Timeout interceptor in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a NestJS TimeoutInterceptor times out?

Consider a NestJS TimeoutInterceptor that throws a timeout error if the request takes longer than 3 seconds. What will the client receive if the handler takes 5 seconds to respond?

NestJS

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

@Injectable()
export class TimeoutInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      timeout(3000),
      catchError(err => {
        if (err.name === 'TimeoutError') {
          return throwError(() => new RequestTimeoutException());
        }
        return throwError(() => err);
      }),
    );
  }
}

A The client receives an empty response with status 200.

B The client receives a 408 Request Timeout error.

C The client waits indefinitely until the handler finishes.

D The client receives a 500 Internal Server Error.

💡 Hint

Think about what the RequestTimeoutException does in NestJS.

The TimeoutInterceptor uses RxJS timeout operator to throw a TimeoutError if the handler takes longer than 3 seconds. This error is caught and transformed into a RequestTimeoutException, which NestJS translates into a 408 HTTP response.

📝 Syntax

intermediate

Identify the syntax error in this TimeoutInterceptor code

Which option contains a syntax error that prevents the TimeoutInterceptor from compiling?

NestJS

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

@Injectable()
export class TimeoutInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      timeout(5000)
    );
  }
}

A The intercept method is missing the return type Observable<any>.

B Missing import of catchError from 'rxjs' causes runtime error.

C timeout operator is used without parentheses: timeout instead of timeout().

D The class is missing the @Injectable decorator.

💡 Hint

Check how the timeout operator is called in RxJS.

The timeout operator must be called as a function with parentheses, e.g., timeout(5000). Using timeout without parentheses is a syntax error.

🔧 Debug

advanced

Why does the TimeoutInterceptor not trigger on slow requests?

A developer wrote this TimeoutInterceptor but it never triggers even when requests take longer than 10 seconds. What is the likely cause?

NestJS

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

@Injectable()
export class TimeoutInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    next.handle().pipe(
      timeout(10000),
      catchError(err => {
        if (err.name === 'TimeoutError') {
          return throwError(() => new RequestTimeoutException());
        }
        return throwError(() => err);
      }),
    );
    return next.handle();
  }
}

A The interceptor returns next.handle() without the pipe, so timeout is never applied.

B The timeout duration is too long to trigger in tests.

C The catchError operator is missing a return statement.

D The interceptor is missing the @Injectable decorator.

💡 Hint

Check what the intercept method returns.

The intercept method calls next.handle().pipe(...) but does not return it. Instead, it returns next.handle() again without the pipe, so the timeout operator is never applied.

❓ state_output

advanced

What is the output when TimeoutInterceptor wraps a fast handler?

Given this TimeoutInterceptor with 2 seconds timeout, and a handler that responds in 1 second, what will the client receive?

NestJS

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

@Injectable()
export class TimeoutInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      timeout(2000),
      catchError(err => {
        throw err;
      }),
    );
  }
}

// Handler code:
// async function handle() { await new Promise(r => setTimeout(r, 1000)); return { message: 'done' }; }

A The client receives a 500 Internal Server Error.

B The client receives a 408 Request Timeout error.

C The client receives an empty response immediately.

D The client receives { message: 'done' } after 1 second.

💡 Hint

The handler finishes before the timeout limit.

Since the handler responds within 1 second, which is less than the 2 seconds timeout, the TimeoutInterceptor lets the response pass through unchanged.

🧠 Conceptual

expert

How does NestJS TimeoutInterceptor integrate with RxJS to handle slow requests?

Which statement best describes how a NestJS TimeoutInterceptor uses RxJS operators to manage request timeouts?

A It uses the RxJS timeout operator to emit an error if the observable does not emit within the specified time, which is caught and transformed into a NestJS exception.

B It uses the RxJS delay operator to postpone the response and cancels the request if it takes too long.

C It uses the RxJS map operator to transform the response into a timeout error if it takes longer than the limit.

D It uses the RxJS retry operator to repeat the request until it finishes within the timeout period.

💡 Hint

Think about how timeout errors are generated and handled in RxJS.

The TimeoutInterceptor applies the RxJS timeout operator to the observable returned by the handler. If the observable does not emit a value within the specified time, timeout emits an error. This error is caught and converted into a NestJS RequestTimeoutException.