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JSON logger for Node.js and browser.


For a long time I have been a big fan of using debug. debug is simple to use, works in Node.js and browser, does not require configuration and it is fast. However, problems arise when you need to parse logs. Anything but one-line text messages cannot be parsed in a safe way.

To log structured data, I have been using Winston and Bunyan. These packages are great for application-level logging. I have preferred Bunyan because of the Bunyan CLI program used to pretty-print logs. However, these packages require program-level configuration – when constructing an instance of a logger, you need to define the transport and the log-level. This makes them unsuitable for use in code designed to be consumed by other applications.

Then there is pino. pino is fast JSON logger, it has CLI program equivalent to Bunyan, it decouples transports, and it has sane default configuration. Unfortunately, you still need to instantiate logger instance at the application-level. This makes it more suitable for application-level logging just like Winston and Bunyan.

I needed a logger that:

  • Does not block the event cycle (=fast).
  • Does not require initialisation.
  • Produces structured data.
  • Decouples transports.
  • Has a CLI program.
  • Works in Node.js and browser.
  • Configurable using environment variables.

In other words,

  • a logger that I can use in an application code and in dependencies.
  • a logger that allows to correlate logs between the main application code and the dependency code.
  • a logger that works well with transports in external processes.

Roarr is this logger.


Producing logs

Roarr logger API for producing logs is the same in Node.js and browser.

  1. Import roarr
  2. Use any of the API methods to log messages.


import log from 'roarr';

Consuming logs

Roarr logs are consumed differently in Node.js and browser.


In Node.js, Roarr logging is disabled by default. To enable logging, you must start program with an environment variable ROARR_LOG set to true, e.g.

ROARR_LOG=true node ./index.js

All logs will be written to stdout.


In a browser, you must implement ROARR.write method to read logs, e.g.

import {
} from 'roarr';
ROARR.write = () => {};

The API of the ROARR.write is:

(message: string) => void;

Example implementation:

import {
} from 'roarr';
ROARR.write = (message) => {

or if you are initializing ROARR.write before roarr is loaded:

// Ensure that `globalThis.ROARR` is configured.
const ROARR = globalThis.ROARR = globalThis.ROARR || {};
ROARR.write = (message) => {

If your platform does not support globalThis, use globalthis polyfill.

Filtering logs


In Node.js, Roarr prints all or none logs (refer to the ROARR_LOG environment variable documentation).

Use roarr filter CLI program to filter the logs that are written to stdout by the program, e.g.

ROARR_LOG=true node ./index.js | roarr filter '{"context.logLevel":{gt:30}}'

Alternatively, use a JSON processor such as jq


In a browser, Roarr calls globalThis.ROARR.write for every log message. Implement your own custom logic to filter logs, e.g.

globalThis.ROARR.write = (message) => {
  const payload = JSON.parse(message);
  if (payload.context.logLevel > 30) {

Log message format

Property name Contents
context Arbitrary, user-provided structured data. See context property names.
message User-provided message formatted using printf.
sequence An incremental ID.
time Unix timestamp in milliseconds.
version Roarr log message format version.


  "context": {
    "application": "task-runner",
    "hostname": "curiosity.local",
    "instanceId": "01BVBK4ZJQ182ZWF6FK4EC8FEY",
    "taskId": 1
  "message": "starting task ID 1",
  "sequence": 0,
  "time": 1506776210000,
  "version": "1.0.0"


roarr package exports a function with the following API:

export type LoggerType =
    context: MessageContextType,
    message: string,
    c?: SprintfArgumentType,
    d?: SprintfArgumentType,
    e?: SprintfArgumentType,
    f?: SprintfArgumentType,
    g?: SprintfArgumentType,
    h?: SprintfArgumentType,
    i?: SprintfArgumentType,
    k?: SprintfArgumentType
  ) => void |
    message: string,
    b?: SprintfArgumentType,
    c?: SprintfArgumentType,
    d?: SprintfArgumentType,
    e?: SprintfArgumentType,
    f?: SprintfArgumentType,
    g?: SprintfArgumentType,
    h?: SprintfArgumentType,
    i?: SprintfArgumentType,
    k?: SprintfArgumentType
  ) => void;

To put it into words:

  • First parameter can be either a string (message) or an object.
    • If 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).

