typescript 39 lines · 5 steps

How a NestJS logging interceptor works

A NestJS interceptor wraps every request to log its method, URL, and timing on both success and failure.

Explained by highlit
NestJS Intermediate interceptors rxjs logging middleware observables
1import {
2 Injectable,
3 NestInterceptor,
4 ExecutionContext,
5 CallHandler,
6 Logger,
7} from '@nestjs/common';
8import { Observable } from 'rxjs';
9import { tap, catchError } from 'rxjs/operators';
10import { throwError } from 'rxjs';
11import { Request } from 'express';
12 
13@Injectable()
14export class LoggingInterceptor implements NestInterceptor {
15 private readonly logger = new Logger(LoggingInterceptor.name);
16 
17 intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
18 const request = context.switchToHttp().getRequest<Request>();
19 const { method, url } = request;
20 const handler = context.getHandler().name;
21 const now = Date.now();
22 
23 this.logger.log(`--> ${method} ${url} (${handler})`);
24 
25 return next.handle().pipe(
26 tap(() => {
27 const elapsed = Date.now() - now;
28 this.logger.log(`<-- ${method} ${url} ${elapsed}ms`);
29 }),
30 catchError((error) => {
31 const elapsed = Date.now() - now;
32 this.logger.error(
33 `<-- ${method} ${url} ${elapsed}ms FAILED: ${error.message}`,
34 );
35 return throwError(() => error);
36 }),
37 );
38 }
39}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Interceptors sit around handler execution, letting you act before and after the response without touching the route logic.
  2. 2RxJS operators like tap and catchError let you observe the response stream for both success and error paths.
  3. 3Capturing a timestamp before next.handle() lets you measure elapsed time once the observable settles.

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