JSFiddle - React, Tailwind, and code Playground

HTML

<div class="dynamicalfields_wrapper">
    <input type="text" name="test[]" value="1" />
    <input type="text" name="test[]" value="2" />
    <input type="text" name="test[]" value="3" />
    <input type="text" name="test[]" value="" />
</div>

CSS

.dynamicalfields_wrapper {width: 400px;}
.dynamicalfields_wrapper input {width: 100%;}

JavaScript

(function ($) {
    $.fn.DynamicalFieldsUpdater = function () {
        return this.each(function (i, elm) {
            var each_elms = $(elm).find('input'),
                last_field = each_elms.last();

            $(last_field).css('background', 'red').live('focus', function () {
                // alert('holy fiddle');
                $(elm).append($(last_field).clone().val(''));
            });
        });
    };
})(jQuery);

$('.dynamicalfields_wrapper').DynamicalFieldsUpdater();