title: Underscore.js find unique values in array of objects; Return unique items and their count
body: <p>I am using _underscore.js to find all unique items in an array, but I can't figure out how to also get the number of unique items returned.</p>
  
  <pre><code>_PERSONARRAY = [{name:"tom",age:7}{name:"john",age:9}{name:"becky",age:2}{name:"sam",age:7}]
  
  _UNIQUEAGEARRAY = _.chain(_PERSONARRAY).map(function(person) { return person.age }).uniq().value();
  </code></pre>
  
  <p>In this case _UNIQUEAGEARRAY will equal:</p>
  
  <pre><code>[7,9,2]
  </code></pre>
  
  <p>What I actually need returned is something like:</p>
  
  <pre><code>[{uniqueAge:7,numberOfPeople:2}{uniqueAge:9,numberOfPeople:1}{uniqueAge:2,numberOfPeople:1}]
  </code></pre>
  
  <p>Thanks for help.  Also, I'm assuming _underscore.js is quick at doing this??  If it's stupid slow tell me cause I'd be open to other solutions.</p>