Very fast and simplistic nodejs logger that logs to the console.





GitHub Stars



Last Commit

3yrs ago






Size (min+gzip)




Type Definitions






License NPM Package NPM Downloads Build Status Test Coverage Code Climate Dependency Status devDependency Status

Stories in Ready

Author: Kurt Pattyn.

Karl is a lightning fast[1] asynchronous logging library with both structured and text based output. It follows the twelve-factor app methodology by only logging to the console. Karl has no external dependencies (except for its tests).

[1]: Benchmarking (benchmarks/benchmark.js) revealed that Karl was around 10 times faster than Bunyan (both with location information disabled; otherwise all defaults enabled), and that it was around 5 times faster with location information enabled.
Karl is also about 5 times faster than directly logging to the console with location information disabled, and about 50% faster with location information enabled.
All tests were conducted on node v0.12.0 running on a MacBook Pro 17" Retina with a Quad-core Core i7 processor with OS X Yosemite.


Karl wants to be a better console logger by also providing source information (filename, functionname and linenumber) and timestamps and by providing structured logging (JSON format) besides text based logging.
Structured logging is ideal when it needs to be consumed by external logging services like Loggly.
By default Karl redirects the console output to go through its own logger. The net result is console output decorated with source information and optional coloring.

Karl is different from other popular logging modules like Bunyan and Winston in that it does not provide routing and filtering of log messages: all messages go always to stdout, from debug() to fatal() messages. We believe that routing and filtering should be done by a log management tool like Logstash, Logplex, Fluentd, and so on and not by the application itself.
This is certainly the case when we think microservices and Docker based applications. See also: The Twelve-Factor App.


$ npm install karl


$ npm install karl --production

for a production only installation (no tests, documentation, ...).

Supported Node Versions

Karl supports Node versions 4.7.1 and later.
It is only tested on LTS versions.


  var karl = require('karl');

  karl.trace("Are you spying on me?");
  karl.debug("Everything looks fine.");
  karl.info("Need some more info huh?");
  karl.warn("Didn't you forget something?");
  karl.error("This definitely went bezerk.");
  karl.fatal("This is the end, my friend!");


{"timestamp":"2015-08-02T18:02:39.456Z","level":"TRACE","hostName":"<hidden>","process":{"name":"karltest","pid":26693},"message":"Are you spying on me?","fileName":"karltest.js","lineNumber":40,"functionName":"<anonymous>"}
{"timestamp":"2015-08-02T18:02:39.456Z","level":"DEBUG","hostName":"<hidden>","process":{"name":"karltest","pid":26693},"message":"Everything looks fine.","fileName":"karltest.js","lineNumber":41,"functionName":"<anonymous>"}
{"timestamp":"2015-08-02T18:02:39.456Z","level":"INFO","hostName":"<hidden>","process":{"name":"karltest","pid":26693},"message":"Need some more info huh?","fileName":"karltest.js","lineNumber":42,"functionName":"<anonymous>"}
{"timestamp":"2015-08-02T18:02:39.456Z","level":"WARN","hostName":"<hidden>","process":{"name":"karltest","pid":26693},"message":"Didn't you forget something?","fileName":"karltest.js","lineNumber":43,"functionName":"<anonymous>"}
{"timestamp":"2015-08-02T18:02:39.456Z","level":"ERROR","hostName":"<hidden>","process":{"name":"karltest","pid":26693},"message":"This definitely went bezerk.","fileName":"karltest.js","lineNumber":44,"functionName":"<anonymous>"}
{"timestamp":"2015-08-02T18:02:39.456Z","level":"FATAL","hostName":"<hidden>","process":{"name":"karltest","pid":26693},"message":"This is the end, my friend!","fileName":"karltest.js","lineNumber":45,"functionName":"<anonymous>"}

All log messages are formatted using util.format(). Hence the following works also:

  var someObject = {
    a: 1,
    b: {
      c: "Owkay"
  karl.info("someObject", someObject);
  karl.info("You have %d options.", 3);


