NodeJS

How to create Intercepter in nestjs

Introduction

In NestJS, interceptors serve functions such as modifying the incoming request, the outgoing response, logging, transformations, and caching. They share a lot of characteristics with middleware, although they are much more exact, such as transforming responses or adding more functionality.

Follow these instructions to make and implement interceptors in NestJS:

Step 1: Create an Interceptor

You may create the file directly or use the NestJS CLI to create the interceptor.

Type the following command into the CLI:

nest g interceptor logging

A new file called logging.interceptor.ts will now be generated.

Step 2: Implement the Interceptor Logic

For interceptors to work, interceptors need to implement the NestInterceptor interface that has an intercept() method. This method comes with two parameters: the context of the execution and a handler which lets one pass the request to the next function in the lifecycle.

Here is all the code for the logging interceptor, which logs the request and the response:

import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';import { Observable } from 'rxjs';import { tap } from 'rxjs/operators';@Injectable()export class LoggingInterceptor implements NestInterceptor { intercept(context: ExecutionContext, next: CallHandler): Observable<any> { const now = Date.now(); console.log('Before the handler...'); return next .handle() .pipe( tap(() => console.log(`After the handler... ${Date.now() - now}ms`)), ); }}

In this case, the intercept method measures time, and can therefore be used for performance monitoring.

Step 3: Apply the Interceptor

An interceptor can be applied at the method level, the controller level, or globally.

Globally: In main.ts:

import { LoggingInterceptor } from './logging.interceptor';import { AppModule } from './app.module';async function bootstrap() { const app = await NestFactory.create(AppModule); app.useGlobalInterceptors(new LoggingInterceptor()); await app.listen(3000);}bootstrap();

Controller Level: For all methods within a controller, you set the interceptor.

import { Controller, UseInterceptors, Get } from '@nestjs/common';import { LoggingInterceptor } from './logging.interceptor';@Controller('example')@UseInterceptors(LoggingInterceptor)export class ExampleController { @Get() getExample() { return 'This is an example'; }}

Method Level: Applying the interceptor to a particular route.

@Get('example')@UseInterceptors(LoggingInterceptor)getExample() { return 'This is an example';}