RequestMethod.POST }) -> Option C

5. Quick Check:

   forRoutes with method filter = selective global middleware [OK]

Hint: Use forRoutes({ path: '*', method: RequestMethod.POST }) for selective global [OK]

Common Mistakes:

• Applying middleware globally without method filter
• Trying non-existent app.useGlobalMiddleware method
• Manually applying middleware on each POST route