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3.3.4 • Public • Published



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Simple pino-based logger for all your writing needs

How to use

Import the logger factory

const loggerFactory = require('cosmas').default;

or with import

import loggerFactory from 'cosmas';

Create logger with default configuration

const logger = loggerFactory; // factory itself is a logger
// or
const logger = loggerFactory();

Create logger with custom configuration

const logger = loggerFactory({
    disableFields: ['error.stack'],
    enableFields: ['req.protocol']

See Options for a list of possible options.

Child loggers

Every logger can be used to create a child logger. Child logger inherits all configuration of its parent and cannot override them.

Child logger can specify its own name which is then concatenated with parent's name. Therefore the child logger name is parentNamechildName.

const parentLogger = loggerFactory('database', { pretty: false });
const childLogger = parentLogger('.updates');

Logger usage

Logger itself is an enhanced and specifically configured pino instance, so you may use all basic pino log methods'hello world')
logger.error('this is at error level')'the answer is %d', 42){ obj: 42 }, 'hello world'){ obj: 42, b: 2 }, 'hello world'){ obj: { aa: 'bbb' } }, 'another')

All pino levels are supported and additionaly there is a warning level which is equivalent to warn level.

Default minimal log level is debug.

All loglevels up to warning (exclusive) - trace, debug and info - are logged to stdout only.

All loglevels from warning up (inclusive) - warning, error, fatal - are logged to stderr only.

Using Sentry

You can enhance cosmas logger with automatic Sentry logging functionality.

Sentry SDK @sentry/node is a peer dependency. If you want cosmas to use it, install it in your project.

import extendSentry from 'cosmas/sentry'

// (1) Let cosmas initialize sentry with provided DSN
const myLogger = loggerFactory();
extendSentry(myLogger, { sentry: 'https://<key><project>' }); // this will CHANGE the logger instance and returns it
// or
const sentryLogger = extendSentry(myLogger, { sentry: 'https://<key><project>' });

// (2) Configure sentry yourself and let cosmas use it
const myLogger = loggerFactory();
extendSentry(myLogger, { sentry: true });

After sentry extension, the interface of log methods will change.

const myLogger = loggerFactory();
myLogger.fatal({ foo: 'bar' }, 'message');

const sentryLogger = extendSentry(myLogger);
sentryLogger.fatal('message', sentryCallback);
sentryLogger.fatal({ foo: 'bar' }, sentryCallback);
sentryLogger.fatal({ foo: 'bar' }, 'message', sentryCallback);

All log methods accept an optional sentryCallback argument. You can use it to pass a function which will obtain a Sentry scope which can be edited that way.

import { Scope } from '@sentry/node';

sentryLogger.fatal('message', (scope: Scope) => {
    // scope.setContext
    // scope.setTags
    // scope.setExtras

When configured, cosmas (additionally to standard logging) captures all logs via Sentry SDK. Logs containing stack are logged as exceptions via captureException (preserves stack trace) and all other messages via captureMessage.

Either way, scope is appropriately set, as well as all payload is passed on in scope's metadata.

By default, Cosmas only logs to Sentry logs with warn or higher level. You can change this behaviour by setting sentryLevel option.

Express middleware

cosmas contains an express middleware which you can use to log all requests and responses of your express application.


const express = require('express');

const logger = require('cosmas').default;
// or
import logger from 'cosmas';

const app = express();
// or
const router = express.Router();

// or

By default, it will log all incoming requests in debug level, all outcoming responses with out property in debug level and all outcoming responses without out property on info level.

If you use it together with logger's error express middleware, it will also log all errors in error level.


All those log messages will contain request and possibly response, error, time from request to response, status code and user-agent, x-deviceid and authorization request headers.

Request and response skipping

You might want to omit some requests or responses from logging completely. Right now, there are two ways to do it and you can even use both at once.

  1. Use options.ignoredHttpMethods to define an array of HTTP methods you want to omit. By default all OPTIONS requests and responses are ommited. See options for details
  2. Use options.skip method to define custom rules for request/response skipping. Set it to a function which accepts an Express's Request and Response and returns boolean. If the return value is true, request (or response) will not be logged. You might want to use matchPath helper to ignore requests based on the req.originalUrl value
const { matchPath } = require('cosmas/utils');
const logger = require('cosmas').default({
    skip: matchPath(/heal.h/),

Environment-specific behavior

cosmas is meant to be used throughout different environments (development, testing, production) and some of its configuration is setup differently based on the environment it runs in. By default, severity (contains current log level) and pkgVersion (contains current version of cosmas) fields are added to logged object.


If the NODE_ENV environment variable is set to test, all logs are turned off (minimal loglevel is set to silent which effectively turns logging off).

Pretty print

If you set pretty option to true, you enable pretty print mode intended for development use. cosmas.pkgVersion, cosmas.loggerName and severity are ommited from the output.


Standard pino log is used and it's optimized for Google Stackdriver logging. That means that default log level is debug, pretty print is turned off and pino's messageKey option is set to message.


Options override both default logger configuration and environment-specific configuration. However, do not forget to specify it during the first cosmas. During it, root logger is created and it cannot be changed later.

  • defaultLevel - set logger's minimal loglevel (default is debug)
  • disableFields - list of paths which will be omitted from the objects being logged (if any)
  • enableFields - list of paths which will not be omitted by default serializers from objects being logged
  • ignoredHttpMethods - list of HTTP methods which will not be logged by express logging middleware at all. Defaults to ['OPTIONS']
  • streams - list of stream objects, which will be passed directly to pino-multistream's multistream function instead of default cosmas stream
  • pretty - if set to true, logger will use pino pretty human-readable logs. This option can be overriden by streams
  • disableStackdriverFormat - if set to false, logger will add severity field to all log objects, so that log levels in Google Stackdriver work as expected. Defaults to false
  • skip - Function to be used in express middlewares for filtering request and response logs. If the function returns true for a given request, no message will be logged. No default value.
  • config - object, which will be passed to underlying logger object. Right now, underlying logger is pino, so for available options see pino API docs
  • sentry - true to enable without configuring or <sentry dsn> to enable and configure with dsn beforehand, false to disable (default)
  • sentryLevel - set minimum level to log to sentry (default warn)

Default serializers

cosmas defines some pino serializers on its own

  • error - logs message, code, stack and data fields
  • processEnv - logs NODE_PATH and NODE_ENV
  • req - logs body, query, url, method, headers.x-deviceid, headers.authorization, headers.user-agent and omits password and passwordCheck from body and query
  • res - logs out, time

Reserved keys

Cosmas uses some object keys for its own purposes. Those keys should not be used in data you send to log functions as they may be overwritten by Cosmas. Those keys are:

  • cosmas.loggerName - used for the name of logger
  • cosmas.pkgVersion - used for the version of cosmas
  • message - used for the log message text


VS Code debugging not showing log messages

This problem is caused by a way VS Code handles console output. Therefore it appears in Winston and pino (underlying library of cosmas) as well.

However, it can be easily solved by adding eithe

"console": "integratedTerminal",


"outputCapture": "std"

to the debug configuration in your launch.json file.




npm i cosmas

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