jQuery: the filter() method

The jQuery filter() method is a DOM utility used to select only certain elements within a jQuery wrapped set. This method accepts either a selector or a callback function to select only the elements that meet a given condition. For example, given the following markup:

<ul id="test">

 <li>A</li>
 <li>1</li>
 <li>B</li>
 <li>2</li>

</ul>

We can select all the even elements in this way:

$('li', '#test').filter('li:nth-child(even)')
                .addClass('test');

Even more useful is the ability of this method to accept a callback function. Let's say that we want to select only the elements whose text content is a digit:

$('li', '#test').filter(function() {

    return /^\d+$/.test($(this).text());
    
}).addClass('test');

As you can see, this second approach is more powerful than the first.