jQuery Plugin. Set each radio button group to the first value.

jquery.radioDefault.js

/*
 * jquery.radioDefault.js
 * Sets the first radio button of each radio button group to checked.
 *
 * Version: 1.0
 * Author: Martin Eriksson
 */

(function ($) {

    $.fn.radioDefault = function () {

        return this.each(function () {

            var context = this,
                names = {};

            // Collect all names.
            //
            $(':radio', context).each(function () {
                if (this.name) {
                    names[this.name] = true;
                }
            });

            // Iterate all names, get the first value, then set the value to the radio group.
            //
            $.each(names, function (key) {
                if (key) {
                    var selector = ':radio[name="' + key + '"]',
                        value = $(selector, context).not(':disabled').first().val();

                    $(selector, context).val([value]).trigger('change');
                }
            });

        });

    };

})(jQuery);