Refer to the Usage documentation for common usage examples.


<T>(routine: () => Promise<T>, context: MessageContextType) => Promise<T>,

adopt function uses Node.js domain to pass-down context properties.

When using adopt, context properties will be added to all all Roarr messages within the same asynchronous context, e.g.

await log.adopt(
  async () => {
    log('foo 0');
    await log.adopt(
      () => {
        log('foo 1');
        baz: 'baz 1',
    bar: 'bar 0',
// {"context":{"bar":"bar 0"},"message":"foo 0","sequence":0,"time":1531914656076,"version":"1.0.0"}
// {"context":{"bar":"bar 0","baz":"baz 1"},"message":"foo 1","sequence":1,"time":1531914656077,"version":"1.0.0"}]


  • adopt method only works in Node.js.
  • You must shim Node.js using domain-parent.


(context: TranslateMessageFunctionType | MessageContextType) => LoggerType,

The child function has two signatures:

  1. Accepts an object.
  2. Accepts a function.

Object parameter

(context: MessageContextType) => LoggerType;

Creates a child logger appending the provided context object to the previous logger context.


import log from 'roarr';
const childLog = log.child({
  foo: 'bar'
log.debug('foo 1');
childLog.debug('foo 2');
// {"context":{"logLevel":20},"message":"foo 1","sequence":0,"time":1531914529921,"version":"1.0.0"}
// {"context":{"foo":"bar","logLevel":20},"message":"foo 2","sequence":1,"time":1531914529922,"version":"1.0.0"}

Refer to middlewares documentation for use case examples.

Function parameter

(translateMessage: TranslateMessageFunctionType) => LoggerType;

Creates a child logger where every message is intercepted.


import log from 'roarr';
const childLog = log.child((message) => {
  return {
    message: message.message.replace('foo', 'bar'),
log.debug('foo 1');
childLog.debug('foo 2');
// {"context":{"logLevel":20},"message":"foo 1","sequence":0,"time":1531914656076,"version":"1.0.0"}
// {"context":{"logLevel":20},"message":"bar 2","sequence":1,"time":1531914656077,"version":"1.0.0"}


Returns the current context.


import log from 'roarr';
const childLogger = log.child({
  foo: 'bar'
// {foo: 'bar'}







Convenience methods for logging a message with logLevel context property value set to a numeric value representing the log level, e.g.

import log from 'roarr';

Produces output:



Roarr logger supports middlewares implemented as child message translate functions, e.g.

import log from 'roarr';
import createSerializeErrorMiddleware from '@roarr/middleware-serialize-error';
const childLog = log.child(createSerializeErrorMiddleware());
const error = new Error('foo');
log.debug({error}, 'bar');
childLog.debug({error}, 'bar');
// {"context":{"logLevel":20,"error":{}},"message":"bar","sequence":0,"time":1531918373676,"version":"1.0.0"}
// {"context":{"logLevel":20,"error":{"name":"Error","message":"foo","stack":"[REDACTED]"}},"message":"bar","sequence":1,"time":1531918373678,"version":"1.0.0"}

Roarr middlwares enable translation of every bit of information that is used to construct a log message.

The following are the official middlewares:

Raise an issue to add your middleware of your own creation.

CLI program

Roarr CLI program provides ability to filter and pretty-print Roarr logs.

CLI output demo

CLI program has been moved to a separate package @roarr/cli.

npm install @roarr/cli -g

Explore all CLI commands and options using roarr --help or refer to @roarr/cli documentation.


A transport in most logging libraries is something that runs in-process to perform some operation with the finalised log line. For example, a transport might send the log line to a standard syslog server after processing the log line and reformatting it.

Roarr does not support in-process transports.

Roarr does not support in-process transports because Node processes are single threaded processes (ignoring some technical details). Given this restriction, Roarr purposefully offloads handling of the logs to external processes so that the threading capabilities of the OS can be used (or other CPUs).

Depending on your configuration, consider one of the following log transports:

  • 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 (e.g. Kubernetes) (written in Ruby).

