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.

_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();

In this case _UNIQUEAGEARRAY will equal:

[7,9,2]

What I actually need returned is something like:

[{uniqueAge:7,numberOfPeople:2}{uniqueAge:9,numberOfPeople:1}{uniqueAge:2,numberOfPeople:1}]

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.