Logging interceptor in NestJS - Mini Project: Build & Apply

Logging interceptor

📖 Scenario: You are building a simple NestJS application. You want to add a logging interceptor that logs when a request starts and when it ends.

🎯 Goal: Create a logging interceptor in NestJS that logs messages before and after a request is handled.

📋 What You'll Learn

Create a logging interceptor class named LoggingInterceptor

Use the CallHandler and ExecutionContext from NestJS

Log a message before the request is handled

Log a message after the request is handled

Print the final log messages to the console

💡 Why This Matters

🌍 Real World

Logging interceptors help track requests in web applications, useful for debugging and monitoring.

💼 Career

Understanding interceptors and logging is important for backend developers working with NestJS or similar frameworks.

1

Create the LoggingInterceptor class skeleton

Create a class called LoggingInterceptor that implements NestInterceptor. Import NestInterceptor, ExecutionContext, and CallHandler from @nestjs/common. Define the intercept method with parameters context: ExecutionContext and next: CallHandler.

NestJS

import { NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';

// Your code here

Need a hint?

Start by importing the required interfaces and create the class with the intercept method signature.

2

Add pre-request logging

Inside the intercept method, add a console.log statement that prints 'Request started' before calling next.handle().

NestJS

import { NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
import { Observable } from 'rxjs';

export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable {
    // Add console.log for request start
    // Your code here
  }
}

Need a hint?

Use console.log before returning next.handle().

3

Add post-request logging using RxJS tap

Import tap from rxjs/operators. Modify the intercept method to return next.handle().pipe(tap(() => console.log('Request ended'))) so that 'Request ended' is logged after the request completes.

NestJS

import { NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
import { Observable } from 'rxjs';
import { tap } from 'rxjs/operators';

export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable {
    console.log('Request started');
    // Use tap to log after request ends
    // Your code here
  }
}

Need a hint?

Use pipe and tap to add a post-request log.

4

Print logs by creating an instance and calling intercept

Create an instance of LoggingInterceptor named interceptor. Call interceptor.intercept with dummy arguments: an empty object for context and an object with a handle method that returns of('response') from rxjs. Import of from rxjs. Print the logs by subscribing to the returned observable.

NestJS

import { NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
import { Observable, of } from 'rxjs';
import { tap } from 'rxjs/operators';

export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable {
    console.log('Request started');
    return next.handle().pipe(
      tap(() => console.log('Request ended'))
    );
  }
}

// Create instance and call intercept
// Your code here

Need a hint?

Create a dummy context and next with a handle method returning of('response'). Subscribe to the observable to see logs.