A simple logger to console or file

Usage no npm install needed!

<script type="module">
  import loggerr from 'https://cdn.skypack.dev/loggerr';



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A very simple logger.


  • Synchronous output (great for cli's, browser and tools)
  • Levels (built-in and customizable)
  • Formatting (built-in and customizable)
  • Loggerr is dependency free (formatters are not)
  • Always captures stack trace on error logs
  • Tiny filesize
  • The cli formatter 🚀

cli formatter example


$ npm install --save loggerr


const log = require('loggerr')
log.error(new Error('My error message'))
// Thu Apr 16 2015 22:05:27 GMT-0500 (CDT) [error] - {"msg":"Error: My error message\n<STACK TRACE>"}

log.info('Something happened', {
  foo: 'info about what happened'
// Thu Apr 16 2015 22:05:27 GMT-0500 (CDT) [info] - {"msg":"Something happened","foo":"info about what happened"}

Log Levels

Each log level can be directed to a different output stream or disabled entirely. The default levels are as follows:

  • emergency
  • alert
  • critical
  • error
  • warning (default)
  • notice
  • info
  • debug

Constants are available for setting and referencing the levels and their streams. These constants are the all uppercase version of the level. Here is an example of setting the log level:

const logger = new Loggerr({
  level: Loggerr.DEBUG

// Thu Apr 16 2015 22:05:27 GMT-0500 (CDT) [debug] - {"msg":"Foo"}

Customize Levels

You can fully customize the levels for your purposes. For example, here we implement pino compatible levels:

const log = new Loggerr({
  level: [ 'trace', 'debug', 'info', 'warn', 'error', 'fatal' ]

log.trace('Example trace log')

See the example of custom levels for cli output.

Log Formatting

Loggerr supports formatting via formatter functions. The default formatter outputs a timestamp, the log level and the messages formatted as json. But you can provide a custom formatter function with the formatter options. Formatter functions take three parameters: date, level, data. Say we want to output the log message with a color based on the level:

const Loggerr = require('loggerr')
const chalk = require('chalk')

const logger = new Loggerr({
  formatter: (date, level, data) => {
    var color
    switch (Loggerr.levels.indexOf(level)) {
      case Loggerr.EMERGENCY:
      case Loggerr.ALERT:
      case Loggerr.CRITICAL:
      case Loggerr.ERROR:
        color = chalk.red
      case Loggerr.WARNING:
      case Loggerr.NOTICE:
        color = chalk.yellow
      case Loggerr.INFO:
      case Loggerr.DEBUG:
        color = chalk.white
    return color(data.msg)

There are a few built-in in formatters:

  • default: Outputs date, level and json
  • cli: Outputs the message and json data, colorized and formatted
  • bunyan: Compatible format to bunyan
  • browser: Relies on console.log, so just returns the data

For these built-in formatters can specify the string name of the formatter for built-in formatters:

const log = new Loggerr({
  formatter: 'cli'

To use the cli formatter you can require it and pass the formatter options:

const log = new Loggerr({
  formatter: require('loggerr/formatters/cli')

Output Streams

You can output each level to it's own stream. The method is simple, just pass an array of streams corresponding to each level as the streams option. The simplest way is to just map over Loggerr.levels, this is how we set the defaults:

new Loggerr({
  streams: Loggerr.levels.map(function (level, i) {
    return i > Loggerr.WARNING ? process.stdin : process.stderr

The most useful reason to specify an output stream to to redirect logs to files. Here is an example of how to do that:

const logfile = fs.createWriteStream('./logs/stdout.log', {
  flags: 'a',
  encoding: 'utf8'

new Loggerr({
  streams: Loggerr.levels.map(() => logfile)