console.log('test',$('.default').attr('id'));

<ul id="sortable">
	<li class="default" id="w1">Item 1</li>
	<li class="default" id="w2">Item 2</li>
	<li class="default" id="w3">Item 3</li>
	<li class="default" id="w4">Item 4</li>
</ul>

var ids = [];
$('.default').each(function(){ ids.push(this.id) });
console.log(ids.toString());