Log Levels

Six levels from most verbose to most critical.

Level Hierarchy

Level	Priority	When to Use
trace	10	Detailed debugging (function entry/exit, variable values)
debug	20	Development debugging (request details, state changes)
info	30	Normal operations (server started, user logged in)
warn	40	Potential issues (deprecated API, high memory)
error	50	Errors (failed request, caught exception)
fatal	60	Critical failures (app crash, unrecoverable)

Level Filtering

Setting minLevel logs that level and all higher priority levels:

const log = createLogger('App', { minLevel: 'warn' });

log.trace('ignored');  // ❌ priority 10 < 40
log.debug('ignored');  // ❌ priority 20 < 40
log.info('ignored');   // ❌ priority 30 < 40
log.warn('logged');    // ✅ priority 40 >= 40
log.error('logged');   // ✅ priority 50 >= 40
log.fatal('logged');   // ✅ priority 60 >= 40

Quick Reference Table

minLevel	trace	debug	info	warn	error	fatal
'trace'	✅	✅	✅	✅	✅	✅
'debug'	❌	✅	✅	✅	✅	✅
'info'	❌	❌	✅	✅	✅	✅
'warn'	❌	❌	❌	✅	✅	✅
'error'	❌	❌	❌	❌	✅	✅
'fatal'	❌	❌	❌	❌	❌	✅

Runtime Level Control

Change log level at runtime:

const log = createLogger('App');

// Start with all levels
log.trace('Detailed info');  // ✅ Logged

// Later, reduce noise
log.setLevel('error');
log.trace('Detailed info');  // ❌ Now ignored
log.error('Problem!');       // ✅ Still logged

Check Before Expensive Operations

Avoid computing expensive data if it won't be logged:

if (log.isLevelEnabled('debug')) {
  // Only compute if debug is enabled
  const debugData = JSON.stringify(largeObject, null, 2);
  log.debug('Full state', { data: debugData });
}

Environment Defaults

Environment	Default minLevel
Development	trace (all logs)
Test	trace (all logs)
Production	info (no trace/debug)

Override:

// Force specific level regardless of environment
const log = createLogger('App', { minLevel: 'debug' });