jQuery “hasParent”

For a clean re-usable solution, consider extending the jQuery.fn object with a custom method used for determining the presence of a particular ancestor for any given element:

// Extend jQuery.fn with our new method
jQuery.extend( jQuery.fn, {
    // Name of our method & one argument (the parent selector)
    within: function( pSelector ) {
        // Returns a subset of items using jQuery.filter
        return this.filter(function(){
            // Return truthy/falsey based on presence in parent
            return $(this).closest( pSelector ).length;
        });
    }
});

This results in a new method, $.fn.within, that we can use to filter our results:

$("li").within(".x").css("background", "red");

This selects all list items on the document, and then filters to only those that have .x as an ancestor. Because this uses jQuery internally, you could pass in a more complicated selector:

$("li").within(".x, .y").css("background", "red");

This will filter the collection to items that descend from either .x or .y, or both.

Fiddle: http://jsfiddle.net/jonathansampson/6GMN5/