Logging interceptor in NestJS

Introduction

A logging interceptor helps you see what happens inside your app by recording requests and responses. It makes debugging and monitoring easier.

You want to track every request coming to your server.

You need to log how long each request takes to process.

You want to record errors or important events automatically.

You want to keep a history of API calls for auditing.

You want to debug issues by seeing input and output data.

Syntax

NestJS

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 request = context.switchToHttp().getRequest();
    const method = request.method;
    const url = request.url;
    const now = Date.now();

    return next
      .handle()
      .pipe(
        tap(() => console.log(`${method} ${url} took ${Date.now() - now}ms`)),
      );
  }
}

The intercept method runs before and after the request handler.

Use context.switchToHttp().getRequest() to access request details.

Examples

This example logs the HTTP method and URL when a request arrives.

NestJS

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 req = context.switchToHttp().getRequest();
    console.log(`Incoming request: ${req.method} ${req.url}`);
    return next.handle();
  }
}

This example logs how long the request took to process after it finishes.

NestJS

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();
    return next.handle().pipe(
      tap(() => console.log(`Request processed in ${Date.now() - now}ms`)),
    );
  }
}

This example combines logging the request method, URL, and processing time.

NestJS

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 req = context.switchToHttp().getRequest();
    const method = req.method;
    const url = req.url;
    const now = Date.now();

    return next.handle().pipe(
      tap(() => console.log(`${method} ${url} took ${Date.now() - now}ms`)),
    );
  }
}

Sample Program

This is a complete logging interceptor that logs the HTTP method, URL, and how long the request took to process.

NestJS

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 req = context.switchToHttp().getRequest();
    const method = req.method;
    const url = req.url;
    const now = Date.now();

    return next.handle().pipe(
      tap(() => console.log(`${method} ${url} took ${Date.now() - now}ms`)),
    );
  }
}

Important Notes

Make sure to register the interceptor globally or in the controller to activate it.

Logging too much data can slow down your app, so log only what you need.

You can customize the interceptor to log request body, headers, or response data if needed.

Summary

Logging interceptors help track requests and responses easily.

They run code before and after your request handlers.

Use them to measure request time and log useful info for debugging.

Practice

1. What is the main purpose of a LoggingInterceptor in a NestJS application?

easy

A. To manage user authentication and authorization

B. To handle database connections automatically

C. To log details about incoming requests and outgoing responses

D. To serve static files like images and CSS

Solution

1. Step 1: Understand the role of interceptors

   Interceptors in NestJS run code before and after request handlers, often used for logging or modifying requests/responses.

2. Step 2: Identify the purpose of LoggingInterceptor

   A LoggingInterceptor specifically tracks and logs request and response details for debugging and monitoring.

3. Final Answer:

   To log details about incoming requests and outgoing responses -> Option C

4. Quick Check:

   LoggingInterceptor = logs requests/responses [OK]

Hint: LoggingInterceptor logs request and response info [OK]

Common Mistakes:

• Confusing interceptors with middleware
• Thinking interceptors handle authentication
• Assuming interceptors serve static files

2. Which of the following is the correct way to implement the intercept method in a NestJS LoggingInterceptor?

easy

A. intercept(context, next) { return next.handle(); }

B. intercept(context, next) { next.handle(); }

C. intercept(context) { return next.handle(); }

D. intercept(next) { return context.handle(); }

Solution

1. Step 1: Recall the intercept method signature

   The intercept method receives two parameters: context and next. It must return next.handle() to continue the request lifecycle.

2. Step 2: Check each option for correct syntax

   intercept(context, next) { return next.handle(); } correctly uses both parameters and returns next.handle(). Options A, C, and D either miss parameters or do not return the observable.

3. Final Answer:

   intercept(context, next) { return next.handle(); } -> Option A

4. Quick Check:

   intercept(context, next) returns next.handle() [OK]

Hint: intercept always returns next.handle() with both parameters [OK]

Common Mistakes:

• Omitting return statement
• Missing one of the parameters
• Calling handle on wrong object

3. Given this LoggingInterceptor code snippet, what will be logged when a request is handled?

intercept(context, next) {
  const now = Date.now();
  return next.handle().pipe(
    tap(() => console.log(`Request took ${Date.now() - now}ms`))
  );
}

medium

A. Logs the time taken to process the request in milliseconds

B. Logs the request URL only

C. Logs the response body content

D. Logs an error message if the request fails

Solution

1. Step 1: Analyze the timing logic

   The code records the current time before the request is handled, then logs the difference after the response is sent.

2. Step 2: Understand the logged message

   The console.log outputs "Request took Xms" where X is the elapsed time in milliseconds.

3. Final Answer:

   Logs the time taken to process the request in milliseconds -> Option A

4. Quick Check:

   LoggingInterceptor measures request duration [OK]

Hint: Look for Date.now() difference to find elapsed time [OK]

Common Mistakes:

• Assuming it logs request URL or body
• Thinking it logs errors only
• Ignoring the timing calculation

4. Identify the error in this LoggingInterceptor code:

intercept(context, next) {
  const now = Date.now();
  next.handle().pipe(
    tap(() => console.log(`Request took ${Date.now() - now}ms`))
  );
}

medium

A. tap operator is not imported

B. Incorrect use of Date.now()

C. Missing context parameter

D. Missing return statement before next.handle()

Solution

1. Step 1: Check the intercept method return

   The intercept method must return the observable from next.handle() to continue the request lifecycle.

2. Step 2: Identify missing return

   The code calls next.handle().pipe(...) but does not return it, so the request will not proceed properly.

3. Final Answer:

   Missing return statement before next.handle() -> Option D

4. Quick Check:

   intercept must return next.handle() observable [OK]

Hint: Always return next.handle() in intercept method [OK]

Common Mistakes:

• Forgetting to return the observable
• Misusing Date.now() for timing
• Ignoring required parameters

5. You want to create a LoggingInterceptor that logs the request method, URL, and the time taken for each request. Which code snippet correctly implements this behavior?

hard

A. intercept(context, next) {
const req = context.getRequest();
const now = Date.now();
next.handle().pipe(
tap(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

B. intercept(context, next) {
const req = context.switchToHttp().getRequest();
const now = Date.now();
return next.handle().pipe(
tap(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

C. intercept(context, next) {
const req = context.switchToHttp().getRequest();
const now = Date.now();
return next.handle().pipe(
map(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

D. intercept(context, next) {
const req = context.switchToHttp().getRequest();
const now = Date.now();
return next.handle().pipe(
tap(() => console.error(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

Solution

1. Step 1: Access request object correctly

   Use context.switchToHttp().getRequest() to get the HTTP request object in NestJS interceptors.

2. Step 2: Use tap operator for side effects

   The tap operator is used to perform logging without altering the response stream. Using map or console.error is incorrect here.

3. Final Answer:

   intercept(context, next) { const req = context.switchToHttp().getRequest(); const now = Date.now(); return next.handle().pipe( tap(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`)) ); } -> Option B

4. Quick Check:

   Use switchToHttp(), tap, and return next.handle() [OK]

Hint: Use switchToHttp().getRequest() and tap for logging [OK]

Common Mistakes:

• Not returning next.handle()
• Using map instead of tap
• Using console.error instead of console.log
• Incorrect request object access