Node.js environment variables

Use environment variables to control roarr behaviour.

Name Type Function Default
ROARR_LOG Boolean Enables/ disables logging. false
ROARR_STREAM STDOUT, STDERR Name of the stream where the logs will be written. STDOUT

When using ROARR_STREAM=STDERR, use 3>&1 1>&2 2>&3 3>&- to pipe stderr output.


Context property names

Roarr does not have reserved context property names. However, I encourage use of the following conventions:

Context property name Use case
application Name of the application (do not use in code intended for distribution; see package property instead).
logLevel A numeric value indicating the log level. See API for the build-in loggers with a pre-set log-level.
namespace Namespace within a package, e.g. function name. Treat the same way that you would construct namespaces when using the debug package.
package Name of the NPM package.

The roarr pretty-print CLI program is using the context property names suggested in the conventions to pretty-print the logs for the developer inspection purposes.

Log levels

The roarr pretty-print CLI program translates logLevel values to the following human-readable names:

logLevel Human-readable name

Using Roarr in an application

To avoid code duplication, you can use a singleton pattern to export a logger instance with predefined context properties (e.g. describing the application).

I recommend to create a file Logger.js in the project directory. Inside this file create and export a child instance of Roarr with context parameters describing the project and the script instance, e.g.

 * @file Example contents of a Logger.js file.
import log from 'roarr';
const Logger = log.child({
  // .foo property is going to appear only in the logs that are created using
  // the current instance of a Roarr logger.
  foo: 'bar'
export default Logger;

Roarr does not have reserved context property names. However, I encourage use of the conventions.


Logging errors

This is not specific to Roarr – this suggestion applies to any kind of logging.

If you want to include an instance of Error in the context, you must serialize the error.

The least-error prone way to do this is to use an existing library, e.g. serialize-error.

import log from 'roarr';
import serializeError from 'serialize-error';
// [..]
send((error, result) => {
  if (error) {
      error: serializeError(error)
    }, 'message not sent due to a remote error');
  // [..]

Without using serialisation, your errors will be logged without the error name and stack trace.

Using with Elasticsearch

If you are using Elasticsearch, you will want to create an index template.

The following serves as the ground work for the index template. It includes the main Roarr log message properties (context, message, time) and the context properties suggested in the conventions.

  "mappings": {
    "log_message": {
      "_source": {
        "enabled": true
      "dynamic": "strict",
      "properties": {
        "context": {
          "dynamic": true,
          "properties": {
            "application": {
              "type": "keyword"
            "hostname": {
              "type": "keyword"
            "instanceId": {
              "type": "keyword"
            "logLevel": {
              "type": "integer"
            "namespace": {
              "type": "text"
            "package": {
              "type": "text"
        "message": {
          "type": "text"
        "time": {
          "format": "epoch_millis",
          "type": "date"
  "template": "logstash-*"

Using with Scalyr

If you are using Scalyr, you will want to create a custom parser RoarrLogger:

  patterns: {
    tsPattern: "\\w{3},\\s\\d{2}\\s\\w{3}\\s\\d{4}\\s[\\d:]+",
    tsPattern_8601: "\\d{4}-\\d{2}-\\d{2}T[\\d:.]+Z"
  formats: [
    {format: "${parse=json}$"},
    {format: ".*\"time\":$timestamp=number$,.*"},
    {format: "$timestamp=tsPattern$ GMT $detail$"},
    {format: "$timestamp=tsPattern_8601$ $detail$"}

and configure the individual programs to use RoarrLogger. In case of Kubernetes, this means adding a log.config.scalyr.com/attributes.parser: RoarrLogger annotation to the associated deployment, pod or container.

Documenting use of Roarr

If your package is using Roarr, include instructions in README.md describing how to enable logging, e.g.

## Logging
This package is using [`roarr`](https://www.npmjs.com/package/roarr) logger to log the program's state.
Export `ROARR_LOG=true` environment variable to enable log printing to stdout.
Use [`roarr-cli`](https://github.com/gajus/roarr-cli) program to pretty-print the logs.


npm i roarr

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