Level - a log level is the severity or priority of a log event (debug, info, etc). Whether an appender will see the event or not is determined by the category’s level. If this is less than or equal to the event’s level, it will be sent to the category’s appender(s).
Category - a label for grouping log events. This can be based on module (e.g. ‘auth’, ‘payment’, ‘http’), or anything you like. Log events with the same category will go to the same appenders. Log4js supports a hierarchy for categories, using dots to separate layers - for example, log events in the category ‘myapp.submodule’ will use the level for ‘myapp’ if none is defined for ‘myapp.submodule’, and also any appenders defined for ‘myapp’. (This behaviour can be disabled by setting inherit=false on the sub-category.) The category for log events is defined when you get a Logger from log4js (
Appender - appenders are responsible for output of log events. They may write events to files, send emails, store them in a database, or anything. Most appenders use layouts to serialise the events to strings for output.
Logger - this is your code’s main interface with log4js. A logger instance may have an optional category, defined when you create the instance. Loggers provide the
error, etc functions that create LogEvents and pass them on to appenders.
Layout - a function for converting a LogEvent into a string representation. Log4js comes with a few different implementations: basic, coloured, and a more configurable pattern based layout.
LogEvent - a log event has a timestamp, a level, and optional category, data, and context properties. When you call
logger.info('cheese value:', edam) the logger will create a log event with the timestamp of now, a level of INFO, a category that was chosen when the logger was created, and a data array with two values (the string ‘cheese value:’, and the object ‘edam’), along with any context data that was added to the logger.