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.