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jQuery conditionals plugin 2007/08/22

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])) {
    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()) {

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();

jquery.if.js (version 0.1)

Comments (2)

  1. Hey Richard,

    Just wanted to let you know that the functionality you are looking for is built into the .filter() method. Filter can take either a selector or a function. Those jQuery core methods can be hard to find sometimes and it is just so easy to quickly write a plugin for what you need. :) Anyways, here are the docs for the filter method:

    Brandon Aaron — 2007/08/22 10:42 am

  2. D'oh! You're right, they're sometimes quite hard to find and I never would have called it "filter". Thanks for the tip, Brandon.

    Richard Crowley — 2007/08/22 10:48 am

Richard Crowley?  Kentuckian engineer who cooks and eats in between bicycling and beering.

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