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log2json2stdout

• What this is it?
• Installation
• Examples
  • Output example
  • Add new log levels
  • Use verbosity instead levels
  • Logging error objects
• LICENSE

What this is it?

This is simple logger for JavaScript programs.

• All logs going to stdout, because apps is not a place where logs should be processed.
• Output in JSON, because it's really easy to parse (except TTY, where output is human readable).
• Fields in JSON object in order: timestamp, level, message, ...rest.
• By default, only 2 levels exported: info & error. You can add own levels, but are we care about other levels in real life?

All this items makes this package perfect for applications in containers.

Installation

npm:

npm install https://github.com/fanatid/log2json2stdout

yarn:

yarn add https://github.com/fanatid/log2json2stdout

By default npm / yarn will install code from master branch. If you want specified version, just add some branch name / commit hash / tag and the end of URL. See Yarn add or npm install for details about installing package from git repo.

Examples

Output example

example.js:

const logger = require('log2json2stdout')

logger.info('hello')

const err = new Error('world')
logger.error({ message: err.message, stack: err.stack })

$ # human-readable output, because output to TTY
$ node example.js
2019-07-10T19:51:23.824Z info: hello
2019-07-10T19:51:23.826Z error: world
Error: world
    at Object.<anonymous> (/home/kirill/projects/log2json2stdout/example.js:5:13)
    at Module._compile (internal/modules/cjs/loader.js:776:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
    at Module.load (internal/modules/cjs/loader.js:643:32)
    at Function.Module._load (internal/modules/cjs/loader.js:556:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:839:10)
    at internal/main/run_main_module.js:17:11
$ # hack for JSON output (STDOUT is not considered as TTY with tee)
$ node example.js | tee
{"timestamp":"2019-07-10T19:54:07.539Z","level":"info","message":"hello"}
{"timestamp":"2019-07-10T19:54:07.540Z","level":"error","message":"world","stack":"Error: world\n    at Object.<anonymous> (/home/kirill/projects/log2json2stdout/example.js:5:13)\n    at Module._compile (internal/modules/cjs/loader.js:776:30)\n    at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)\n    at Module.load (internal/modules/cjs/loader.js:643:32)\n    at Function.Module._load (internal/modules/cjs/loader.js:556:12)\n    at Function.Module.runMain (internal/modules/cjs/loader.js:839:10)\n    at internal/main/run_main_module.js:17:11"}
$ # JSON output can be converted to human-readable with log2json2stdout2tty
$ node example.js | tee | log2json2stdout2tty
2019-07-10T19:51:23.824Z info: hello
2019-07-10T19:51:23.826Z error: world
Error: world
    at Object.<anonymous> (/home/kirill/projects/log2json2stdout/example.js:5:13)
    at Module._compile (internal/modules/cjs/loader.js:776:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
    at Module.load (internal/modules/cjs/loader.js:643:32)
    at Function.Module._load (internal/modules/cjs/loader.js:556:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:839:10)
    at internal/main/run_main_module.js:17:11

Add new log levels

If you still want use more levels than info / error, you can add new functional in your own project:

const logger = require('log2json2stdout')

logger.emerg = (obj) => logger('emerg', obj)
logger.alert = (obj) => logger('alert', obj)
logger.crit = (obj) => logger('crit', obj)
// logger.error = (obj) => logger('error', obj) <-- already exists
logger.warning = (obj) => logger('warning', obj)
logger.notice = (obj) => logger('notice', obj)
// logger.info = (obj) => logger('info', obj) <-- already exists
logger.debug = (obj) => logger('debug', obj)

and if you want add colors for levels in TTY, you should change ttyGetColoredLevel:

logger.ttyGetColoredLevel = (level) => {
  switch (level) {
    case 'emerg':
    case 'alert':
    case 'crit':
      return logger.chalk.red.bold(level)

    case 'error':
      return logger.chalk.red(level)

    case 'warning':
    case 'notice':
      return logger.chalk.yellow(level)

    case 'info':
      return logger.chalk.green(level)

    default: return level
  }
}

Use verbosity instead levels

In comprasion to more log levels (or to additional), we can use similar idea which used in debug. Logging will depends from CLI flags / ENV vars:

const logger = require('log2json2stdout')

const namespaces = Object.create(null)
logger.create = (namespace) => {
  if (!namespaces[namespace]) {
    function newLogger (...args) {
      newLogger.log(...args)
    }
    Object.assign(newLogger, logger)
    if (!process.env[`MYAPP_LOG_${namespace}`]) newLogger.log = () => {}

    namespaces[namespace] = newLogger
  }

  return namespaces[namespace]
}

// and now create logger for specified namespace
const loggerThingX = logger.create('THING_X')
loggerThingX.info('log info event from thing x')

So, now if we defined ENV MYAPP_LOG_THING_X we will see log message, if not defined there will no logs about THING_X.

In addition, if we want see how much time has passed from last message we can modify logObj function:

const prettyMs = require('pretty-ms')

function diffTime (time) {
  if (time === undefined) return process.hrtime()

  const diff = process.hrtime(time)
  return prettyMs(diff[0] * 1e3 + diff[1] / 1e6)
}


// logObj = newLogger.logObj
// let ts = null
// newLogger.logObj = (obj) => {
//   if (ts === null) ts = diffTime()
//   else obj.message = `${obj.message} +${diffTime(time)}`
//   logObj(obj)
// }

Logging error objects

Sometimes errors do not have nice messages and it would be good log them in little different way than just string.

logger.errorObj = (message, err) => {
  const obj = { message }

  for (const name of Object.getOwnPropertyNames(err)) {
    switch (name) {
      // do not rewrite message
      case 'message':
        obj.errmsg = err.message
        break

      default:
        obj[name] = err[name]
        break
    }
  }

  logger.error(obj)
}

// } catch (err) {
//   logger.errorObj(`new error at block X: ${err.message.toUpperCase()}`, err)
// }

LICENSE MIT