{"timestamp":"2015-08-02T17:56:32.186Z","level":"INFO","hostName":"<hidden>","process":{"name":"karltest","pid":26603},"message":"someObject { a: 1, b: { c: 'Owkay' } }","fileName":"karltest.js","lineNumber":48,"functionName":"<anonymous>"}
{"timestamp":"2015-08-02T17:58:41.671Z","level":"INFO","hostName":"<hidden>","process":{"name":"karltest","pid":26632},"message":"You have 3 options.","fileName":"karltest.js","lineNumber":48,"functionName":"<anonymous>"}

Customize Formatting

The output above is not easy to read for humans.

Karl can be instructed to output in plain text as well, by setting the json option to false.

  json: false
karl.info("I like reading log files in plain English.");


[INFO] <hostname-hidden> 2015-08-02T18:17:16.738Z - karltest[<anonymous>@karltest.js(43)]: I like reading log files in plain English.

To make the log output even more human digestable, the output can be colored by setting the colorize option to true. The colorize options will output error and fatal messages in red, warning messages in yellow and all other messages in the current text color of the terminal.

  json: false,
  colorize: true
karl.error("I like reading log files in red.");
karl.warning("I like reading log files in yellow.");

Output cannot be shown in color as GitHub markdown does not support colored text.

Note: If output will be processed by an external log management tool, it is advized to turn coloring off.

Customize Information

By default, Karl logs a timestamp, the log level, the hostname, the location from where the log method was called along with the message itself. Fetching location information is an expensive operation, making the logging around 4 times slower (20 microseconds vs 5 microseconds per call on my MacBook). Karl provides the includeLocationInformation option to turn location information on or off.

karl.setOptions({ includeLocationInformation: false }); //turns location information off
karl.setOptions({ includeLocationInformation: true });  //turns location information on (default)

Location information is included by default and must be disabled explicitly.

Console Redirection

By default Karl redirects all console output to its own logger.
As a consequence, all console messages are decorated with location information and a timestamp.
Karl also adds a debug() and trace() method to the console.

console.info("Hi there!") is exactly the same as karl.info("Hi there!"). The console.log() method is redirected to karl.info().

To disable this redirection, set the redirectConsole option to false.

karl.setOptions({ redirectConsole: false });

Log Enrichtment

Applications can add extra information to a message at the moment it is logged.
One example usage is adding request information to the log messages in an express application.

  enrich: addRequestInformation,
  json:   true

function addRequestInformation(msg) {
  //fetch request information from somewhere, e.g. by using continuation local storage
  const request = context.get("request");
  msg.headers = request.headers;

app.use((req, res, next) => {
  console.log("Entering application");
{"timestamp":"2015-08-02T18:02:39.456Z","level":"DEBUG","hostName":"<hidden>","process":{"name":"karltest","pid":26693},"message":"Entering application","fileName":"karltest.js","lineNumber":41,"functionName":"<anonymous>", "headers": { "host": "localhost", ... }}


Karl catches any uncaught exception (see Event 'uncaughtException').
When such an exception occurs, Karl logs a fatal log message (including stack trace) and then gracefully shuts down the process by emitting a SIGINT signal.


Unit Tests

$ npm test

Unit Tests with Code Coverage

$ npm run test-cov

This will generate a folder coverage containing coverage information and a folder coverage/lcov-report` containing an HTML report with the coverage results.

$ npm run test-ci

will create a folder 'coverage' containing lcov formatted coverage information to be consumed by a 3rd party coverage analysis tool. This script is typically used on a continuous integration server.


$ npm run benchmark



$ npm run check-style

will run the jscs stylechecker against the code.

Static Code Analysis


$ npm run code-analysis

will run jshint to analyse the code.

Code Documentation


$ npm run make-docs

will run jsdoc to create documentation.



Rate & Review

Great Documentation0
Easy to Use0
Highly Customizable0
Bleeding Edge0
Responsive Maintainers0
Poor Documentation0
Hard to Use0
Unwelcoming Community0