• Incerceptors
• common/interceptors/transform.interceptor.ts

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) => ({
        success: true,
        data,
        timestamp: new Date().toISOString(),
      })),
    );
  }
}

• Now Apply it Globally on On All Routes

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { TransformInterceptor } from './common/interceptors/transform.interceptor';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalInterceptors(new TransformInterceptor());
  await app.listen(process.env.PORT ?? 3000);
}
bootstrap();

Output - add timestamps to response

Concept -

An Interceptor wraps around the controller, It runs before and after conroller ( so it can see request and response)

Request → Middleware → Guard → Interceptor(before) → Contoller → Interceptor(after) → response

• used for response shaping, Timing and Logging, Caching and Transforming Output etc ….
• use ful for consitency response shapes

now lets work on before usage - Caching

• Do the Work Once → save the result for a while → hands out the saved copy instead of redoing it
• tll - how long an entry lives
• In Memory for Learning and Redis in production