Logging exceptions in NestJS - Mini Project: Build & Apply

Logging Exceptions in NestJS

📖 Scenario: You are building a simple NestJS application that needs to handle errors gracefully. When an error happens, you want to log the error message so you can understand what went wrong later.

🎯 Goal: Learn how to catch exceptions in a NestJS service and log the error messages using NestJS's built-in Logger.

📋 What You'll Learn

Create a service method that throws an error

Add a Logger instance to the service

Catch the error inside the method and log the error message

Print a message indicating the error was logged

💡 Why This Matters

🌍 Real World

Logging errors helps developers find and fix problems quickly in real applications.

💼 Career

Knowing how to log exceptions is essential for backend developers and DevOps engineers to maintain reliable services.

1

Create a service method that throws an error

Create a class called AppService with a method performTask that throws an error with the message 'Something went wrong!'.

NestJS

class AppService {
    // Your code here
}

Hint

Define a class with a method that uses throw new Error('Something went wrong!').

2

Add a Logger instance to the service

Inside the AppService class, add a private property called logger that is an instance of Logger from @nestjs/common. Initialize it with new Logger(AppService.name).

NestJS

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

class AppService {
    private logger = new Logger(AppService.name);

    performTask() {
        throw new Error('Something went wrong!');
    }
    // Your code here
}

Hint

Import Logger and create a private property logger initialized with new Logger(AppService.name).

3

Catch the error and log the error message

Modify the performTask method to use a try-catch block. Inside try, throw the error as before. Inside catch, use this.logger.error(error.message) to log the error message.

NestJS

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

class AppService {
    private logger = new Logger(AppService.name);

    performTask() {
        // Your code here
    }
}

Hint

Wrap the error throw in try and catch it to log the message with this.logger.error(error.message).

4

Print a message indicating the error was logged

After calling performTask() on an instance of AppService, print 'Error logged successfully' to the console.

NestJS

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

class AppService {
    private logger = new Logger(AppService.name);

    performTask() {
        try {
            throw new Error('Something went wrong!');
        } catch (error) {
            this.logger.error(error.message);
        }
    }
}

const appService = new AppService();
// Your code here

Hint

Create an instance of AppService, call performTask(), then print 'Error logged successfully'.

Practice

1. What is the main purpose of logging exceptions in a NestJS application?

easy

A. To increase the app's memory usage

B. To speed up the application runtime

C. To track and fix errors by recording error details

D. To prevent users from accessing the app

Solution

1. Step 1: Understand the role of exception logging

   Logging exceptions records error details like messages and stack traces to help developers find and fix bugs.

2. Step 2: Identify the correct purpose

   Among the options, only tracking and fixing errors matches the purpose of logging exceptions.

3. Final Answer:

   To track and fix errors by recording error details -> Option C

4. Quick Check:

   Logging exceptions = track and fix errors [OK]

Hint: Logging exceptions helps find bugs by saving error info [OK]

Common Mistakes:

• Thinking logging improves app speed
• Confusing logging with security features
• Believing logging increases memory unnecessarily

2. Which of the following is the correct way to log an error with NestJS Logger inside a catch block?

easy

A. Logger.error(error.message, error.stack, 'MyContext')

B. Logger.log(error.message, 'MyContext')

C. Logger.warn(error.stack, 'MyContext')

D. Logger.debug(error.message, error.stack)

Solution

1. Step 1: Recall Logger.error method signature

   Logger.error takes three arguments: message, stack trace, and optional context string.

2. Step 2: Match the correct syntax

   Logger.error(error.message, error.stack, 'MyContext') correctly uses Logger.error with message, stack, and context. Others use wrong methods or missing parameters.

3. Final Answer:

   Logger.error(error.message, error.stack, 'MyContext') -> Option A

4. Quick Check:

   Logger.error(message, stack, context) = Logger.error(error.message, error.stack, 'MyContext') [OK]

Hint: Use Logger.error with message, stack, and context [OK]

Common Mistakes:

• Using Logger.log or Logger.warn instead of Logger.error
• Omitting the stack trace argument
• Passing arguments in wrong order

3. Given this NestJS code snippet inside a service method:

try {
  throw new Error('Failed operation');
} catch (error) {
  Logger.error(error.message, error.stack, 'MyService');
}

What will be logged?

medium

A. An info log with stack trace only

B. A warning log with message 'Failed operation' only

C. No log will be created because throw stops execution

D. An error log with message 'Failed operation' and stack trace under 'MyService' context

Solution

1. Step 1: Understand the try-catch behavior

   The error is thrown and immediately caught by catch block, so execution continues inside catch.

2. Step 2: Analyze Logger.error call

   Logger.error logs an error message with stack trace and context 'MyService'. This creates an error-level log entry.

3. Final Answer:

   An error log with message 'Failed operation' and stack trace under 'MyService' context -> Option D

4. Quick Check:

   Throw caught and logged as error with message and stack [OK]

Hint: Catch errors to log message and stack with Logger.error [OK]

Common Mistakes:

• Thinking throw stops all logging
• Confusing log levels (warn vs error)
• Ignoring the context parameter

4. Identify the error in this NestJS exception logging code:

try {
  // some code
} catch (error) {
  Logger.error(error.stack, error.message, 'AppModule');
}

medium

A. Arguments to Logger.error are reversed; message should come first

B. Logger.error cannot be used inside catch blocks

C. Missing await keyword before Logger.error

D. Logger.error requires only one argument

Solution

1. Step 1: Check Logger.error argument order

   Logger.error expects (message, stack, context). Here, stack and message are swapped.

2. Step 2: Confirm correct usage

   Swapping arguments causes wrong log output or errors. Correct order is message first, then stack trace.

3. Final Answer:

   Arguments to Logger.error are reversed; message should come first -> Option A

4. Quick Check:

   Logger.error(message, stack, context) order matters [OK]

Hint: Logger.error needs message first, then stack, then context [OK]

Common Mistakes:

• Swapping message and stack arguments
• Thinking Logger.error is async needing await
• Assuming Logger.error takes only one argument

5. You want to log exceptions in a NestJS global exception filter to capture all errors with context 'GlobalFilter'. Which code snippet correctly implements this?

hard

A. catch(exception: any) {
Logger.log(exception.message, 'GlobalFilter');
}

B. catch(exception: any) {
Logger.error(exception.message, exception.stack, 'GlobalFilter');
}

C. catch(exception: any) {
Logger.warn(exception.stack, 'GlobalFilter');
}

D. catch(exception: any) {
Logger.debug(exception.message, exception.stack);
}

Solution

1. Step 1: Understand global exception filter logging

   Global filters catch all exceptions; logging must use Logger.error with message, stack, and context.

2. Step 2: Evaluate each option

   catch(exception: any) { Logger.error(exception.message, exception.stack, 'GlobalFilter'); } correctly calls Logger.error with message, stack, and 'GlobalFilter' context. Others use wrong log levels or miss stack/context.

3. Final Answer:

   catch(exception: any) { Logger.error(exception.message, exception.stack, 'GlobalFilter'); } -> Option B

4. Quick Check:

   Global filter logs errors with Logger.error and context [OK]

Hint: Use Logger.error with message, stack, and context in global filters [OK]

Common Mistakes:

• Using Logger.log or Logger.warn instead of Logger.error
• Omitting stack trace or context
• Using Logger.debug which is too low level