# Interceptors — NestJS

Source: https://www.geekswithgeeks.com/en/nestjs/nestjs-interceptors

> Add cross-cutting concerns: logging, response transformation, error handling.

## Write an interceptor

Interceptors wrap request/response using `NestInterceptor` and `Observable` streams.

```typescript
import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

@Injectable()
export class TransformInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      map(data => ({ data, timestamp: new Date() }))
    );
  }
}
```

## Apply interceptors

Use `@UseInterceptors()` on methods, classes, or globally.

```typescript
@UseInterceptors(TransformInterceptor)
@Get()
findAll() {
  return [{ id: 1, name: 'Ada' }];
}

// output: { data: [...], timestamp: Date }
```