Logging Using EventEmitters in Node.js

January 15, 2014

I've been working a lot in node.js lately for a work project. Javascript as a language is an odd duck (pun not intended). It's got all these incredibly powerful features - dynamic typing, inheritance-by-prototype, functions as first-class objects - but it has some really odd anachronisms, including having to use the C-style for (var i = 0; i < length; i++) loop to iterate over arrays. Node adds a lot to the language as well, such as EventEmitters, which are very powerful. This afternoon I found a nifty new use for them: logging.

One of the issues that always seems to come up is the fact that logging is one of those things that is both local and global - it's local, in that you want to do the logging at the place where the event you're logging occurs, but global in that you want your logging configured globally - you don't want each and every class/module to have to "know" about logging. One consequence of this, especially with respect to testing, is that you end up having to configure loggers for your unit tests, which, in a way, makes them no longer "unit" tests at all. Optimally, what you want is a situation where you're logging locally, but if logging hasn't been set up by anyone, the logs just go into /dev/null. Basically, you want your logging to involve sending out logging "events", which are either captured by something, or not. EventEmitters give that to you for free.

All EventEmitters are, for those of you unfamiliar with them (but familiar with OO terminology) are an implementation of the observer pattern, but a really lightweight and easy to use one. I won't go into details on how it works, if you're interested, look at the docs (or, even better, check out eventemitter2, which adds wildcards and namespaces to it). What I want to talk about is how to leverage it to make logging nicer.

For instance, if you have a logging package that you're using (I'm using winston), you just create a module containing a class that is an EventEmitter:

var EventEmitter = require('events').EventEmitter;
var util = require('util');

function LogEmitter() {
  EventEmitter.call(this);
}

util.inherit(LogEmitter, EventEmitter);

Then, create an instance of your emitter, and you make your logging method emit an event:

var logEmitter = new LogEmitter();

module.exports.log = function(level, message) {
  logEmitter.emit('logging', level, message);
}

This just emits a logging event when log() is called, passing the parameters along.

Finally, you also have something listening if someone initializes logging:

var initialized = false;
module.exports.initialize = function() {
  if (!initialized) {
    initialized = true;
    logEmitter.on('logging', function(level, message) {
      // Log the message through your logging package here
    });
  }
}

Then, you're good to go. Just distribute logging.log() calls throughout your code. If something in the code calls logging.initialize(), great, your messages get logged. If not (like, say, in a unit test), the messages go into the bitbucket.

Jack Lund

Jack Lund

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