Logging interceptor in NestJS - Step-by-Step Execution

Concept Flow - Logging interceptor

Request received

↓

Interceptor intercepts request

↓

Log request details

↓

Pass control to handler

↓

Handler processes request

↓

Interceptor intercepts response

↓

Log response details

↓

Send response back

The interceptor catches the request, logs info, lets the handler run, then logs the response before sending it back.

Execution Sample

NestJS

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

@Injectable()
export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    console.log('Before handler');
    return next.handle().pipe(tap(() => console.log('After handler')));
  }
}

This interceptor logs a message before and after the request handler runs.

Execution Table

Step	Action	Log Output	Handler Called	Response Sent
1	Request received by interceptor	'Before handler'	No	No
2	Pass control to handler	Nothing	Yes	No
3	Handler processes request	Nothing	Yes	No
4	Response intercepted	Nothing	Yes	No
5	Log after handler	'After handler'	Yes	No
6	Send response back	Nothing	Yes	Yes

💡 Response sent back after logging post-handler message

Variable Tracker

Variable	Start	After Step 1	After Step 5	Final
Log Output	''	'Before handler'	'After handler'	'After handler'
Handler Called	false	false	true	true
Response Sent	false	false	false	true

Key Moments - 2 Insights

Why do we see 'Before handler' logged before the handler runs?

Because the interceptor logs 'Before handler' immediately when intercept() is called, before passing control to the handler (see execution_table step 1).

Why is 'After handler' logged only after the handler finishes?

The interceptor uses RxJS tap operator on the handler's observable to log after the handler completes (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, what is logged at step 3?

A 'Before handler'

B Nothing

C 'After handler'

D 'Handler running'

💡 Hint

Check the 'Log Output' column at step 3 in the execution_table.

At which step is the handler actually called?

A Step 2

B Step 1

C Step 5

D Step 6

💡 Hint

Look at the 'Handler Called' column in execution_table to find when it changes to 'Yes'.

If we remove the tap operator, what changes in the logs?

A 'Before handler' will not log

B Both logs will still appear

C 'After handler' will not log

D No logs will appear

💡 Hint

The tap operator triggers 'After handler' log after handler finishes, see code in execution_sample.

Concept Snapshot

Logging Interceptor in NestJS:
- Implements NestInterceptor interface
- intercept() logs before passing control
- Uses next.handle() to call handler
- Uses RxJS tap() to log after handler completes
- Helps track request and response flow

Full Transcript

A logging interceptor in NestJS catches incoming requests before they reach the handler. It logs a message 'Before handler' immediately. Then it passes control to the handler using next.handle(). After the handler finishes processing, the interceptor logs 'After handler' using the RxJS tap operator on the observable returned by the handler. Finally, the response is sent back to the client. This flow helps developers see when requests start and finish processing.