Roarr! the perfect JSON logger for Node.js and browser

Zero-configuration, out of process transports and adheres to the Twelve Factors

Running services in high-concurrency you need to be able to pin point when and what breaks.

Existing loggers

  • I can use in an application code and in dependencies
  • allows me to correlate logs between the main application code and the dependency code
  • works well with transports in external processes
Roarr is loud

Strict API


ROARR_LOG=true node ./index.js

The logger function

  • The first parameter can be either a string (message) or an object. If the first parameter is an object (context), the second parameter must be a string (message).
  • Arguments after the message parameter are used to enable printf message formatting. Printf arguments must be of a primitive type (string | number | boolean | null). There can be up to 9 printf arguments (or 8 if the first parameter is the context object).
import log from 'roarr';


log('bar %s', 'baz');

const debug = log.child({
level: 'debug'


quuz: 'corge'
}, 'quux');
{"context":{},"message":"bar baz","sequence":1,"time":1506776210000,"version":"1.0.0"}

Inspecting the logs

CLI program output.

Usage patterns

Using Roarr in an application

* @file Example contents of a Logger.js file.
import Roarr from 'roarr';
import ulid from 'ulid';
// Instance ID is useful for correlating logs in high concurrency environment.
// See `roarr augment --append-instance-id` option as an alternative way to
// append instance ID to all logs.
const instanceId = ulid();
// The reason we are using `global.ROARR.prepend` as opposed to `roarr#child`
// is because we want this information to be prepended to all logs, including
// those of the "my-application" dependencies.
global.ROARR.prepend = {,
application: 'my-application',
export default Roarr.child({
// .foo property is going to appear only in the logs that are created using
// the current instance of a Roarr logger.
foo: 'bar'

Using Roarr in a dependency

* @file Example contents of a Logger.js file.
import Roarr from 'roarr';export default Roarr.child({
domain: 'database',
package: 'my-package'

Filtering logs

ROARR_LOG=true node ./index.js | jq 'select(.context.logLevel == "warning" or .context.logLevel == "error")'
ROARR_LOG=true node ./index.js | jq 'select(.context.package == "usus")' | roarr pretty-print

Manipulating the log message context globally

import log from 'roarr';
import foo from 'foo';

const taskIds = [

for (const taskId of taskIds) {
global.ROARR = global.ROARR || {};
global.ROARR.prepend = {

log('starting task ID %d', taskId);

// In this example, `foo` is an arbitrary third-party dependency that is using
// roarr logger.

log('successfully completed task ID %d', taskId);

global.ROARR.prepend = {};
{"context":{"taskId":1},"message":"starting task ID 1","sequence":0,"time":1506776210000,"version":"1.0.0"}
{"context":{"taskId":1},"message":"successfully completed task ID 1","sequence":2,"time":1506776210000,"version":"1.0.0"}


  • Beats
    For aggregating at a process level
    Written in Go
  • logagent
    For aggregating at a process level
    Written in JavaScript
  • Fluentd
    For aggregating logs at a container orchestration level such as Kubernetes
    Written in Ruby

The future of Roarr

What about the name?

You’re gonna hear me roar!



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Gajus Kuizinas

Founder, engineer interested in JavaScript, PostgreSQL and DevOps. Follow me on Twitter for outbursts about startups & engineering.