jQuery Cookbook - Cdn.oreilly.com - O'Reilly

These filters can also be used with regular CSS syntax. For example, selecting all text
input elements plus all elements can be done in the following way:
jQuery(':text, textarea');
2.10 Selecting an Element with Specific Characteristics
Problem
You need to select an element based not only on its relationship to other elements or
simple attribute values but also on varying characteristics such as programmatic states
not expressible as selector expressions.
Solution
If you’re looking for an element with very specific characteristics, selector expressions
may not be the best tool. Using jQuery’s DOM filtering method (filter()), you can
select elements based on anything expressible within a function.
The filter method in jQuery allows you to pass either a string (i.e., a selector expression)
or a function. If you pass a function, then its return value will define whether certain
elements are selected. The function you pass is run against every element in the current
selection; every time the function returns false, the corresponding element is removed
from the collection, and every time you return true, the corresponding element is not
affected (i.e., it remains in the collection):
jQuery('*').filter(function(){
return !!jQuery(this).css('backgroundImage');
});
The preceding code selects all elements with a background image.
The initial collection is of all elements (*); then the filter() method is called with a
function. This function will return true when a backgroundImage is specified for the
element in question. The !! that you see is a quick way of converting any type in Java-
Script to its Boolean expression. Things that evaluate to false include an empty string,
the number zero, the value undefined, the null type, and, of course, the false Boolean
itself. If any of these things are returned from querying the backgroundImage, the function
will return false, thus removing any elements without background images from
the collection. Most of what I just said is not unique to jQuery; it’s just JavaScript
fundamentals.
In fact, the !! is not necessary because jQuery evaluates the return value into a Boolean
itself, but keeping it there is still a good idea; anyone looking at your code can be
absolutely sure of what you intended (it aids readability).
2.10 Selecting an Element with Specific Characteristics | 47