Question

How to build function to return object in jquery like this:

<ul class="clearfix">
  <li data-year="2011" data-month="1">Jan</li>
  <li data-year="2011" data-month="2">Feb</li>
  <li data-year="2012" data-month="3">Mar</li>
  <li data-year="2012" data-month="4">Apr</li>
  <li data-year="2012" data-month="5">May</li>
</ul>

Search for li item, find data-year and return an object with count of equal items, like that
{2011: 2, 2012: 3}

Answer 1

function obj(ul) {
    return {
               2011: $('li[data-year="2011"]', ul).length,
               2012: $('li[data-year="2012"]', ul).length
           }
}

console.log(obj($('ul')))   

http://jsfiddle.net/x2n7t/2/

Update:

var obj = {};

$('ul li[data-year]').each(function(){
   var year = $(this).data('year');
   obj[year] = obj[year] ? obj[year] + 1 : 1
})

http://jsfiddle.net/x2n7t/3/