jQuery provides a ton of ways to select sets of elements but pretty much no way to pare down a selection in an arbitrary way. Fortunately for me it’s easy to extend jQuery, so I wrote a plugin that allows you to apply a function to your set of elements to pare the set down before applying an action. Source first, talk later:

jQuery.fn.if = function(arg) {
    var elems = [];
    var all = this.get();
    var ii = all.length;
    for (var i = 0; i < ii; ++i) {
        if (arg.apply(all[i])) {
            elems.push(all[i]);
        }
    }
    return jQuery(elems);
};

jquery.if.js (version 0.1)

The plugin takes as its argument a function that must return a boolean value. This function will be applied to every argument in the current set. Those that the function finds true will be returned in the final subset.

The use-case that inspired this quickie plugin is this: I wanted an action to happen to form fields that were empty, which previously required a bit of work. If I had more work to do with each match or a more complex matching requirement, this could get ugly.

$('input.promptable').focus(function() {
    var to = $(this);
    if ('' == to.val()) {
        to.val(this.id);
        to.addClass('prompt');
    }
});

With this plugin, the chaining action jQuery is known for is not interrupted. I can select all input elements with the promptable class and pare down that selection to only those whose value is empty. On the set returned there I can set values and add classes without breaking the chain.

$('input.promptable').focus(function() {
    $(this).if(function() {
        return '' == $(this).val();
    }).val(this.id).addClass('prompt');
});

jquery.if.js (version 0.1)