Why interceptors add cross-cutting logic in NestJS - Visual Breakdown

Concept Flow - Why interceptors add cross-cutting logic

Request Received

↓

Interceptor Runs Before

↓

Controller Handles Request

↓

Interceptor Runs After

↓

Response Sent

Interceptors run code before and after the main request handler, letting you add shared logic around many requests.

Execution Sample

NestJS

async intercept(context, next) {
  console.log('Before');
  const result = await next.handle().toPromise();
  console.log('After');
  return result;
}

This interceptor logs messages before and after the controller handles the request.

Execution Table

Step	Action	Output/Effect
1	Request arrives, interceptor intercepts	Logs 'Before'
2	Interceptor calls next.handle() to continue	Controller processes request
3	Controller returns response	Response captured by interceptor
4	Interceptor logs 'After'	Logs 'After'
5	Interceptor returns response	Response sent to client

💡 Interceptor finishes after controller response is handled and returned

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
result	undefined	undefined	Promise pending	Resolved response data	Returned response data

Key Moments - 2 Insights

Why does the interceptor run code both before and after the controller?

Because interceptors wrap the controller call, they can add logic before calling next.handle() and after awaiting it, as shown in steps 1 and 4 of the execution_table.

What happens if the interceptor does not call next.handle()?

The controller never runs, so the request is blocked. This is clear in step 2 where next.handle() triggers controller execution.

Visual Quiz - 3 Questions

Look at the execution_table, what is logged first when a request arrives?

A 'After'

B 'Before'

C Controller response

D Nothing

💡 Hint

Check Step 1 in the execution_table where the interceptor logs 'Before'

At which step does the controller actually process the request?

A Step 1

B Step 4

C Step 2

D Step 5

💡 Hint

Step 2 shows the interceptor calling next.handle(), which triggers the controller

If the interceptor skipped logging 'After', which step would change?

A Step 4

B Step 1

C Step 3

D Step 5

💡 Hint

Step 4 is where 'After' is logged by the interceptor

Concept Snapshot

Interceptors wrap controller calls to add shared logic.
They run code before and after the controller.
Use next.handle() to continue to the controller.
Useful for logging, timing, error handling.
They add cross-cutting logic without changing controllers.

Full Transcript

Interceptors in NestJS add cross-cutting logic by running code before and after the main controller handles a request. When a request arrives, the interceptor first runs its 'before' code, then calls next.handle() to let the controller process the request. After the controller returns a response, the interceptor runs its 'after' code and finally returns the response to the client. This wrapping behavior allows interceptors to add shared features like logging or timing around many requests without changing each controller. The execution table shows the step-by-step flow: logging 'Before', calling the controller, receiving the response, logging 'After', and sending the response. Variables like the response result change from undefined to resolved data as the interceptor awaits the controller. Key points include that interceptors must call next.handle() to continue processing, and they can run code both before and after the controller. This pattern helps keep code clean and reusable.

Practice

1. What is the main reason to use interceptors in NestJS?
Interceptors help to add logic that applies to many parts of the app without repeating code. Why is this useful?

easy

A. They add shared logic before and after main code, like logging or timing.

B. They replace controllers to handle HTTP requests directly.

C. They store data permanently in the database.

D. They generate frontend UI components automatically.

Solution

1. Step 1: Understand the role of interceptors

   Interceptors run code before and after the main handler, allowing shared tasks like logging or error handling.

2. Step 2: Compare with other options

   Controllers handle requests, databases store data, and UI components are unrelated to interceptors.

3. Final Answer:

   They add shared logic before and after main code, like logging or timing. -> Option A

4. Quick Check:

   Interceptors add shared logic = A [OK]

Hint: Interceptors wrap main code to add shared tasks [OK]

Common Mistakes:

• Thinking interceptors handle HTTP requests directly
• Confusing interceptors with database or UI roles
• Assuming interceptors replace controllers

2. Which of the following is the correct way to implement an interceptor in NestJS?

easy

A. Implement the NestInterceptor interface and define the intercept() method.

B. Create a class with a handle() method and decorate it with @Controller().

C. Use @Injectable() on a service without any methods.

D. Write a function that returns a promise without any interface.

Solution

1. Step 1: Recall NestJS interceptor pattern

   Interceptors must implement the NestInterceptor interface and define the intercept() method.

2. Step 2: Check other options

   Controllers use @Controller() and handle() is not an interceptor method; services need methods; plain functions lack interface.

3. Final Answer:

   Implement the NestInterceptor interface and define the intercept() method. -> Option A

4. Quick Check:

   Interceptor interface + intercept() = B [OK]

Hint: Interceptors always implement NestInterceptor with intercept() [OK]

Common Mistakes:

• Confusing interceptors with controllers or services
• Missing the intercept() method
• Not using the NestInterceptor interface

3. Given this interceptor code snippet:

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'))
    );
  }
}

What will be printed to the console when a request is handled?

medium

A. Only After handler

B. Before handler, then After handler

C. Only Before handler

D. No output printed

Solution

1. Step 1: Analyze the intercept method flow

   The interceptor logs 'Before handler' immediately, then calls next.handle() which runs the main handler.

2. Step 2: Understand the tap operator

   The tap operator runs after the main handler completes, logging 'After handler'.

3. Final Answer:

   Before handler, then After handler -> Option B

4. Quick Check:

   Logs before and after handler = C [OK]

Hint: Logs before then after main handler due to tap() [OK]

Common Mistakes:

• Thinking only one log prints
• Confusing order of logs
• Ignoring the tap() operator effect

4. You wrote this interceptor but it does not log anything:

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

@Injectable()
export class SilentInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    console.log('Intercepted');
    next.handle();
  }
}

What is the problem?

medium

A. The import statements are incorrect.

B. The console.log statement is after next.handle(), so it never runs.

C. The class is missing the @Controller() decorator.

D. The intercept method does not return the observable from 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 flow.

2. Step 2: Identify missing return

   Here, next.handle() is called but its result is not returned, so the request does not proceed and logs may not appear.

3. Final Answer:

   The intercept method does not return the observable from next.handle(). -> Option D

4. Quick Check:

   Missing return next.handle() = D [OK]

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

Common Mistakes:

• Forgetting to return next.handle()
• Misplacing console.log after next.handle()
• Confusing decorators for interceptors

5. You want to measure the time taken by any request handler in your NestJS app. Which interceptor approach correctly adds this cross-cutting timing logic?

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

@Injectable()
export class TimingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    const start = Date.now();
    return next.handle().pipe(
      tap(() => {
        const elapsed = Date.now() - start;
        console.log(`Request took ${elapsed}ms`);
      })
    );
  }
}

hard

A. This interceptor logs time but does not call next.handle(), so no handler runs.

B. This interceptor measures time only before the handler, missing after completion.

C. This interceptor correctly measures time before and after the handler using tap().

D. This interceptor should use setTimeout instead of tap() to measure time.

Solution

1. Step 1: Understand timing measurement

   The interceptor records start time before calling next.handle() and uses tap() to run code after the handler completes.

2. Step 2: Confirm correct usage of RxJS tap()

   The tap() operator runs side effects after the observable emits, so it correctly logs elapsed time after the request finishes.

3. Final Answer:

   This interceptor correctly measures time before and after the handler using tap(). -> Option C

4. Quick Check:

   Timing with tap() after handler = A [OK]

Hint: Use tap() after next.handle() to measure elapsed time [OK]

Common Mistakes:

• Measuring time only before handler
• Not returning next.handle() observable
• Using setTimeout instead of RxJS operators