Log levels and when to use them in Express

Introduction

Log levels help organize messages by importance. They make it easier to find problems and understand what your app is doing.

When you want to record normal app actions like user visits or data saved.

When you need to warn about something unusual but not breaking the app.

When an error happens that stops part of your app from working.

When debugging to see detailed info about how your app runs.

When tracking security or critical failures that need immediate attention.

Syntax

Express

logger.info('User logged in')
logger.warn('Disk space low')
logger.error('Database connection failed')
logger.debug('User data: %o', userData)

Replace level with the actual log level like info, warn, error, or debug.

Use placeholders like %o to insert objects or variables in logs.

Examples

Use info for general messages about app status.

Express

console.info('Server started on port 3000')

Use warn to alert about potential issues.

Express

console.warn('Memory usage is high')

Use error when something goes wrong.

Express

console.error('Failed to load user profile')

Use debug for detailed info useful during development.

Express

console.debug('Request headers: %o', req.headers)

Sample Program

This Express app uses different log levels to show how to log info, warnings, and errors. The morgan middleware logs HTTP requests automatically.

Express

import express from 'express';
import morgan from 'morgan';

const app = express();

// Use morgan middleware for logging HTTP requests
app.use(morgan('combined'));

app.get('/', (req, res) => {
  console.info('Info: Home page accessed');
  res.send('Hello, world!');
});

app.get('/warn', (req, res) => {
  console.warn('Warning: /warn route was called');
  res.send('This is a warning example');
});

app.get('/error', (req, res) => {
  console.error('Error: Something went wrong on /error');
  res.status(500).send('Error occurred');
});

app.listen(3000, () => {
  console.info('Server running on http://localhost:3000');
});

Important Notes

Use info for normal operations to keep track of app flow.

Use warn to highlight things that might cause problems later.

Use error to log serious issues that need fixing.

Use debug only during development to avoid cluttering logs in production.

Summary

Log levels help you organize messages by importance.

Use info, warn, error, and debug for different situations.

Good logging makes it easier to find and fix problems in your app.

Practice

1. Which log level in Express is best used to record normal operation messages that confirm things are working as expected?

easy

A. info

B. error

C. debug

D. warn

Solution

1. Step 1: Understand the purpose of log levels

   Log levels help separate messages by importance. 'info' is for normal operations.

2. Step 2: Match log level to message type

   'info' is used to confirm the app is running fine, unlike 'error' or 'warn' which indicate problems.

3. Final Answer:

   info -> Option A

4. Quick Check:

   Normal operation messages = info [OK]

Hint: Info logs show normal app activity clearly [OK]

Common Mistakes:

• Confusing 'info' with 'error'
• Using 'debug' for normal messages
• Thinking 'warn' is for normal info

2. Which of the following is the correct way to log a warning message using Express's console object?

easy

A. console.info('This is a warning')

B. console.error('This is a warning')

C. console.debug('This is a warning')

D. console.warn('This is a warning')

Solution

1. Step 1: Identify the correct console method for warnings

   Express uses standard console methods; 'warn' is for warnings.

2. Step 2: Match method to message type

   console.warn() logs warning messages, unlike console.error() which logs errors.

3. Final Answer:

   console.warn('This is a warning') -> Option D

4. Quick Check:

   Warning messages use console.warn() [OK]

Hint: Use console.warn() for warnings, not error or info [OK]

Common Mistakes:

• Using console.error for warnings
• Using console.info for warnings
• Using console.debug for warnings

3. What will be the output of this Express logging code snippet?

console.info('Server started');
console.warn('Memory usage high');
console.error('Failed to connect to DB');
console.debug('User ID: 1234');

medium

A. Only 'Failed to connect to DB' will appear

B. Only 'Server started' and 'Memory usage high' will appear

C. All four messages will appear in the console

D. Only 'User ID: 1234' will appear

Solution

1. Step 1: Understand default console behavior in Express

   By default, all console methods (info, warn, error, debug) print messages to the console.

2. Step 2: Analyze the code snippet output

   All four console calls will output their messages unless filtered by a logging library, which is not shown here.

3. Final Answer:

   All four messages will appear in the console -> Option C

4. Quick Check:

   Default console logs all levels [OK]

Hint: Default console logs all messages unless filtered [OK]

Common Mistakes:

• Thinking debug logs are hidden by default
• Assuming error logs suppress others
• Confusing info and warn visibility

4. You want to log an error in Express but accidentally write console.warn('Error occurred'). What is the main issue with this?

medium

A. The code will cause a syntax error

B. The message will be logged as a warning, not an error

C. The message will not appear in the console

D. The app will crash

Solution

1. Step 1: Identify the difference between warn and error logs

   console.warn logs warnings; console.error logs errors. They differ in severity.

2. Step 2: Understand the impact of using warn instead of error

   Using console.warn will log the message but mark it as a warning, not an error, which may mislead debugging.

3. Final Answer:

   The message will be logged as a warning, not an error -> Option B

4. Quick Check:

   Wrong log level changes message severity [OK]

Hint: Use console.error for errors, not console.warn [OK]

Common Mistakes:

• Thinking warn causes errors
• Expecting syntax error
• Assuming message won't show

5. In an Express app, you want to log detailed information only when debugging is enabled, but always log warnings and errors. Which approach correctly uses log levels to achieve this?

hard

A. Use console.debug for detailed info, console.warn for warnings, and console.error for errors; enable debug logs conditionally.

B. Use console.info for all logs and filter messages manually.

C. Use console.error for all messages to ensure visibility.

D. Use console.warn for detailed info and console.debug for errors.

Solution

1. Step 1: Assign log levels by message importance

   Detailed info fits debug, warnings fit warn, and errors fit error.

2. Step 2: Enable debug logs conditionally

   Debug logs can be turned on or off based on environment or settings, while warnings and errors always log.

3. Final Answer:

   Use console.debug for detailed info, console.warn for warnings, and console.error for errors; enable debug logs conditionally. -> Option A

4. Quick Check:

   Conditional debug, always warn/error = correct logging [OK]

Hint: Debug logs conditional; warn and error always on [OK]

Common Mistakes:

• Logging all as info
• Using error for all messages
• Mixing warn and debug levels incorrectly