title: Trigger event when an object's property changes
body: <p>Unsure about the correct terminology for this, but I'm working with a few APIs in NodeJS, as well as Mongoose. I have a stack of Mongoose records which have data from the Google Maps API being loaded into them, and I want to create a listener for each record to watch when a particular field is filled.</p>
  
  <p>I can't do this via simple callbacks, as the APIs are rate limited and require an interval queuing system.</p>
  
  <p>Instead I'd like something like:</p>
  
  <pre><code> var obj1 = {name: 'rec1', mapData: {}, fieldData: {} };
   obj1.on('mapDataComplete', function(){...});
  </code></pre>
  
  <p>Where <code>mapDataComplete</code> is emitted when <code>obj1.mapData</code> changes from its initial state.</p>