Stream transformer that prefixes lines with timestamps and other things.

Usage no npm install needed!

<script type="module">
  import strongLogTransformer from '';



A stream filter for performing common log stream transformations like timestamping and joining multi-line messages.

This is not a logger! But it may be useful for rolling your own logger.


Install strong-log-transformer and add it to your dependencies list.

npm install --save strong-log-transformer


When installed globally the sl-log-transformer CLI utility is exposed. It is primarily used for testing, but it can also be used as an alternative to awk or sed for jobs such as timestamping every line of another process's output. This can be useful for cron jobs, for example.

$ npm install -g strong-log-transformer
$ sl-log-tranformer --help
Usage: sl-log-transformer [options]

Stream transformer that prefixes lines with timestamps and other things.

   --format FORMAT        default: "text"
   --tag TAG              default: ""
   --mergeMultiline       default: off
   --timeStamp            default: off

Line Merging

In order to keep things flowing when line merging is enabled (disabled by default) there is a sliding 10ms timeout for flushing the buffer. This means that whitespace leading lines are only considered part of the previous line if they arrive within 10ms of the previous line, which should be reasonable considering the lines were likely written in the same write().


Here's an example using the transformer to annotate log messages from cluster workers.

var cluster = require('cluster');

if (cluster.isMaster) {
  // Make sure workers get their own stdout/stderr streams
  cluster.setupMaster({silent: true});

  // require log transformer module
  var transformer = require('strong-log-transformer');

  // Following the 12-factor app model, we pipe to stdout, but we could easily
  // pipe to any other stream(s), such as a FileStream for a log file.

  // stdout is plain line-oriented logs, but we want to add timestamps
  var info = transformer({ timeStamp: true,
                           tag: 'INFO' });
  // stderr will only be used for strack traces on crash, which are multi-line
  var error = transformer({ timeStamp: true,
                            tag: 'ERROR',
                            mergeMultiline: true });

  // Each worker's stdout/stderr gets piped into our info and erro transformers
  cluster.on('fork', function(worker) {
    console.error('connecting worker');

  //... cluster fork logic goes here ...

} else {
  //... worker code here ...

  console.log('new worker, this line will be timestamped!');
  throw new Error('This will generate a multi-line message!');

When we run the example code as example.js we get:

$ node example.js
connecting worker
2014-06-08T18:54:00.920Z INFO new worker, this line will be timestamped!
2014-06-08T18:54:00.926Z ERROR /Users/ryan/work/strong-log-transformer/e.js:33\n    throw new Error('This will generate a multi-line message!');\n          ^
2014-06-08T18:54:00.926Z ERROR Error: This will generate a multi-line message!\n    at null._onTimeout (/Users/ryan/work/strong-log-transformer/e.js:33:11)\n    at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)