# Middleware — NestJS: Building Scalable Node.js APIs

Source: https://www.skillbyai.com/en/nestjs/nestjs-middleware

> Run logic before route handlers with middleware classes and apply them to chosen routes in a module's configure method.

## Write middleware

Middleware is a function or class that runs before/after a route handler.

```typescript
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(`${req.method} ${req.path}`);
    next();
  }
}
```

## Apply middleware

Apply in `configure()` method of a module, specifying routes to intercept.

```typescript
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer
      .apply(LoggerMiddleware)
      .forRoutes('users'); // or use { path: 'users', method: RequestMethod.GET }
  }
}
```

## Functional middleware

A middleware is a plain function of `(req, res, next)`.

- Call `next()` or the request hangs.
- `consumer.apply(...).forRoutes('users')` attaches it to one route prefix.

```ts
import { Module, NestModule, MiddlewareConsumer } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

function logger(req: Request, _res: Response, next: NextFunction) {
  console.log(`${req.method} ${req.url}`);
  next();
}

@Module({})
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer.apply(logger).forRoutes('users');
  }
}
```

Quick check

**Quiz:** In the request lifecycle, which runs first?

- [ ] Guards
- [ ] Pipes
- [x] Middleware

*Answer:* Middleware. Order is middleware, guards, interceptors (before), pipes, handler.