Middleware & Filters

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In the NestJS request lifecycle, where does a middleware function run?

javascript

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): void {
    console.log(`${req.method} ${req.originalUrl}`);
    next(); // hand control to the next middleware / route handler
  }
}

ABefore the route handler, with access to the request and response objects

BAfter the route handler returns its response

COnly inside the database layer, wrapping Prisma queries

DExclusively during application bootstrap, never per-request

Which method must a class implement to satisfy the NestMiddleware interface?

javascript

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

@Injectable()
export class AuthHeaderMiddleware implements NestMiddleware {
  use(req: Request, _res: Response, next: NextFunction): void {
    req.headers['x-traced'] = 'true';
    next();
  }
}

Ahandle(req, res)

Bintercept(context, next)

Cuse(req, res, next)

Dtransform(value, metadata)

How is class-based middleware registered in NestJS?

javascript

import { Module, NestModule, MiddlewareConsumer } from '@nestjs/common';
import { LoggerMiddleware } from './logger.middleware';

@Module({})
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer): void {
    consumer.apply(LoggerMiddleware).forRoutes('*');
  }
}

ABy adding it to the providers array of the module

BBy passing it to @Module({ middleware: [...] })

CBy decorating the controller with @Middleware()

DBy implementing NestModule and applying it inside the configure(consumer) method

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