Log levels and when to use them in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding log levels in Express apps

Which log level is best suited for recording detailed information useful only during development and debugging in an Express app?

A error

B warn

C debug

D info

💡 Hint

Think about which level gives the most detailed messages for developers.

The debug level is used for detailed diagnostic information useful during development. Other levels like error or warn are for more serious issues or warnings.

❓ component_behavior

intermediate

Log output behavior with different levels

Given an Express app configured to log only 'warn' and above, which log messages will appear in the console?

Express

logger.error('Error happened');
logger.warn('Warning issued');
logger.info('Informational message');
logger.debug('Debug details');

A Only 'Error happened' appears

B All four messages appear

C Only 'Informational message' and 'Debug details' appear

D Only 'Error happened' and 'Warning issued' messages appear

💡 Hint

Remember that logging levels filter out messages below the set level.

When the log level is set to warn, only warn and error messages are shown. info and debug are ignored.

📝 Syntax

advanced

Correct usage of log levels in Express middleware

Which code snippet correctly logs an error message inside an Express error-handling middleware?

Express

app.use((err, req, res, next) => {
  // log error here
  next(err);
});

A logger.error(`Error: ${err.message}`);

B logger.warn('Error: ' + err.message);

C logger.info(`Error: ${err.message}`);

D logger.debug('Error: ' + err.message);

💡 Hint

Errors should be logged with the highest severity level.

Errors should be logged with error level to highlight serious problems. Using warn, info, or debug is less appropriate for errors.

🔧 Debug

advanced

Identifying incorrect log level usage

What is wrong with this Express logging code snippet? logger.info('User login failed due to invalid password');

A It should use 'warn' level because login failure is a warning, not an error

B It should use 'debug' level because it's only useful during development

C It should use 'error' level because login failure is a serious issue

D There is no problem; 'info' is the correct level

💡 Hint

Consider how serious a login failure is for the app's security.

A login failure due to invalid password is a warning about user input, not a system error. So warn is more appropriate than info or error.

❓ state_output

expert

Log output filtering with dynamic log level changes

An Express app starts with log level 'info'. During runtime, the log level changes to 'error'. Which log messages will be output before and after the change? Code snippet: logger.info('Server started'); logger.error('Database connection failed'); // log level changes to 'error' logger.warn('Cache miss'); logger.error('Unhandled exception');

A Before change: 'Server started', 'Database connection failed'; After change: 'Cache miss', 'Unhandled exception'

B Before change: 'Server started', 'Database connection failed'; After change: 'Unhandled exception' only

C Before change: 'Server started'; After change: 'Cache miss', 'Unhandled exception'

D Before change: 'Database connection failed'; After change: 'Cache miss' only

💡 Hint

Remember which levels are shown at 'info' and 'error' settings.

At 'info' level, info and error messages appear. After changing to 'error', only error messages appear, so warn is filtered out.

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