How to to log JS errors from a client into kibana?

When you say client, I’m assuming here that you mean a logging client and not a web client.

First, make it a habit to log your errors in a common format. Logstash likes consistency, so if you’re putting text and JSON in the same output log, you will run into issues. Hint: log in JSON. It’s awesome and incredibly flexible.

The overall process will go like this:

1. Error occurs in your app
2. Log the error to file, socket, or over a network
3. Tell logstash how to get (input) that error (i.e. from file, listen over network, etc)
4. Tell logstash to send (output) the error to Elasticsearch (which can be running on the same machine)

In your app, try using the bunyan logger for node. https://github.com/trentm/node-bunyan

node app index.js

var bunyan = require('bunyan');
var log = bunyan.createLogger({
  name: 'myapp',
  streams: [{
    level: 'info',
    stream: process.stdout // log INFO and above to stdout
  }, {
    level: 'error',
    path: '/var/log/myapp-error.log' // log ERROR and above to a file
  }]
});

// Log stuff like this
log.info({status: 'started'}, 'foo bar message');

// Also, in express you can catch all errors like this
app.use(function(err, req, res, next) {
   log.error(err);
   res.send(500, 'An error occurred');
});

Then you need to configure logstash to read those JSON log files and send to Elasticsearch/Kibana. Make a file called myapp.conf and try the following:

logstash config myapp.conf

# Input can read from many places, but here we're just reading the app error log
input {
    file {
        type => "my-app"
        path => [ "/var/log/myapp/*.log" ]
        codec => "json"
    }   
}

# Output can go many places, here we send to elasticsearch (pick one below)
output {

  elasticsearch {
    # Do this if elasticsearch is running somewhere else
    host => "your.elasticsearch.hostname"
    # Do this if elasticsearch is running on the same machine
    host => "localhost"
    # Do this if you want to run an embedded elastic search in logstash
    embedded => true   
  }

}

Then start/restart logstash as such: bin/logstash agent -f myapp.conf web

Go to elasticsearch on http://your-elasticsearch-host:9292 to see the logs coming in.