npm

Add dynamic instrumentation to emitters

shimmer does a bunch of the work necessary to wrap other methods in a wrapper you provide:

var EventEmitter = require('events').EventEmitter;
var wrapEmitter = require('emitter-listener');
 
var ee = new EventEmitter();
 
var id = 0;
 
wrapEmitter(
  ee,
  function mark(listener) {
    listener.id = id++;
  },
  function prepare(listener) {
    console.log('listener id is %d', listener.id);
  }
);

Mandatory disclaimer

There are times when it's necessary to monkeypatch default behavior in JavaScript and Node. However, changing the behavior of the runtime on the fly is rarely a good idea, and you should be using this module because you need to, not because it seems like fun.

wrapEmitter(emitter, mark, prepare)

Wrap an EventEmitter's event listeners. Each listener will be passed to mark when it is registered with .addListener() or .on(), and then each listener is passed to prepare to be wrapped before it's called by the .emit() call. wrapListener deals with the single listener vs array of listeners logic, and also ensures that edge cases like .removeListener() being called from within an .emit() for the same event type is handled properly.

The wrapped EE can be restored to its pristine state by using emitter.__unwrap(), but this should only be used if you really know what you're doing.