Log4js

Tips

• By default, log4js level for default category is set to OFF, thus not outputting any logs. To turn on logging, level must be set other than OFF.
• Levels: TRACE -> DEBUG -> INFO -> WARN -> ERROR -> FATAL

Glossary

Appenders

• Appenders serialise logs to some form of output.
• Like files, emails, data over the network.
• Frequently used appenders: stdout, stderr, file (stdout and stderr seems to be identical).
• Appenders do not specify level.

Categoris

• Categories groups appenders.
• Categories can be inherited by x.y syntax(which will inherit from x category).
• Categories specifies level.

Layout

• Defines structure of each line of code.
• See syntax.

Example

Simplest

var log4js = require("log4js")

var logger = log4js.getLogger() // returns default category with no param specified
// default category is stdout with level TRACE
logger.level = "debug" // default level is OFF - which means no logs at all.
logger.debug("Some debug messages")

File and stdout combined

const log4js = require("log4js")

log4js.configure({
  appenders: {
    out: { type: "stdout" },
    app: { type: "file", filename: "app.log", maxLogSize: 100 * 1024 * 1024, backups: 3 },
    // maxLogSize is in byte
    // Reaching max size will create a new file and rename old files to xx.log.1, xx.log.2, xx.log.3... The bigger number the older.
    // backups: number of old log iles to keep during rolling(excluding hot/current file)
  },
  categories: {
    default: { appenders: ["out"], level: "trace" }, // getLogger() gets default category
    special: { appenders: ["out", "app"], level: "debug" }
  }
})

const logger = log4js.getLogger('special')
// note that if file appender is defined, log4js will always create the file no matter if file appender is utilized.

logger.debug('Some debug information.')

Print error stack trace

logger.error(new Error(errorMessage).stack)