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Logger + multiplexer for JSON based logs, based on Cobalt ruby gem by ktlacaelel (


Cobalt is a simple logger multiplexer that works with a JSON based format. You can instantiate it with a set of loggers or add them later (see API reference below). When logging anything, Cobalt will attempt to generate an Object that conforms to the Cobalt Log Format Definition (if required) and passes it to every logger it has by calling their log method.

Example of instantiating a cobalt logger:

In node:


In the browser just require the files. Then:

this.logger = new Cobalt.Console({
  from : "Breezi Client",
  loggers : [ new Cobalt.Logger.JsConsole({
              formatter : Cobalt.Formatter.Token,
              formatterOpts : {
                formatString : "{{_from}}: {{message}}",
                ansiColor : true

This code will create an instance with a JsConsole logger that uses the Token formatter (See loggers and formatters below).

Cobalt works in a browser or inside node, so feel free to use cobalt all over! (Also, see the socket logger below for info on connecting cobalt loggers)

Quick API Reference

  • addLogger(logger): Adds a logger to the cobalt instance.
  • removeLogger(logger): Removes a logger from the cobalt instance.
  • buildLog(item, level=7): Generates a cobalt log object as if you had logged item (it will do this automatically when you log anything)
  • extendLog(object): Creates a dummy log object and extends it with object.
  • buildSeparator: Generates a cobalt log object that defines a separator
  • log, info, notice, warn, error: Generates a log object with the appropriate severity level and sends it to all loggers.
  • separator(): Generates a separator log object and sends it to all loggers.
  • space(lines): Logs an empty string lines times
  • indent(): Increases the indent level globally.
  • indent(callback): Increases the indent level for anything logged from inside the callback.
  • outdent()/outdent(callback): Same as indent, but decreases indent level.
  • color(): Changes the color globally. †
  • color(callback): Changes the color for anything logged from inside the callback. †
  • now(): Returns the current time in microseconds, using or process.hrtime() if available. If not, falls back to miliseconds.

† Cobalt doesn't really care about formatting or colors, but it allows you to set the _color property in the generated object. In the end, it's up to the formatter to decide if it will use this property. However, this maintains the old cobalt API and gives you flexibility in how you color your logs.


Cobalt doesn't depend on any particular logger, and the loggers it expects to receive is any object that responds to the log method. However, since it would pass a JSON object instead of a string, this may result in unexpected behavior for loggers that don't expect it. To ease the use of Cobalt with existing loggers, cobalt includes a couple of loggers that you can use out of the box.


This logger communicates the Javascript console present in web browsers or node with cobalt. It uses the logLevel to trigger the appropriate method (e.g. info vs warn vs error). You can also initialize it with a formatter, to convert the log object to a string:

 new Cobalt.Logger.JsConsole({
     formatter : Cobalt.Formatter.Token,
     formatterOpts : {
         formatString : "[{{_timestamp}}] {{message}} (@{{_from}})"

What this does is: it will trigger the method format on formatter passing the logObject and formatterOpts. This means that a formatter is any object that responds to format(logObject, formatterOpts). It expects a string to be returned.


This logger communicates a file via a writable stream, and is intended only for node. Like the JSConsole logger, you can also initialize it with a formatter to convert the log object to a string:

 new Cobalt.Logger.File({
     formatter : Cobalt.Formatter.Token,
     formatterOpts : {
         formatString : "[{{_timestamp}}] {{message}} (@{{_from}})"

What this does is: it will trigger the method format on formatter passing the logObject and formatterOpts. This means that a formatter is any object that responds to format(logObject, formatterOpts). It expects a string to be returned.


This logger sends the log object to a socket using Socket.IO. It does not format the output. To catch the log from the recipient, you have to listen for the log event, and from there you can pass it to another Cobalt instance or do whatever you want with it.

More Loggers?

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(logObject) is a valid logger:

// A valid, very minimalistic logger 
var simpleLogger = {
  log : function (logObject) {


Cobalt itself makes no assumptions about the output of the logger and just passes the object to every logger it has. However, it is clear that loggers may want to manipulate this object. As shown in the JsConsole, a formatter should respond to the format method and receive a logObject and an optsObject. However, as this is not a core part of Cobalt, this is only a recommendation (as this is the way the included JsConsole/File loggers do it) and it is up to the logger on how to transform the object it receives.


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

'[{{_timestamp}}][{{_logLevelString}}]{{_from}}: {{_message}}'

Where _timestamp is converted to ISO.

Example output:

cobalt.log("hello world");
// -> [2015-01-09T16:02:23.102Z][INFO] Generic Cobalt Logger : hello world


The Token formatter is a more advanced, but still fairly simple formatter. It takes a formatString and interpolates the properties of the object. By default it transforms anything inside double curly braces {{likeThis}}, however you can set a custom replaceRule.

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
  • separatorLength : How long to print separators. Defaults to 60.
  • isoDate : Whether or not to convert _timestamp to ISO date. Defaults to true.
  • separatorType : The string to use for the separator. Defaults to "="
  • ansiColor : Whether to use ANSI colors for output. Defaults to false. This options depends on colors


  • formatString: A string that defines the format of the output. It is a string with double curly braces denoting fields. For example: "[{{_timestamp}}] {{message}} (@{{_from}})" would attempt to extract the _timestamp, message and _from fields to create a string similar to this: "[124896126491.123] Testing the logger (@Client Application)" (defaults to "{{message}}")
  • ansiColor: A boolean value, when true will output the string in ANSI color depending on the severity level (defaults to false)

More Formatters?

As with loggers, cobalt itself does not worry about these things. However, if you wish to make a formatter that is exchangable with Token, you just need to create an object that responds to theformat(logObject, optionsObject) method:

// A valid, very minimalistic formatter 
var simpleFormatter = {
  format : function (logObject, options) {
    if (options.showDate) {
      return "[" + Date(logObject._timeStamp) + "" + logObject.message
    } else {
      return logObject.message;
logger.addLogger(new Cobalt.Logger.JsConsole({
  formatter: simpleFormatter,
  formatterOpts : {
    showDate : true

The Cobalt Log Format

The Cobalt Log (CoLog) format is a JSON based log format used with cobalt. It is partly inspired in Greylog's GELF format, but with very notorious differences. The CoLog requires a header with certain fields that allow cobalt and its pieces to handle it. All header fields are prefixed with an underscore. Other than those fields, you can put whatever you want in the object; It's up to the loggers to make sense of the structure and display it in a way that makes sense.

You can attempt to build this structure on your own, or let cobalt build it for you. Any object you pass for logging will be converted. However, if you build it on your own you have two options: The first one is use buildLog to create a log object for "item" as if you had logged "item" or you can use extendLog that will create a dummy log object and extends it with whatever object you pass to it.

Required Fields

  • _version : The version of cobalt this is designed to work with
  • _timestamp : A timestamp in microseconds.
  • _cobaltLog [true] : Cobalt will check for the _cobaltLog to decide if transformation will happen or not.

Optional Fields

  • _from: The sender of the log (Defaults to Generic Cobalt Logger)
  • _level: The level of the log (Defaults to 7)
  • _levelString: The string corresponding to the log level (e.g. 7 -> DEBUG, 3 -> ERROR, 0 -> CRIT)
  • _indentLevel: The indent level of the log
  • _color: The color of the log
  • _separator: If true, indicates that this is a separator and holds no valuable information.