Dependencyless log multiplexer for Javascript
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Cologne is a logger multiplexer that uses a JSON log format inspired in gelf. It can be instantiated with several loggers, or they can be changed after the fact.


Install from npm

$ npm install --save cologne

Create an instance

const { Cologne, Loggers, Formatters } = require('cologne');

const co = new Cologne({
  from: 'Special Worker Logger',
  loggers: [
    new Loggers.Console({
      formatter: new Formatters.Token({
        formatString: '[{{_timestamp}}]{{_from}}: {{message}}'

This example would create a cologne instance with a console logger that uses a Token formatter. (More on loggers and formatters below.);

#Quick API Reference

  • addLogger(logger): Adds a logger to the cologne instance.
  • removeLogger(logger): Removes a logger from the cologne instance.
  • buildLog(item, level, [meta]): Generates a cologne log object as if you had logged an item (it will do this automatically when you log anything.) level defaults to 6. You can optionally send it an object to extend the object with.
  • log, info, notice, warn, error: Generates a log object with the appropriate severity level and sends it to all loggers.


Cologne loggers are any object that responds to the #log() method. This methoud should be able to receive any number of arguments and log them independently. Similar to how you can send multiple arguments to the browser console.

#log() will receive any number of Cologne Log Objects. For a detailed reference of this format, see further below.

Cologne includes two loggers out of the box:

  • Loggers.Console logs to the JS console
  • Loggers.File appends to a file


This logger communicates with the Javascript console. It uses the log level to trigger the appropriate method, so error logs would go to stderr as expected when calling console.error.

This logger can be sent a formatter, which is an object that responds to the #format() method: it should receive a cologne log object and respond with a string.

new Loggers.Console({
  formatter : new Formatters.Token({
    formatString: '[{{_timestamp}}]{{_from}}: {{message}}'


This logger opens a writable stream to a file, to which it will append everything. Like the Console logger it supports a formatter property that will respond to the #format() method.

It MUST include a file property on initialization, otherwise it will throw an exception.

new Loggers.File({
  file: '/var/log/server_log.log',
  formatter : new Formatters.Token({
    formatString: '[{{_ansi:_level}}{{_timestamp}}{{_ansi:reset}}]{{_from}}: {{message}}'

#More Loggers?

We're working on a socket logger. It's separate so you don't have to install the socket dependencies if you don't want to.

You can build your own logger easily for any method of transport you find necessary (e.g. mail, database, twitter, etc). Any object that responds to #log() is a valid logger:

// A valid, very minimalistic logger
const simpleLogger = {
  log: function(...logs) {

    for (const log of logs) {

  _doSomeMagic: function(log) {

    console.log(log + "... but magical!");



Cologne doesn't need formatters to work, and in fact they're optional in the included loggers. But if you would like to make your logs prettier, then you can use one of the included formatters or roll your own.

Formatters are objects that respond to the #format() method. It will receive a single cologne log object (see fields it includes below), and it should return a string.

We include some formatters so you can get running real quicklike:

  • Formatters.Simple a simple predefined formatter
  • Formatters.Token a formatter that lets you define format strings that it will use to build your final log.


This is the lazy formatter, it just outputs the string in the following format:

'[{{_timestamp}}][{{_levelString}}]{{_from}}: {{message}}'

Where _timestamp is converted to ISO.

#Accepted Options
  • colorize : whether or not to add color. False by default.

By default we don't colorize the output, but if you enable the flag this formatter will add a bit of color in the level string. Red for error, crit, alert, and emerg; yellow for warn; blue for info; green for debug; and white for everything else.

new Formatters.Simple({
  colorize: true

#Example Output

co.log("hello world");
// -> [2016-01-21T05:50:36.505Z][INFO] Server Logger: hello world


The token formatter lets you build strings with simple tokens. When instantiating, you can specify a formatString to interpolate properties from the logObject. The default version looks for tokens inside double curly braces like {{message}} or {{_level}}. If you don't like it, you can specify your own.

#Accepted Options
  • formatString : The string used to replace. Defaults to "{{message}}"
  • replaceRule : The regex rule to use for replacement of tokens in the formatString. Defaults to /{{(.*?)}}/g
  • isoDate : Whether or not to convert _timestamp to ISO date. Defaults to true. Otherwise it'll use the raw timestamp.
new Formatters.Token({
  formatString: '[{{_timestamp}}]{{_from}}: {{message}}'
#ANSI tokens

If you want to add color to your logs, you can use the special _ansi token. It has several options which you can call like {{_ansi:red}} and {{_ansi:reset}}. Here's a list of all the ansi stuff you can use:

  • bold: makes text bold
  • italics: makes text italics
  • underline: makes text underlined
  • inverse: inverts foreground and background
  • strikethrough: strikethrough text
  • bold_off, italics_off, underline_off, inverse_off, and strikethrough_off: turn off the specified effect.
  • black, red, green, yellow, blue, magenta, cyan, white, and default: change the foreground color of your text.
  • black_bg, red_bg, green_bg, yellow_bg, blue_bg, magenta_bg, cyan_bg, white_bg, and default_bg: change the background color of your text.
  • reset: makes everything normal again.
  • _level: this is a special code that will set a color depending on the level of the log: debug gets green, info and notice blue, warn is yellow, and anything worse is red.

#More Formatters?

You can create your own formatters by creating an object that responds to the #format() method, knows how to handle cologne log objects and returns a string.

Here's an example of a logger that surrounds a log with sparkles:

const sparkleFormatter = {
  format: function(logObject) {

    return `✨${logObject.message}✨`;

logger.addLogger(new Loggers.Console({
  formatter: sparkleFormatter

#The Cologne Log Format

The cologne log format is a JSON based log format, based on the cobalt log format, which is in turn based on Graylog's GELF. However, where GELF treats all internal fields without a prefix, and all user fields with a prefix, we do it backwards so it's easier to extend the object with metadata from existing objects.

You could try to build it on your own, but you can use #buildLog() to build it without logging.


  • _timestamp : A bigint timestamp in nanoseconds
  • _cologneLog : This is how we know if the log is already formatted and ready to go. This field is a string containing the version of cologne log format it's using. It's 2.0.0 right now.
  • _from: The sender of the log (Defaults to Generic Cologne Logger)
  • _level: The level of the log (Defaults to 6)
  • _levelString: The string corresponding to the log level (e.g. 7 -> debug, 3 -> error, 0 -> emerg)

#A word on Log Levels

The log levels in cologne correspond to the syslog levels, and the levelStrings correspond to the priority keywords:

  • 0 -> emerg
  • 1 -> alert
  • 2 -> crit
  • 3 -> error
  • 4 -> warning
  • 5 -> notice
  • 6 -> info
  • 7 -> debug

This is useful when deciding how to log. You could even have a logger filter out unnecessary levels (eg. If you have a reporting logger that only reports error or worse.)

#Further Improvements

  • Improve the API for buildLog
  • More loggers & formatters (will not be distributed in core cologne)
  • Improve tests