Logging exceptions in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output when an exception is logged using NestJS Logger?

Consider this NestJS service method that throws an exception and logs it using the Logger service:

import { Logger } from '@nestjs/common';

const logger = new Logger('MyService');

function riskyOperation() {
  try {
    throw new Error('Something went wrong');
  } catch (error) {
    logger.error('Operation failed', error.stack);
  }
}

riskyOperation();

What will be the output in the console?

NestJS

import { Logger } from '@nestjs/common';

const logger = new Logger('MyService');

function riskyOperation() {
  try {
    throw new Error('Something went wrong');
  } catch (error) {
    logger.error('Operation failed', error.stack);
  }
}

riskyOperation();

A

[MyService] ERROR Operation failed
Error: Something went wrong
    at riskyOperation (...stack trace...)

B

[MyService] WARN Operation failed
Error: Something went wrong
    at riskyOperation (...stack trace...)

C

[MyService] LOG Operation failed
Error: Something went wrong
    at riskyOperation (...stack trace...)

D

[MyService] DEBUG Operation failed
Error: Something went wrong
    at riskyOperation (...stack trace...)

💡 Hint

Remember that logger.error logs messages with the ERROR level.

The Logger.error method logs messages with the ERROR level, so the output prefix is [MyService] ERROR. The message and the error stack are printed.

🧠 Conceptual

intermediate

Which NestJS built-in class is best for catching and logging all exceptions globally?

In NestJS, you want to catch all exceptions thrown anywhere in your app and log them centrally. Which built-in class should you extend or use?

A Interceptor

B ExceptionFilter

C Pipe

D Guard

💡 Hint

Think about the class designed to handle exceptions.

The ExceptionFilter class is designed to catch exceptions globally or locally and can be used to log errors centrally.

❓ Configuration

advanced

How to configure NestJS Logger to write exceptions to a file?

You want to log exceptions to a file instead of the console in a NestJS app. Which approach is correct?

A Create a custom LoggerService implementing LoggerService interface and override error() to write to a file.

B Set environment variable LOG_TO_FILE=true and NestJS will auto-log to a file.

C Use the built-in Logger with a config option <code>file: 'errors.log'</code> in main.ts.

D Call <code>Logger.error('message', { file: 'errors.log' })</code> to write to a file.

💡 Hint

Think about how to customize logging behavior in NestJS.

NestJS Logger does not support file logging out of the box. You must create a custom LoggerService that implements the LoggerService interface and override methods like error() to write logs to a file.

❓ Troubleshoot

advanced

Why does the stack trace not appear when logging exceptions with NestJS Logger?

You use logger.error('Failed', error.message) inside a catch block, but the stack trace is missing in logs. Why?

A The error object is lost after the catch block ends.

B Logger.error automatically hides stack traces for security.

C Because <code>error.message</code> is a string without stack info; you should pass <code>error.stack</code> instead.

D You must enable stack trace logging in NestJS config file.

💡 Hint

Check what property contains the stack trace.

The error.message is just the error text. The stack trace is in error.stack. Passing error.message to logger.error will not show the stack trace.

🔀 Workflow

expert

Order the steps to implement global exception logging in NestJS

Put these steps in the correct order to set up global exception logging in a NestJS app.

A 1,2,4,3

B 4,1,2,3

C 4,2,1,3

D 1,4,2,3

💡 Hint

Think about class creation, dependency injection, method override, then registration.

First create the filter class (1), then inject Logger in constructor (4), override catch() to log (2), and finally register globally (3).