Isomorphic logger based on Elastic Common Schema


  • Tiny <1KB size gzip
  • Works in Node.js and in Browser
  • Built-in Typescript support

How to install


npm install eo-logger --save

How to use in browser

For start using required to create instance of class Logger

import { Logger } from 'eo-logger/dist/client/logger';

export const logger = new Logger();

Now you can import logger in any place of your application and use one of next methods

logger.error – for errors

logger.debug – for debug some information

logger.warning – for warnings

logger.collectMetrics – for collect performance metrics of your web application (this method available only for client logger)

By default logger transport just display ECS.Message in console.
If you want to collect logs, you have to implement your own Transport class.
Actually it is quite simple, look at example below:

import { ECS, Transport } from 'eo-logger';
import { Logger } from 'eo-logger/dist/client/logger';

class MyTransport extends Transport {
  public send(message: ECS.Message): void {
    fetch('/frontend-logs', {
      method: 'post',
      body: JSON.stringify(message),

export const logger = new Logger({
  transport: new MyTransport(),

How to use with express

  1. Create api endpoint for collect logs
import * as express from 'express';

const app = express();
app.use(express.json());'/frontend-logs', (req, res) => {
  return res.status(200).send();
  1. Add @elastic/elasticsearch
yarn add @elastic/elasticsearch
  1. Create elasticsearch client and transport instance
import * as express from 'express';
import { Client } from '@elastic/elasticsearch';
import { Transport as ElkTransport } from 'eo-logger/dist/server/logger';

const app = express();
app.use(express.json());'/frontend-logs', (req, res) => {

  return res.status(200).send();

const elasticClient = new Client({ node: 'http://localhost:9200' });

const elkTransport = new ElkTransport({
  client: elasticClient,
  indexBase: 'frontend-logs',
  maxQueueSize: 5,


Logger contains 3 main components:

  • Context
  • Transport
  • Formatter

Client utilities

getPerformanceMetrics – function returns object with performance metrics of page speed loading

Server utilities

parseUserAgent – function returns ECS.UserAgent object with information based on user-agent string

parseGeo – function return ECS.Geo object with information based on ip string

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